Wet cleaning of historical textiles: surfactants and other wash bath additives
Ágnes Tímár-Balázsy
Abstract
The paper is a review of the literature relating to the use of water, surfactants and other additives
for cleaning historical textiles. Ethical considerations are introduced and the common types of soil
that occurs on historical textiles are characterized. The review covers publications on the
advantages and disadvantages of wet cleaning and discusses the properties of water, detergents
and surface active agents. The paper underlines the importance of the HLB value, critical micelle
concentration, solubility of surfactants, Krafft and cloud point in conservation. Washing processes
using surface active agents, the connection between the chemical structure of surfactants and
detergency, the role of soil/dirt anti-redeposition agents, foam, pH, washing time and temperature
in cleaning are discussed and the composition of solutions for washing historical textiles are given.
The paper introduces the dependence of rinsing on the adsorption of surfactant to textiles and
reviews the use of vacuum suction in wet cleaning. The efficiency of washing and the effect of
washing on fibres, textiles and dyes is assessed and the review ends with references to
biodegradation of surfactants and a list of selected case studies.
Introduction
Wet cleaning is a widely used conservation treatment for historical textiles. It is a cleaning method
using water as a solvent alone and as a solvent and/or medium for surface active and soil antiredeposition agents and other additives. The use of buffers and agents involving chemical reactions
between the dirt and the agent (such as sequestering agents, bleaching agents and enzymes), are not
included in this review.
Before the 1990s many textile conservators may have agreed with Durian-Rees who said that 'dirt is
not part of old age' [1]. In the last decade of the twentieth century several publications call textile
conservators' attention to the danger of removing valuable curatorial information from historical
textiles by wet cleaning, for example Eastop and Brooks [2, 3], Hall and Barnett [4], Dodds [5],
Brooks et al. [6, 7], Johansen [8], Stauffer [9] and Windsor [10]. However, wet cleaning has many
advantages and removing harmful dirt can serve conservation as discussed by Timar-Balazsy et al.
[11].
Soiling on historical textiles
Dirt or soiling is an undesirable matter adhering to surfaces and influencing their appearance.
According to Akar [12], carpet sweepings contain particles with a diameter from 1 to over 20 pm,
consisting of inorganic materials, cellulose fibres, animal fibres, oils and resinous materials.
McKinnon and McLaughlin [13] focus on soil's adherence to fibre. Clothing in contact with human
skin picks up greasy human sebum containing about 31% free fatty acids, 29% triglycerides (fats
and oils), 15% fatty alcohols and cholesterol, 21% hydrocarbons and 3.3% short chains fats and oils
as characterized by Patterson and Grindstaff [14]. These authors also make connections between
soil release, soil type and fibres, yarn and fabric geometry. According to Powe [15], the low melting
point of sebum (30 °C) is due to the complex mixture of lipids quite different from body fat. The
strong bonding of fat to polyester fibres has been investigated by Weber et al. [16].
The damaging effect of various stains has been discussed by Timar-Balazsy and Eastop [17, pp.
157-62] who characterize soils according to their potential to cause damage to textiles. Armstrong
et al. [18] deal with the problem of removing carbon black, while Carter [19] gives an overview of
iron stains on textiles. Jordan [20] reports on the removal of lime plaster from medieval woollen
embroidery and Hutchins [21] and Hersh et al. [22] discuss the effect of deterioration products on
fibres. The ageing process of oil stains on textiles has been described by both Andrasik [23] and
Moreland [24] and it has been found that coloured organic substances, such as dyes, inks and
pigments increase the light-sensitivity of fibres. Micro-organisms, such as bacteria and micro-fungi,
cause biological deterioration, characterized by Caneva et al. [25] and as Ballard [26] describes, the
stiffness of synthetic resins on a textile artefact may increase on ageing, thereby inducing
mechanical damage in the object.
Particulate dirts, such as dust, sand, earthy material and corrosion products, may be attracted by the
negatively-charged surface of textiles or may bond to the textile by rather weak electrical forces, as
described by Rice [27], According to Patterson and Grindstaff [14], the removal of such particulate
dirt is largely size-dependent: particles of 0.2 pm or less are almost impossible to remove from
textiles by-wet cleaning. Large particles (up to 5 pm) may also be difficult to remove. TimarBalazsy and Eastop [17, p. 159] refer to deterioration products of fibres, body oils, perspiration,
finishes or adhesives, water stains, dyes and stains originating from fruits and micro-fungi as
molecular soiling. Soiling forming a large mass on textiles includes greasy or oily dirt, proteins or
polysaccharides, synthetic adhesives and paints.
Three main types of 'dirt' are distinguished by Matteini et al. [28] according to their different
response to various cleaning methods. Dirt has also been characterized according to electrostatic
attraction and secondary bonds (van der Waals/dispersion, dipole and hydrogen bonds) by
Moncrieff and Weaver [29, pp. 16-21] and Hofenk de Graaff [30). Kissa [31, p. 384] formulates this
as follows:
'The removal of a soil particle from a substrate during laundering involves breaking an adhesive
bond between the particle and the fiber. The strength of this bond, and consequently the energy
required to detach the particle, depends on the attractive (mainly Van der Waals) forces and the
contact area between the soil particle and the fiber surface.'
Saito et al. [32] correlate the adhesion of oily dirt with washability by applying the surface energy
analysis method to the detergency system. Smith and Sherman [33] give a detailed review of the
effect of fibre surface characteristics and fabric construction on soil release, and distinguish
between 'micro-occlusion', in which fine soil particles became entrapped in the small crevices of
fibres, and 'macro-occlusion', in which soil particles became entrapped between fibres in the yarn
and between yarns in the weave. They found that 'micro-occlusion' has much more influence in the
case of natural fibres than for nylon, which has a smooth surface.
Advantages and disadvantages of wet cleaning
Rice [34] emphasizes that water dissolves most of the yellow and acidic deterioration products of
natural fibres; it also acts as a 'plasticizer' for the polymers of fibres, thereby improving the
flexibility and softness of the textile. According to Cooke [35], water eliminates creases and
wrinkles (elastic-deformations) in textiles by relaxing strains in fibres, yarns and fabrics. FluryLemberg [36, 37] finds it very important that in a wet state reorientation of a textile's yarns and
fibres is easier and so its original texture and dimensions may be 'recovered'. Rice [38] discusses
possible dimensional and colour changes, as well as dye 'bleeding' problems during wet cleaning.
As noted by Tímár-Balázsy and Eastop [17, p. 194], historical textiles may undergo further
deterioration during wet cleaning.
Water
Moncrieff and Weaver [29, pp. 75-7] characterize the properties of water. The fractional solubility
parameters (ability to form dispersion, dipole and hydrogen secondary bonds) of water is published
by Torracca [39]: (dispersion forces) N (fd)=18, (dipole bonds) D (fp)=28 and (hydrogen bonds) W
(fh)=54, which indicates its very high polarity. Many salts, originating from burial conditions for
example, can be dissolved in water, except ferric (Fe3+) compounds, which are non-water-soluble
according to Rice [27]. Polar organic soils, such as sugars, some types of polysaccharides (e.g..gum
arable) and proteins (e.g. animal glue), dissolve in water. Seth-Smith and Wedge [40] report on the
removal of animal glue from tapestry fragments.
The purity of water used for wet cleaning in conservation is an important factor. Rain water, coming
through the atmosphere may form acids with acidic gaseous pollutants and may carry paniculate
soils and bacteria. In tap water various compounds dissolved from the ground and from water pipes
can be present. The presence of cations, such as calcium (Ca2+), magnesium (Mg2+), sodium (Na + ),
potassium (K+), manganese (Mn2+, ferrous (Fe2+), and ferric ion (Fe3+) in a washing solution is
usually undesirable. There are three main reasons for this:
The cleaning power of the washing solution for soils containing the given ion will be reduced.
Compounds of heavy and transition metals are catalysts for chemical reactions contributing to
further deterioration of textiles.
Compounds of these metals may turn into coloured compounds by photo-oxidation or form
coloured compounds with other soiling on the textile.
The Guild of Cleaners and Launderers [41] gives an example: the maximum concentration of iron
compounds that does not cause obvious harm to textiles is two parts of iron in ten million parts of
water. However, in the case of wool the tolerance is lower: anything in excess of one half part per
ten million can cause yellowing and discoloration of wool textiles. The presence of metal ions with
two or three positive charges prevents the removal of ionic dirt or polar organic compounds because
of their attraction for the negatively or partially negatively-charged parts of dirt molecules, thus the
'washfastness of the soiling' is improved, as noted by Hofenk de Graaff [30, 42]. To avoid this
phenomenon, she advises that the concentration of cations with two or three positive charges must
be very low (less than 10-5 g-ion/litre) in solutions used to wash historical textiles.
Anions in tap water may also originate from water soluble-compounds (salts) found in the ground,
from sulphates (SO42-), carbonates (CO3 2-), hydrogen carbonates (HCO3- ) nitrates (NO3-) and
chlorides (Cl-).
Water hardness is caused by water-soluble compounds of calcium and magnesium. 'Hard' water is
the term used to describe water that does not form a lather with soap but instead forms an insoluble
precipitate, called 'lime-soap'. This whitish scum is formed from calcium and magnesium ions with
the anion of the soap; it deposits in the small pores on textile fibres causing an undesirable greying
of the fabric. According to Walker [43], calcium ions also form salts with anionic surfactants:
calcium alkyl sulphonates and sulphates have Krafft points that are generally higher at the
temperature used for wet cleaning in textile conservation, hence precipitation of the surfactant onto
the textile may occur (see below for a discussion of Krafft point). The calcium salt of sodium lauryl
sulphate, for instance, is insoluble below 50 °C. Arai [44] studied the effect of the concentration and
the kind of detergent in hard water. A linear relationship between the concentration of detergent and
water hardness at maximum oil removal efficiency was found.
Filtering, softening, deionization, reverse osmosis and distillation can be applied for purification of
tap water. Bede [45] suggests that conductivity measurements can be used to monitor the presence
of minerals in water. In theory, pure water has a resistance of 18 megohms/cm at 25 °C, and
contains 0.028 ppm total dissolved solids. Pure water is considered by some conservators to be too
aggressive and thus will dissolve too much soiling (e.g. fibre/finish/ degradation products) and
therefore some conservators prefer a purity of 1 to 4 megohms/cm. Distilled and deionized water
are aggressive solvents capable of achieving the maximum solubility of a given compound.
According to Heald [46], deionized water can be damaging to historical textiles.
The 'softening' of water refers to the process of exchanging the calcium and magnesium ions in
'hard' water for sodium ions. There are two common methods of water softening: ion exchange with
ion-exchange resins and formation of non-soluble or soluble compounds of calcium and magnesium
with a chemical additive. Sequestering agents (also referred to as chelating agents and complexbuilders) form co-ordinate bonds with metal ions to give a complex. Phenix and Burnstock [47]
characterized their role in conservation. According to Hofenk de Graaff [42, 48] the calcium and
magnesium ions are held strongly in the complexes and therefore they cannot replace the sodium of
soap or other washing agents. Adding polyphosphates often results in an alkaline pH of the washing
solution. Di-, and tetrasodium salts of ethvlene diamine tetra-acetic acid (EDTA) are effective
sequestering agents. The complex formation of disodium salt of EDTA with calcium (or
magnesium) results in an increase of protons (hydrogen/hydroxonium ions) in the solution due to
the exchange of the hydrogen to calcium (or magnesium). As the pH of the washing solution
containing this softening agent will decrease strict control of pH is required.
Detergents, detergency
Detergents added to water create washing solutions. Their role is threefold: promoting wetting of
the textile; dislodging the dirt and separating it from the fibres; keeping the dirt in a dispersion
and/or emulsion. The definition given by Davidson and Milwidsky [49, p. 1] for a detergent is a
formulation comprising essential constituents (surface active agents) and subsidiary constituents
(builders, boosters, fillers and auxiliaries). According to Neiditch [50, p. 9]:
'Detergency refers to the process of cleaning the surfaces of a solid material by means of a liquid
bath involving a physico-chemical action other than simple solution. Generally it is considered to
be an unusually enhanced cleaning effect of a liquid hath caused by the presence of a special agent,
the detergent.'
Surface tension of water and surface active agents
Water exhibits surface tension at the liquid-air interface and interfacial tension at the liquid-liquid
or liquid-solid interfaces, according to Moore [51]. This interfacial tension hinders water from
penetrating and wetting textiles. In a body of water the electric forces of attraction (especially
hydrogen bonding) operate in all directions and each molecule is held in equilibrium. At the surface
of water however, there are no forces acting from the air side and hence the equilibrium is
disturbed. The energy accumulated in the surface molecules of water is manifested as surface
tension. Surface tension is recorded as Newtons per metre and the surface tension of water is very
high: 72 mN/m.
Surface active agents (surfactants) reduce the surface tension of water and other liquids. When
added to water, surfactants will more or less cover the surface of the liquid. They are not as strongly
attracted to the inner water molecules as the water molecules were previously. According to both
Niven [52] and Durham [53], the surface tension of water is reduced to the range of 25-40 mN/m in
the presence of a surface active agent.
Surfactants are organic compounds with molecules having a hydrophobic (water repelling) nonpolar tail and a hydrophilic (water attracting) polar head or tail. As the
attraction forces (hydrogen bonds) between water molecules are much stronger than those between
water and the hydrophobic tail of the surfactant, the water molecules tend to 'squeeze out' the
hydrophobic tail of the surface active agent. A generally coherent layer of the detergent will cover
the surface, so that the non-polar tails are in the air while the hydrophilic head/part of the detergent
is attracted and dissolved by the water molecules. As the hydrophobic tails of the detergent
molecules are pushed out, the water spreads and wets the surface of the textile.
Davidson and Milwidsky [49] give an overview of surfactants, which are divided into four groups
depending on the character of the hydrophilic part: anionic, non-ionic, cationic and amphoteric
surfactants. Members of the first two groups provide a wide range of washing agents, while cationic
(or cation active) surface active agents are applied as optical brighteners, fungicides, softening,
antistatic or colour fixing agents. Amphoteric surfactants, containing both acid and basic groups in
their molecules, act either as anionics or as non-ionics depending on the pH, and have not gained
importance in either industry or textile conservation. Characterization of surfactants given below is
based on publication by Jakobi and Ldhr [54], Linfield [55] and Schick [56].
Anionic surfactants
The hydrophilic heads of anionic surfactants ionize to a positively-charged cation, while the residue
of the surface active agent becomes a negatively-charged ion (anion). Soaps are metallic salts of
fatty acids with 14-18 carbon atoms in their hydrophobic tail. They ionize in water to a positive
metal ion and an anion.
Marseilles soap contains a mixture of sodium and ammonium salt of stearic acid (C17H33COONa +
C17H33COONH4) and was recommended for washing historical textiles in soft water by Hofenk de
Graaff [57], As outlined above, natural soaps can form a scum (lime-soap) in hard water, thereby
reducing or inactivating their detergency and precipitating on fabrics. Natural soaps can cause
alkaline pH in washing solutions and their cleaning efficiency is not good in cold water.
Early anionic surfactants had a close resemblance to soap. Alkyl sulphates, also known as fatty
alcohol sulphates (FAS), originally were made from fatty alcohols. Both primary and secondary
alkyl sulphates contain 11-18 carbon atoms.
Sodium and ammonium salts of alkyl sulphates have excellent water-solubility and may
form water-soluble salts with the calcium or magnesium ions of hard water, thus they do not form
insoluble lime-soap precipitates. Orvus WA, a surfactant produced by Proctor & Gamble and used
widely in North America, is sodium dodecyl/lauryl sulphate.
Alkyl ether sulphates (AES) exhibit unique characteristics, such as very low sensitivity to water
hardness, high solubility and storage stability at low temperature in liquid formulations.
Sodium C12-14 n-alkyl diethylene glycol ether sulphates, for example, demonstrate increased
detergencv performance (e.g. on wool) as the water hardness increases. This is a result of the
positive electrolyte effects attributable to calcium/magnesium ions. Walker [43] found the best
anionic detergents for cleaning historical textiles in this group have a chain length of C12-15 and 2-3
mols of EO.
Alkyl (phenol-polyethene-glycol) sulphates (APPGS) have rarely been used in conservation, apart
from Levapon (Bayer).
sodium (phenol-polyethene-glycol-ether) sulphate
Sodium alkyl or alkane sulphonates (SAS) may be linear or have a branched chain:
primary
sodium alkyl sulphonate
secondary
sodium alkyl sulphonate
Straight and branched-chain alkyl aryl sulphonates (AAS) contain
their alkyl chain:
10-18
carbon atoms
in
branched chain alkyl aryl sulphonate
The α-olefin sulphonates (AOS) also contain hydroxy-alkane sulphonates as a result of partial
reaction with water.
α -olefin sulphonates
hydroxy-alkanesulphonates
According to Hofenk de Graaff [48], α -olefin sulphonates are well known to conservators for their
property of being less irritating to skin and having little sensitivity to water hardness.
Another important class of anionic surfactants is the a-sulpho fatty acid esters (SFAE), particularly
the methyl derivatives.
Good detergencv performance is attained only with products having a rather long hydrophobic part.
One of the interesting detergencv properties of a-sulpho fatty acid methylesters is their exceptional
dispersion power with respect to lime soap.
Fatty acid methyl taurides, such as Hostapon T (Hoechst) are known for their excellent foaming
properties.
sodium oleic methyl tauride
Straight chain (linear) and branched alkylbenzene sulphonates (ABS, LAS), exist. Until the mid1960s, this was the largest class of the synthetic surfactant and was most prominently represented
by tetrapropylenebenzene-sulphonate (TPS):
LAS has very high foaming ability; however, it is sensitive to water hardness: the detergencv power
of LAS diminishes as the hardness of water increases. Smith et al. [58] attribute this sensitivity to
the formation of Ca(LAS)2 on addition of calcium ions to LAS. Initially the micelles can solubilize
the Ca(LAS)2 but, in the presence of a higher concentration of calcium, the micelles capacity
becomes exhausted.
Natural anionic surfactants are not widely used in conservation cleaning. However, the search for a
biodegradable surfactant to replace Synperonic N, manufactured by ICI (see below), led to an
investigation of the seaweed funori, a marine algae of the Gloiopeltis genus, by Takami [59]. The
main component of the mucilage extracted from the dried sheets is a partially sulphated and
methylated polysaccharide, named funoran. Using a torsion balance to measure the surface tension
of eleven different concentrations of funori (0.1 to 1.5% v/v) at 20 "C, she found that it decreases
the surface tension of water from the 73 mN/m to 54 mN/m, which means that it has some surface
activity but less than most synthetic surfactants (24-40 mN/m). Takami's initial findings indicate
that a 0.1% funori anionic surfactant solution may be suitable for washing historical textiles, subject
to further tests.
Non-ionic surfactants
The term 'non-ionic surfactant' chiefly refers to polyoxyethylene and polyoxypropylene derivatives,
but other surfactants are also included in this category, such as anhydrohexitol derivatives, sugar
esters, fatty alkanol amides and fatty amine oxides. Non-ionic surface active agents do not ionize in
water. The proportion of hydrophilic parts to the hydrophohic tails is different from that of anionic
surfactants: in non-ionic surfactants the polar part/tail can be as large or even larger than the nonpolar tail.
Embree [60] mentions the use of soap bark (soap wort, Saponana) for washing and Cains [61]
records that the soapwort plant was called 'radicula' by the ancient Greeks and Romans, who used it
for cleaning wool. Shashoua [62] reports on using saponin for washing silk and Czerwinske [63]
carried out experiments with Saponin DAB 9 obtained from the bark of the Chilenian Quillaja tree
(Quillaja saponaria Molina) and of the roots of saponin (Saponana officinalis L). This is a non-ionic
substance, which decreases the surface tension of water from 73 mN/m to 20 mN/m at 20 "C in a
concentration of 1.5 g/litre. Tsujii [64] classifies saponin as two types, steroid and triterpenoid, in
which hydrophilic saccharides (glucose, galactose, rhamnose, xylose, pentose, etc.) are attached to
hydrophobic steroids and triterpene.
Most synthetic non-ionic surfactants used in conservation are of the ethylene oxide class. If the
polymerization of ethylene oxide is carried out in the presence of organic compounds containing
easily exchangeable hydrogen (e.g. fatty alcohol) the product will be an ethylene oxide adduct. The
proportion/size of the hydrophilic and hydrophobic part can be 'tailored' to the planned role of the
surfactant in washing. Wash effectiveness shows an initial increase with an increasing degree of
ethoxylation, but a point is then reached after which the wash effectiveness declines markedly.
Dillan [65] found that narrow-range1 ethoxylates contain less unreacted fatty alcohol and other
water insoluble species and they are capable of forming aqueous solutions with much lower cloud
points than their broad-range counterparts.
The general formula of alkylphenol polyglycol ethers (APEO) is as follows:
These surfactants, which have exceptional detergency properties and, in particular, oil and fat
removal characteristics, show low biodegradibility (for a discussion of biodegradability, see below)
Synperonic N is an ethylene oxide adduct (EO = 8) with nonylphenol.
HLB
The detergency of a surfactant depends greatly on the balance (B) of the molecular weight of the
hydrophobic (H) portion to that of the hydrophilic (lipophylic, L) portion. Porter [66] asserts that
calculation of the HLB number was first proposed in 1949. HLB = % of the hydrophilic group
(molar) divided by 5. The maximum HLB number is 20 and represents a completely water-soluble
surfactant, while an HLB of zero represents a completely water-insoluble product. Boring and Ewer
[67] make a connection between HLB value and application:
Table 1 Connection between HLB values, appearance on adding surfactant to water (after Porter
[66] and Boring and Ewer [67])
It should be noted that an increase in temperature will bring about a phase inversion from an oil in
water (OAV) to a water in oil (W/O) emulsion due to the non-ionic surfactant becoming less watersoluble as the temperature increases. Delcroix and Bureau [68] suggest that for non-ionic
surfactants, it is better to use a mixture of surfactants with different HLB values, giving the desired
proportions, than to use a single one that has the required HLB value. A strongly hydrophobic
surfactant has a low HLB, usually less than 10. A highly hydrophilic surfactant has a value higher
than 10. For example, polyethylene nonylphenols have an HLB of about 13 and sodium lauryl
sulphate has an HLB value of approximately 4.
According to Kravetz via a personal communication with Walker [43], solutions of linear primary
alcohol ethoxylates C12-15, EO 3 to 4 mols, blended and mixed with varying HLB values are
efficient for washing historical textiles. Surfactants of HLB 10 to 12 are effective for oily soil and
those with a higher HLB, 13 to 15, for particulate soil. Walker recommends a two- or three-step
washing process, starting with a lower and ending with a higher HLB surfactant.
Wetting properties of surfactants
In the presence of a surface active agent, the interfacial tension between water and textile is reduced
and the textile is wetted. Various surface active agents show various wetting properties, depending
on their chemical and stereo-chemical structure (e.g. the length of hydrophobic and hydrophilic
parts, straight or branched chains and the presence of an aromatic ring in the molecule). The wetting
property of a surfactant is characterized by the measurement of the 'rim angle' and 'contact angle',
described by Bigler [69].
The 'rim angle' is the tangent of the angle between the solid and the liquid surface measured in the
air. It increases as the wetting increases. Ward and Benerito [70] define 'contact angle' as the angle
between the solid surface and the tangent of the liquid surface as it approaches the solid, the angle
being measured in the liquid. The contact angle decreases as the wetting increases.
1
When reacting detergent-range primary alcohols (C12-14) with ethylene oxide and achieving a
narrow distribution of EO moles.
The rolling-up process by surfactants in the case of liquid dirt, such as oil, can be characterized so
that the fibre surface is wetted by the oil in the starting state and then it will be wetted by the
aqueous phase. This process, which results in cleaning, can be followed by the decrease of the rim
angle and an increase of the contact angle between the oil stain and fibre while it is rolled up by a
surfactant solution, as depicted by Lange [71, p. 158]. At the beginning of the rolling-up process the
oil is a flat stain on the surface of the textile. As it is lifted up step by step it curls in a ball-shaped
oil particle and separates from the surface of the fabric.
Critical micelle concentration
The total number of surfactant molecules that enter the liquid surface is determined by a balance of
forces: between those squeezing out the hydrophobic tails and the repelling forces between likecharged parts of the surfactant molecules. When a surface active agent is added to water, so that its
concentration gradually increases, the number of the surfactant ions at the surface increases up to a
certain critical concentration of the detergent. The excess surfactant molecules (i.e. those added
since the critical concentration was reached) cannot access the surface or stay individually in
solution but form micelles within the body of the liquid. Hence the term critical micelle
concentration (cmc).
Hutchinson and Shinoda [72, p. 13] define micelle as 'hydrated surfactant in liquid state'. Since the
micelles are small compared with the wavelength of light, the solution is transparent. According to
the above authors, micelles have a solubilization power: when the surfactant solution is above its
cmc, the solubility of a third additive is markedly greater than in pure water. Wentz [73] also
attributes the main mechanism of soil removal in aqueous systems to this solubilization, by stating
that non-polar substances are solubilized in the interior of the micelles. Skagerlind [74] published
an illustrative figure showing that surfactant molecules are present in single form below, and in
micelle form above cmc.
Micelles are aggregates of a number of surfactants. The hydrophobic tails of the surfactant
molecules tend to cluster so that they are isolated from the water; in other words, the water
molecules squeeze out the non-polar tails of the detergent molecules. The hydrophilic parts of the
surfactant align themselves facing the water. Many suggested configurations for micelles have been
suggested, such as spherical, ellipsoid, lamellar and cylindrical; the micelle configuration is thought
to depend on the chemical composition and stereo-chemical structure of the surfactant.
The micelles themselves are stable entities; however, they continually break up and reform in a
process of equilibrium. With anionic surfactants the outer layer of the micelle is negatively-charged,
with non-ionics, the micelles have no charge. According to Delcroix and Bureau [68], the number
of molecules in a micelle at room temperature is 40-499 for a non-ionic surfactant and 20-300 for
an anionic one.
As mentioned by Taylor [75], the cmc varies markedly according to the character of the surfactant
as it is affected by temperature. At low concentrations the amount of micelle formation is negligible
and, at higher concentrations, the point of equilibrium is to a very great extent independent of the
total concentration. The reverse applies to surface tension: at low surfactant concentrations
the reduction in surface tension is dramatic, but it does not change above the critical micelle
concentration.
Hofenk de Graaff [30] remarks that the effectiveness of a washing agent increases up to the critical
micelle concentration but decreases after cmc has been reached. Lange [71, p. 173] distinguishes
between the mechanism of single surfactant molecules and micelles in cleaning. He explains that
the increase of cleaning efficiency up to the cmc is due to adsorption of the single surfactant
molecules onto the dirt. Thus, solubilization of the dispersed dirt is a purely micellar phenomenon.
It therefore does not occur unless the concentration is higher than the cmc. The actual mechanism
depends both on the surfactant and the type of soil. It seems to be a general experience that a strong
increase in cleaning efficiency occurs up to the cmc and only a very slight increase above the cmc.
The critical micelle concentration of any one class of surfactant is reduced as the size of the
hydrophobic tail increases, or the hydrophilic part decreases in size. The closer the polar group to
the centre of the hydrophobic tail (in secondary alkyl sulphates/sulphonates), the higher the cmc.
The presence of more ionic groups in one surfactant molecule also causes an increase in cmc. The
cmc also depends on the cation of the surfactant. It reduces cmc with decreasing attraction forces
between the detergent ion and the cation: the reduction in cmc is smaller with sodium than with
potassium. According to Juhasz and Lelkesne Eros [76, pp 126-38], with non-ionic surfactants the
cmc increases as the hydrophilic part (EO) becomes larger.
From these observations it is clear that non-ionics produce lower surface tensions than anionics at
equivalent concentrations. Also, at comparable hydrophobic chain size, non-ionics form micelles
more readily; this is probably because, without an ionic charge, there is no barrier to aggregation, as
noted by Taylor [75, pp. 10-11]. One of the advantages of using non-ionic surfactants is that the
critical micelle concentration is very low, in the order of 0.05-0.5 g/litre in comparison to anionic
ones with a cmc of 0.3-3 g/litre, thus, the amount of surfactant required is reduced, thereby
increasing liquor clarity and rinsability.
According to Juhasz and Lelkesne Eros [76, p. 138], in aqueous solutions the cmc of anionic
surfactants is reduced in the presence of salts of the surfactant's cation. In addition, the repelling
forces between the polar parts of non-ionics are reduced by certain cations. This may explain why
sequestering agents providing cations on ionizing reduce the cmc of the detergent. Dirt containing a
similar cation to those of the anionic surfactant may have a similar effect, that is such soiling may
reduce the cmc of anionic surfactants. Thus, when wet cleaning archaeological textiles
contaminated with sodium compounds, a lower concentration of an anionic washing agent may be
as effective as one of its cmc.
Solubility of surfactants, Krafft point and cloud point
The solubility of surfactants depends largely on the length and proportion of their hydrophobic and
hydrophilic parts (HLB), as well as the number and position of ionic or polar groups. According to
Davidson and Milwidsky [49, p. 10], lauryl alcohol reacted with ten molecules of ethvlene
oxide is completely water-soluble and a good detergent, while one with less that five molecules of
ethylene oxide would be insoluble.
Temperature is another factor that determines the solubility of surfactants. Below a certain
temperature, the apparent solubility of an anionic surfactant drops dramatically. In contrast, the
solubility of non-ionic surfactants drops considerably above a certain temperature. The temperature
at which the solubility of a surfactant decreases sharply and the undissolved detergent molecules
appear in the form of a whitish cloud, is called the 'cloud point'. In a 'cloudy' solution only the
soluble (invisible) portion of the surfactant carries out its surface activity. Zika [77, p. 27/324] gives
the following definition: 'nominal cloud point: the temperature at which a cloud of insoluble
surfactant first begins to form in a 1% aqueous solution of the surface active agent'.
The solubility of anionic surfactants depends basically on the length of the hydrophobic chain. If
there are fewer than 10 carbon atoms in the non-polar chain, the detergent may be too soluble to
form micelles suitable for soil removal. If it contains more than 18 carbon atoms, the anionic
surfactant molecule is too large to be soluble at reasonable working temperatures. A surfactant with
unsaturated hydrophobic chains is more soluble than a similar saturated chain compound. Rice [78]
gives an example: a sodium oleate surfactant (C17H33iCOONa) with an unsaturated chain dissolves
in cooler water than its saturated counterpart, sodium stearate surfactant (C17H35COONa).
The solubility of anionic surfactants increases as the temperature rises and hence cloudy solutions
clear as their temperature is increased. There is a critical solubility temperature, called 'Krafft point':
above this temperature the solubility of the anionic surfactant increases dramatically with increasing
temperature. Micelles cannot form below Krafft point. Walker [43] provides an example: the Krafft
point of SDS is about 10 °C. At 12 °C its solubility is 0.02%. This increases to 0.2% at 16 °C and to
3% at 17 °C.
Hutchinson and Shinoda [72] formulate that at low temperatures the precipitate of anionic
surfactants will be in equilibrium with the saturated solution of singly dispersed surfactant
molecules. In contrast, at high temperatures the precipitate becomes transformed to a liquid state. If
the temperature cannot be varied under practice conditions, another equivalent effect may be to
change the molecular structure of the surfactant or to add a third component in order to depress the
Krafft point. Branching or unsaturation in a hydrocarbon chain causes a marked reduction of the
Krafft point.
In contrast, the cloud point temperature of non-ionic surfactants can be rather low and the 'cloud'
appears above the cloud point temperature. The solubility of non-ionics decreases as temperature
rises. Different theories are advanced to explain this particular cloud point phenomenon, described
by Zika [77] in a paper in 1969. One states that the hydrogen bonds, formed between the
polyethylene oxide part of the detergent and the water molecules, break with increasing
temperatures. The higher the temperature the more hydrogen bonds break. As a result, the surfactant
becomes insoluble. Another theory claims that with increasing temperature the micelles of the nonionic surfactant grow larger, to the point where they can actually be seen in the form of a 'cloud'.
Water hardness greatly influences the solubility of detergents. Those anionics that form non-watersoluble compounds with calcium or magnesium (such as soap, alkyl benzene sulphonates or
secondary alkyl sulphonates) have a much lower cloud point than other anionics. Non-ionics are not
usually sensitive to hard water and it is the presence of electrolytes that lowers their cloud point.
Jakobi and Lohr [54, p. 57] note that in the presence of pure non-ionic surfactants the cloud point
can be reduced greatly by the addition of several grams of electrolytes.
Washing process with surface active agents
In general, the wash effectiveness of anionics increases with increasing chain length, as described
by Jakobi and Lohr [54, p. 42]. For example, surfactants bearing n-alkyl groups show a linear
relationship between the number of carbon atoms in the surfactant molecule and the logarithm of
the amount of surfactant adsorbed on activated carbon or kaolin. The structure of the hydrophobic
residue also has a significant effect on surfactant properties. Surfactants with little branching in
their alkyl chains generally show good wash effectiveness but relatively poor wetting
characteristics, whereas more highly branched surfactants are good wetting agents but have
unsatisfactory detergency. For compounds containing an equal number of carbon atoms in their
hydrophobic residues, wetting power increases markedly as the hydrophilic groups move to the
centre of the chain or as branching increases, but a simultaneous decrease in adsorption and
washing power occurs.
The micelles formed around the cmc provide reserves of surfactant molecules, which are not only
available for instant mobilization, but also have the power to solubilize substances, such as fat,
which do not dissolve in water. Taylor [75, p. 7] describes fat as being 'solubilized' within the
hydrophobic interior of the micelle.
In wet cleaning textiles, a surfactant solution is applied and soil removal is promoted by careful
agitation. According to Kissa [79, p. 763]:
'The process of washing (soil release) consists of three consecutive stages: an induction stage,
during which water diffuses into the soiled textile but soil release is slow; a rapid soil release stage,
during which 'rolling up' and dislodgement of soil and water diffusion are rapid; and a final stage,
during which soil retention in the textile remains essentially constant'.
The hydrophobic tail of a surfactant penetrates hydrophobic soiling, while the polar part of a nonionic penetrates polar soiling. Through the penetration of surfactant molecules, the soiling on a
textile surface is dislodged (deflocculated) into small particles. The four stages of this process are
depicted in an illustration in a paper by Moncrieff et al. [29, p. 84].
Anionic detergents require a long time to penetrate negatively-charged soils, such as clay or carbon
black, due to the mutually repelling forces present. Diffusion of the surfactant into the textile is
hindered by the repelling action between the anionic surfactant and textile, which also has partial
negative charges on its surface due to the polar groups of the fibre polymers. Anionic detergents
dissolve soiling particles of non-polar character in the micelles and they bond to polar soils by
secondary dipole and hydrogen bonds with their polar heads. In the case of polar soils, a second
layer of surfactant molecules joins the first layer and thereby a double anionic surfactant layer
prevents polar soiling particles from aggregation.
According to Zika [77], non-ionic surfactants are equally good at penetrating both non-polar and
polar soils due to the equal length of non-polar and polar parts and the absence of any charge. Nonionic surfactants bond to polar soils by dipole and hydrogen secondary bonds with their polar parts
and by van der Waals bonds to non-polar soiling with their non-polar parts.
Non-ionics penetrate soiling and textile quite quickly, especially if the hydrophobic part is of a
straight chain type. Non-ionic detergents with 12-14 carbon atoms in their alkyl chain and 10 EO
groups are excellent at penetration, but octyl- to dodecylphenol detergents with 10 EO groups are
also very effective at soil penetration.
Applying a mixture of anionic and non-ionic surfactants in the same washing solution has the
advantage of forcing off soiling by the anionic surfactant and at the same time penetrating various
soiling by the non-ionic surfactant. Berol 784 (Berol Nobel), for example, is a mixture of an anionic
(alkyl aryl sulphonate) with a non-ionic (fatty alcohol ethoxylate) surfactant. Gentle and Muller [80]
combined anionic and non-ionic surfactants in the same washing solution and achieved a good
result in terms of cleanness. Lewis [81 ] experimented by mixing Synperonic A5 non-ionic and
SDS anionic surfactants and found its efficiency better than either single surfactant for washing
wool. Stauffer [82] provided the author with leaflets on two mixed surfactants used by German
textile conservators: Invadin LUN (Ciba-Geigy) and Kieralon OLB (BASF).
Chemical structure of surfactants and detergency
Stupel [83J lists surfactants according to their increasing cleaning power: primary fatty alcohol
sulphate —>alkyl polyglycol ether —>alkyl aryl sulphonate (dodecilbenzene sulphonate) —
>secondary fatty alcohol sulphate (tridecylsulphate) —>fatty acid condensation product (oleyl
methyl taurine) —>alkyl sulphonate.
Jones [84] reported on variables affecting efficiency of amonic surfactants in soil removal: straight
alkyl chains on benzene sulphonates are superior to branched chains; effective detergency of
benzene sulphonates begins with an alkyl chain length of 10 carbon atoms and improves to a
maximum at 14-16 atoms; p-alkylbenzene sulphonates are superior to o-alkyl compounds; straightchain carboxylates and sulphonates have similar detergent activity.
Harris [85] and Schonfeldt [86] investigated surfactant effects and summarized that optimum soil
removal activity for non-ionic surfactants is produced by condensing ethylene oxide with a normal
straight chain aliphatic hydroxy compound with 12-14 carbon atoms in the chain and about 10
ethylene oxide units. They found that branched alkyl chains give less efficient detergent action than
straight chains and that aromatic derivatives, such as octyl- and dodecylphenol, provide effective
non-ionic surfactants when condensed with about 10 mols of ethylene oxide. Non-ionic
surfactants soluble in dilute solution at room temperature can become insoluble at higher
temperatures and their detergent action is optimized close to this condition, while low levels of nonionic surfactants form micelles in water so that the amount of these compounds needed for optimum
soil removal is less than that of anionic surfactants. The authors noted that effective soil
solubilization shown by non-ionic surfactants is an additional removal mechanism not available
with ionic materials and that non-ionic and anionic surfactants combined in detergent mixtures can
give more effective soil removal than either surfactant alone.
When evaluating surfactant groups according to their cleaning power one has to bear in mind that
the actual effect of the surfactant chosen depends on the individual surfactant selected from a
particular group, the type of textile and soiling and other components of the washmg solution as
well as the washing temperature, pH, mechanical action and the duration of washing.
Soil/dirt redeposition and soil/dirt anti-redeposition agents
In the last stage of washing the role of the surfactant is to keep the dislodged soil particles in a
stable suspension, dispersion or emulsion and prevent soil redeposition on the textile. According to
Rice [27], an average strongly-adhered, plate-like clay soil particle is about 0.1 µm in diameter.
Carbon deposits that exhibit strong greying power appear to be about 0.05 µm in diameter. Having
been broken down by the surfactant, the soils turn into much smaller particles, which can deposit in
the surface crevices of vegetable fibres, at the junctures of animal fibres with scales and in the
complex surface structure of some synthetics. Such fine, redeposited soiling results in a dull,
uniformly grey appearance, which is very difficult to remove. Rice [27, p. 13] discusses the
problem in detail.
Hofenk de Graaff [30] characterizes washing as a process of equilibrium where the amount of soil
'rolled up' by the detergent is in equilibrium with the amount of redeposited soil:
The equilibrium is shifted in the direction of the upper arrow if dirt is held strongly in the washing
solution. Redeposition of soiling can be prevented if a washing solution loaded with dirt is replaced
before the equilibrium is shifted towards the lower arrow.
Hence, like-charged 'soil-surfactant micelles' repel each other and are repelled by the textile,
anionics usually act well in preventing soil-redeposition. The dirt-carrying properties of anionic
surfactants are usually excellent if the surfactant is present above its cmc. Non-ionics solubilize or
retain mixed non-polar and polar soils (i.e. greasy dirt) in a stable dispersion, whether or not they
are present below or above their cmc.
Juhász and Lelkesne Eros [76, p. 239] state that the use of a mixture of anionic and non-ionic
detergents in the same washing solution has advantages for soil-carrying. For example, the anionic
detergent sulpho-succinate does not have very good washing properties but is excellent in soilcarrying; this applies also to anionic fatty acid-alkanol-amides, which also promote the stability of
foams.
Patterson and Gnndstaff [14] list special soil anti-redeposition agents, such as polyvinyl alcohol,
polyethylene-glycol, polyvinyl pyrrohdone as well as sodium carboxy methylcellulose hydrophilic
polymers. The most common soil carrier used in washing historical textiles is the carboxy
methylcellulose (CMC) and its sodium salt (SCMC or NaCMC). A paper by Lange [71, p. 155]
refers to several theories about the mechanism of inhibiting soil redeposition by CMC and SCMC,
such as reinforced electrical repulsion, competitive adsorption, protective colloid action and steric
protection.
The usual recommended concentration of SCMC in a washing solution is 0.01% of the quantity of
the surfactant. Complete dissolution of SCMC requires a rather long time (about 24 hours). Smith
and Lamb [87] recommend SCMC with a small degree of polymerization (DP = 200-500) and with
a low degree of substitution (DS = 0.6-0.8) for soil-carrying purposes. ]akobi and Lohr [54, p. 90]
mention the use of carboxymethyl starch (CMS), as well as non-ionic cellulose ethers. The
cellulose-based soil anti-redeposition agents are particularly effective with cellulose-containing
fibres. These agents form a barrier layer on the surfaces of cellulosic fibres. The advantage in soilcarrying is a disadvantage in separating cellulose-based soil anti-redeposition agents from cellulosic
textiles, which require repeated rinsing at rather high temperatures.
Sequestering agents also act as soil carriers, partly by forming complexes with the metal ions of
dirts, partly by their dispersing, emulsifying and stabilizing properties, which are limited in
comparison to surfactants.
Composition of washing solutions for historical textiles and methods of washing
Commercial washing powders and liquids are unsuitable for cleaning historical textiles due to the
presence of many unwanted additives, such as complex builders, optical brighteners, enzymes,
corrosion and foam inhibitors, bleaching agents, stabilizing agents, dyestuffs, fillers and perfumes.
These are described in papers by Lehmann [88] and Hofenk de Graaff [57].
In an unpublished report Wyeth [89] lists specific surfactant properties desirable for a conservation
cleaning agent: effective lowering of interfacial tension; good wetting power; low cmc; high
solubility at low temperatures (i.e. low Krafft/cloud point); efficiency at neutral pH range; low
sensitivity to water hardness; good detergency; soil anti-redeposition capabilities; rinsability;
neutral odour; favourable handling characteristics; acceptable biodegradability; storage stability;
reasonable price and availability in small quantities.
Hofenk de Graaff [48] recommends various formulations for washing historical textiles taking into
account the fibres of the textile, nature of the dirt, quality of the water and the foaming property of
the detergent. Collins [90] advocates non-ionics and natural soap for washing undyed cotton and
linen. Shashoua [62] describes a so-called 'Standard Washing Solution' used for experimental
cleaning purposes. Boring and Ewer [67] surveyed wet cleaning and found that anionic surfactants
were used in a concentration of 0.2-0.5% and non-ionics in a concentration of 0.05-0.02% at 20-27
°C. For washing, the use of deionized water and for rinsing the use of tap water was reported.
Hogberg [91] reports an opposite approach, using tap water for washing and deionized water for
rinsing.
Washing solutions for historical textiles normally contain a surfactant, either an anionic surfactant
in a concentration of 0.5 to 1.0 g/litre, or 0.1-0.5 g/litre of a non-ionic surfactant as well as distilled,
deionized, demineralized or soft water. General formulations for detergents are summarized by
Daniels and Shashoua [92). The Canadian Conservation Institute have produced two notes [93] and
a report [94] on the wet cleaning of textiles, recommending a 0.5% concentration of anionic
detergent for cleaning textiles in cultural heritage collections.
In general, non-ionic and anionic surfactants combined in a single detergent mixture result in more
effective soil removal than any single surfactant alone, especially in the conditions appropriate for
treating historical textiles, according to Patterson and Gnndstaff [14]. What is probably the first
published report on using both kind of detergents for cleaning a piece of historical textile dates from
1966, when Rice [95] used a fatty alcohol sulphonate anionic detergent in the first wash bath and an
ethylene oxide condensate non-ionic in the second. Cox et al. [96] investigated the interaction
between LAS and non-ionic surfactants and found that the addition of low levels of a lauryl rangehigh EO non-ionic surfactant significantly lowers cmc and causes the formation of micelles
containing predominantly non-ionic molecules. Non-ionic surfactant enhances LAS hard water
performance by preventing the loss of LAS via Ca(LAS)2 precipitation. The non-ionic surfactant
acts as a micelle promotion agent, while LAS remains responsible for surface and mterfacial
properties. Davis [97, pp. 159-63] reports on using a 2 g/litre solution of Synperonic N non-ionic
surfactant mixed with a solution of 0.5 g/litre sodium dodecyl sulphate, anionic surfactant. For
washing a severely soiled curtain she used a solution of 0.5 g/litre Synperonic N, 0.5 g/litre sodium
dodecyl sulphate and 2.00 g/litre SCMC.
Hofenk de Graaff [98] lists the following additives as sometimes also being used in washing
solutions; a soil-carrier of sodium carboxy methyl cellulose (SCMC) in a usual concentration of
0.05-0.1 g/litre; a sequestering agent, which can be added to soften water and/or to remove heavy
soils containing metals, in a usual concentration of 0.5 to 2.0 g/litre and a buffer, added to maintain
the pH of the washing solution when treating highly acidic textiles.
In washing solutions for historical textiles the use of distilled, demineralized, deionized or soft
water is recommended. Giuntini and Bede [99] used deionized water without any detergent for
washing a group of Paracas mantles, as did Kajitani [100] when wet cleaning a Munghal court robe.
Distilled water alone was recommended for wet cleaning of archaeological textiles by Zongyou
[1011. A 0.2% solution of Synperonic N non-ionic surfactant containing 0.005% CMC
was used by Kiefer [102] for cleaning a shattered silk brocade.
Cussel [103] compared the British and the French methods of wet cleaning and noted that there is
little difference between them, although in the UK Synperonic (ICI), and in France Tinovetine
(Ciba-Geigy), are the preferred surfactants. A detergent formula containing Synperonic N non-ionic
surfactant, SCMS and sodium tripolyphosphate was published by Glover [104] in a report on the
textile conservation methods in north-western England.
Burgess [105] suggests that textiles made of cellulosic fibres should be washed and rinsed in a
solution containing 20 to 200 ppm of magnesium sulphate (MgSO4) dissolved in distilled water.
This reduces the loss of calcium and magnesium from the cellulose (hemicellulose and pectin) and
thereby improves the stability of the polymers of the vegetable fibres. In a personal communication,
Hofenk de Graaff [106] felt that this method should be subject to further debate, as the presence of
calcium and magnesium ions in the wash bath hinders soil removal and, unlike paper, the quantity
of calcium and magnesium in cellulosic fibres is very low. Shenai [107] recommends using nonionic, rather than anionic, surfactants to wash wool because they have a lack of substantivity to
wool. Delcroix [108] gives a sophisticated mathematical process for determining the ideal
concentration of a surfactant used for wet cleaning historical textiles for the Mai son Chevalier in
Aubusson, France.
Washing temperature
The washing temperature has a great influence on the solubility (i.e. cloud point) of surfactants. The
solubility of anionics increases as the temperature rises. Surfactants, with long polar chains, for
example non-ionics, dissolve readily in cold water. The solubility of non-ionics decreases as the
temperature rises. Alkyl sulphate anionics with 14-16 carbon atoms in their chain are excellent
washing agents with good micelle formation and soil dispersing properties. Their use requires the
temperature of the washing solution to be above 40 "C to reach the critical micelle concentration.
This temperature is too high for textile conservation purposes because of the damaging effect of
swelling, shrinking, felting or hydrolysis of degraded fibres, as well as bleeding of dyes or
dissolution of too many degradation products. Thus, anionic detergents are often used in
conservation at lower temperatures, i.e. below their cmc.
According to Morris and Prato [109], the effect of the washing temperature on the removal of
particulate and oily soiling depends on the fibre type too. Generally from both cotton and polyester
fabric the dirt removal improves as washing temperatures increase from 10 to 54 °C. Removal of
non-polar oily soil from polyester fabric was a notable exception, where soil removal was inversely
related to wash temperature. Myers [110] distinguishes between wet cleaning and laundering,
stating that the latter term refers to cleaning at high temperatures.
Washing time
To prevent too much swelling or hydrolysis of degraded fibres, the duration of washing of historical
textiles is normally reduced as much as possible. The use of a suction table in wet cleaning can
reduce the time available for fibre swelling. On the other hand, according to the three consecutive
stages of washing (induction time, rapid soil release stage and final stage), it is not advisable to stop
washing before the rapid soil release stage is reached.
The induction time of washing is usually shorter with non-ionic surfactants than with anionics due
to the lack of repelling ionic forces between the textile, soil and surfactant. Naturally, the induction
time depends on the fibres, the thickness and structure of the textile, the hydrophobic property of
the soils, the temperature of the washing process and the components of the washing solution.
Complete wetting can be achieved in minutes or may take hours, depending on the above factors. In
wet cleaning of historical textiles the rapid soil removal period should start and reach equilibrium
within a reasonable time. The washing process for historical textiles should stop before the end of
the rapid soil release stage in order to prevent soil redeposition, as mentioned above with the
'equilibrium process theory' of washing. If a single washing solution is insufficient to achieve the
required soil removal, the use of two, or more, washing solutions is recommended instead of
soaking the textile longer in the same bath. With a second or subsequent bath the rapid soil release
stage starts again, and the risk of dirt redeposition is therefore reduced.
When analysing the answers to the questionnaire 'Operation Wetclean', Howell and Farnsworth
[111, p. 55] concluded that 'rinsing times are longer than wash times, wool tends to be soaked
before washing but silk does not, the longest treatment time tends to fit into a working day,
although some conservators were doing very long days.'
The pH of the washing solution
Anionic surfactants require complete ionization for optimum washing efficiency. Anionics in the
form of sodium salt ionize better in alkaline conditions than in acid ones. According to Hofenk de
Graaff [48], fatty acid methyl ester a-sulphonates are said to be exceptions as they are stable
between pH 3 and pH 10, which makes them excellent for cleaning textiles that have become acidic.
Sequestering agents can act as buffers in a washing solution as they often provide mildly alkaline
conditions which promote ionization of anionic surfactants. The degree of ionization depends on the
presence of anions in the washing solution (e.g. hydrogen carbonate ions), which results in weakly
acidic solutions, as well as the counter-ions of anionics (e.g. sodium ions), which result in strongly
basic solutions. Thus, the combined use of sequestering agents with anionic surfactants may result
in a wash solution of alkaline pH. If the fibres and dyes are not sensitive to alkaline conditions,
there are several advantages to using a washing solution with a mildly alkaline pH for cleaning nondegraded and non-alkali-sensitive textiles. These advantages include: improving the cleaning power
of the anionic surfactant; breaking down fatty soils by saponification; stabilizing anionic surfactants
and neutralizing acids released into the wash bath from the textile and soiling.
Non-ionic detergents do not cause a change in the pH of washing solutions because they do not
ionize. They are usually effective in acid conditions. The pH of the washing (and rinsing) solutions
will change throughout a wet cleaning process. Cartwright and Colombini [112] emphasize
the importance of monitoring the pH of washing solutions throughout every stage of the washing
process.
The role of lather (foam) in washing
The lathering (foaming) properties of a surfactant can be characterized by the volume of foam
produced from a unit volume of washing agent prepared in standard conditions. Foams are
dispersions of air bubbles in water, where the liquid is deformed into thin films. These films of
water separate the air bubbles. Fine solid particles from the washing solution float into the foam.
The relationship between the cleaning power of surfactants and their foaming properties is not
close; however, surfactant foam decreases the amount of dispersed soil in the washing solution by
holding soil in the foam and thereby inhibits soil redeposition. The application of foam to the
surface of textiles has the advantage of 'drawing up1 the soil into the foam without lengthy soaking
of the textile in the washing solution. However, this cleaning method of using foam alone has
drawbacks: if too much foam is used it can be difficult to rinse out the detergent completely.
Juhasz and Lelkesne Eros [76, pp. 220-6] list surfactants with good foaming properties: fatty acid
soaps and primary alkyl sulphates with 12-14 carbons containing above 90% sulphate groups,
sodium salts of fatty acid-methyl-taurides (e.g. Hostapon T) and sodium alkyl benzene-sulphonates
provide stable foam (i.e. the foam bubbles do not collapse over time). Non-ionic surfactants (except
saponin) provide much less foam than anionics.
Adsorption of surfactants to textiles and rinsing
Adsorption of surfactants is not completely reversible because anionics irreversibly chemisorb onto
wool and silk, as observed by both Aickin [113] and Holt and Onorato [114]. Alkyl sulphate and
alkyl sulphonate ions react through ion exchange with positively-charged amino and imino end
groups of proteins. The actual number of sites available for sorption is pH dependent, increasing
with decreasing pH. Aickin [115] found that up to 2.5% owf (weight of fabric) of sodium alkyl
sulphate was retained by wool fibres under neutral pH conditions. He likened the alkyl sulphate ion
to a colourless dye. Mauersberger [116] states that surfactants chemisorb less onto silk than onto
wool and chemisorption of alkyl sulphates onto cotton is proportional to its protein content, most of
which is located within the lumen. Freeland [117] warns that with longer washing time and higher
concentrations, surfactants penetrate into the cells of wool cortex and into the lumen and the
amorphous regions of cotton fibres. If they go deep they remain after rinsing. Holt et al. [118, 119]
conclude that under normal wool dyeing conditions, anionic surfactants show good substantivity to
wool. The factors that influence this are liquor ratio, length of surfactant side-chain and pH. Spei
and Holzem [120] made a connection between the molecular length of anionics and their deposition
in keratin fibre.
Weatherburn and Bayle [121] give an overview of the subject. Surfactant affinity for the textile
surface increases as the size of the hydrophobic part increases and the length of the hydrophilic
head group/part decreases. The presence of a benzene ring increases the strength of the
hydrophobic bonding, making rinsing more difficult. Electrolytes reduce the electric double layer at
the liquid/solid interface, thereby increasing the adsorption of anionic surfactants.
Rhee and Ballard [122-4] investigated the adsorbance of the anionic Orvus WA and reported that
wool adsorbed nine times the level adsorbed by cotton. Colorimetric test methods showed that silk
adsorbed Orvus WA 2.73±0.3% owf. Depending on the length of the rinsing time and level of
temperature, desorption can be encouraged. Left unrinsed, the hydrophobic character of silk
significantly increased while, after thorough rinsing, its hydrophobic character was normal.
The aim of rinsing is to remove the 'surfactant-soil micelles' and the remaining surfactant molecules
and soil-carriers. If left in the textile, surfactants and soil carriers, such as SCMC, attract and aid the
diffusion of environmental soils as well as other deteriorating pollutants.
Rinsing problems have been discussed by Hofenk de Graaff [125]. To optimize rinsing, it should be
carried out at the solubility temperature of the surfactant, the soil-carrier and other constituents of
the washing solution. When considering only optimum rinsing efficiency, the recommended
temperature for rinsing anionic surfactants, at above 40 °C, is rather high for historical textiles
(unless sequestering agents arc added to allow the use of lower temperatures); with non-ionics a
lower temperature (25-30 °C) is effective. Soil-carriers may be added to the first rinse solution if the
textile is heavily soiled. At lower temperatures shorter rinsing times (to avoid saturation) and
regular changes of rinse baths are required. The duration of rinsing, as with the duration of washing,
depends on many factors, notably the thickness and structure of the textile, the possibility of
agitation and whether rinsing is carried out in running water or in still baths. The use of hard water
for rinsing may result in calcium and magnesium ions replacing soil in the 'surfactant-soil micelle'
and soil redeposition may occur. Hence, the use of soft or deionized water at least in the first two
rinse baths is recommended to help to prevent soil redeposition.
Comparing the rinsing properties of anionic surfactants and non-ionics shows that anionics can be
rinsed out more easily-due to the repelling forces between the textile and washing agent. However,
the concentration of anionics in the washed textile may be rather high. Non-ionic surfactants,
having long polar parts, may be bonded to the textile with considerable strength and may be
impossible to remove completely. Despite this, their concentration is particularly low, so any
residue in the textile will be at a very low concentration.
Use of vacuum suction in wet cleaning
Vacuum suction tables were introduced to textile conservation in the late 1970s by Perkinson [126].
Columbus [127] described the washing methods at the Textile Museum in Washington DC in 1967
and recommended the use of vacuum suction tables in wet cleaning of historical textiles. Landi
[128] refers to vacuum suction tables as essential equipment for textile conservation workrooms.
Smith [129], Diebholz [130], Christiensen [131], Hutchinson [132], Fletcher [133], Ashton [134]
and Hackett [135] recommend this equipment for cleaning historical textiles. Howell [136]
introduces the vacuum suction table of the Hampton Court Palace Textile Conservation Studio,
purchased in 1994. Barnett [137] describes the use of a domestic water-extracting vacuum cleaner
in the wet cleaning of carpets and tapestries. Harper [138] provides a thorough characterization of
the vacuum suction table of the Textile Conservation Centre, UK, and its use for wet cleaning
historical textiles. Both Maes [139] and Bosworth [140] describe tapestry cleaning by aerosol
suction.
A vacuum suction table has been used in Germany for wet cleaning historical textiles as presented
by Helbig [141]. Keyserlingk and Vuori [142] reported the use of a custom-made vacuum wash
table for wet cleaning of oversized textiles and announced the availability of the table specifications
from the Canadian Conservation Institute [143].
Efficiency of washing
For research purposes, washing effectiveness has been tested using textiles soiled with standard-soil
mixture, for example by Eastaugh and Stevens [144], Boring and Ewer [67, 145], Ewer and Rudolf
[146], Reponen [147], Lewis [81] and Ginn et al. [148].
A number of approaches has been utilized to characterize the detergent process, including
correlating detergency and electrokinetic phenomena by Rutkowski [149]. Cramer [150] provides a
range of methods for evaluating soil removal. The degree of cleanness of a textile can be
determined by optical assessments and measuring actual soil content. Optical assessments relate to
subjective visual investigation, reflectometry and the photometric and colorimetric attributes, such
as whiteness, greyness, yellowness. Actual soil content is usually measured in simulation tests and
include microscopic/Scanning Electron Microscopic (SEM) examination, gravimetry, solvent
extraction, radiotracer methods in clay and in organic constituents, including artificial sebum. Soils
such as iron oxide can be determined by chemical analysis.
Eastaugh [151] tried to make a connection between percentage soil removal and detergent
formulations. Radioisotope techniques are recommended by Shebs [152], while Morris and Prato
[109] used X-ray fluorescence analysis as a quantitative measure for determining paniculate soil
removal from fabrics, as well as colour measurements. Meek [153] studied the action of soap and
alkali alone in removing fatty dirt from fibres with a vertical microscope fitted with a reflex viewer
and camera. Gentle and Muller [80] controlled the effectiveness by using a video microscope xlOO.
When carrying out experimental washing of archaeological textiles Wolf [154] used visual
examination and low magnification photography, as well as SEM, the latter also used by Obendorf
[155, 156], and Pradhan [157]. Cooke et al. [158] carried out a research program studying the
efficiency of four different cleaning systems for degraded linen, using ob]ective colour
measurement to assess soil removal and colour change, tensile testing to estimate changes in
strength, image analysis to measure changes in fibre and yarn diameter and yarn spacing, weight
loss and SEM to assess soil removal and fibre damage.
Walker [43] reports on the first studies of spectrophotometric techniques that measured the
differences in light remission between soiled and cleaned textiles. Colour measurement with a
spectrophotometer before and after washing and calculation of changes in colour (delta E values),
whiteness, lightness/darkness or yellowness, including the comparison of the colour of a textile to
the Grey Scale, serve to draw conclusions about cleaning effectiveness. These techniques have been
applied widely to monitor washing effectiveness, for example by Boring and Ewer [148], Reponen
[146] and Rhee and Ballard [123]. However, Kissa [159] found that the delta E of remission often
does not correlate with the degree of soil removal.
The washing effect is normally expressed by comparing the 'whiteness' of a textile before and after
treatment. The degree of 'whiteness' of a fabric can be determined by measuring the
ultraviolet/visible reflectance spectrum of the textile. The percentage of light reflected from a
textile can be compared to a standard of magnesium oxide [48]. The reflectance values of treated
and untreated samples can be compared in the same way. Shashoua [62, 160] measured both the
percentage reflectance of light at 500 nm and at the wavelength of maximum reflectance (colour of
the fabric) before and after cleaning. The washing power was mathematically calculated and it was
found that the higher the reflectance, the more effective the washing.
Effect of washing on fibres and textiles
Tensile strength testing has been used to measure the residual strength of textiles. Wolf and Hughes
[154] did not find significant differences between washed and unwashed yarns. Burgess [161]
compared the long-term stability of naturally aged cotton textile fibres washed in distilled-deionized
water, tap water and solutions of calcium bicarbonate (20 ppm) and calcium sulphate (20 ppm),
using gel permeation chromatography and viscosimetry. On the basis of the result of measuring the
degree of polymerization (DP) of cellulosic fibres before and after treatment and accelerated ageing
she-found that fibres washed with distilled-deionized water or calcium bicarbonate solution showed
greatly increased deterioration relative to the control. Hutchins [162] investigated the effect of wet
cleaning on cotton by measuring weight change, stating that part of the decrease in weight after
washing comes from the dissolved deterioration products, while, with the aid of SEM, Pradhan
observed dirt particles disappearing from the surface of the fibres after washing [163]. Wallenborg
examined changes in dimensions, weight, colour, pH, fibre and chemical deposits on the surface on
seventeenth century cotton [164].
Golikov and Ustinov [165] carried out microscopic investigations on fibres to establish an
appropriate cleaning solution. Asnes [166] illustrates damage to cotton fibres after wet cleaning
treatments using SEM micrographs. Hansen and Derelian describe the effects of wet cleaning on
silk tapestries. Based on measuring tensile properties, they found '...silk threads of tapestries which
became significantly stronger following the washing procedure, but it is extremely unlikely for this
to be a random occurrence' [167, p. 9S\. Their study did not address the effect of water on long-term
strength or the negative effects of swelling of fibres.
Washfastness and colour change of dyes
Masschelein-Kleiner [168] investigated the solubility of colourants in washing solutions. According
to Duff et al. [169], the washfastness of dyes is determined by several factors, such as the strength
of the bonds between the dye and the fibre, the size, shape, and levelling characteristics of the dye,
the pH and the actual composition of the washing solution, the temperature and duration of the
treatment and the mechanical treatment during washing. In the American Standard [170] five
classes can be used for characterizing washfastness: 1 very poor, 2 poor, 3 fair, 4 good, 5 excellent.
Tíimár-Balázsy and Eastop [17] give a short overview on the washfastness of direct, acid, basic,
mordant and vat dyes.
Wentz [73] dyed wool with natural colorants (alkanet, annato, brazilwood, cochineal, cutch, henna,
indigo, lac dye, logwood, madder, weld) to investigate their colour change and washfastness during
wet cleaning. The colour change ratings were obtained by visually comparing the dyed samples
after treatment with the American Association of Textile Chemists and Colonsts (AATCC) Grey
Scale for colour change. The staining rates (i.e. degree of bleeding) were made on the white fabrics
by using the AATCC Grey Scale for staining. The findings led to the conclusion that the anionic
sodium alkyl sulphate surfactant Orvus WA and the non-ionic ethoxylated nonylphenol surfactant,
Tergitol NPX, do not cause significant colour changes or staining with the natural colourants tested.
Daniels [171] gives a detailed explanation of the reasons for colour change of a number of natural
dyes in various pH conditions. Vago [172] reports on the serious colour change of a cochineal dyed
wool garment during the cleaning of its silver braiding with sodium hydrogen carbonate solution.
The red colour turned to violet due to the change of pH into the alkaline region [17, pp. 143-6].
Dirks [173] describes the thorough washfastness testing of an American quilt and the use of the
results in the decision-making concerning its treatment. Bruselius-Scharff [174] not only provides a
detailed evaluation on the washfastness of synthetic dyes occurring on historical textiles but also
makes suggestions for the treatment of bleeding dyes. Oger [175] investigated the washfastness of
modern direct dyes on support fabrics and yarns used for conservation and found that they bleed
considerably in washing; the use of these colourants appears to be incompatible with the practical
requirements of restoration and conservation.
Biodegradation of surfactants
The ease with which surfactants degrade plays an important role in their selection as concern about
their long-term environmental effects increases. An overview on the connection between surfactants
and environment has been published by Thomas [176] in 1999. Towards the end of the twentieth
century serious doubts about the biodegradability of alkyl phenol ethylene oxide non-ionic
surfactants have been expressed, noted, for example, by Davidson and Milwidsky [49, pp. 185-7]. It
appears that fatty alcohol ethoxylates are biodegradable but this reduces somewhat with increasing
amounts of ethylene oxide. Schick [177] opposes the use of alkyl phenol ethoxylates because of
health consequences, slow biodegradation and relative difficulty of rinsing out. In an unpublished
typescript Potter [178] highlighted the possibility of banning Synperonic N in the UK as early as
1992 and Gentle and Muller [80] report that the use of Synperonic N was first banned in Sweden
due to its partial biodegradability. Nonylphenols have been identified as oestrogenic compounds,
which have been linked to both male infertility and breast cancer. Daniels [179] also called
conservators attention to the necessity of replacing Synperonic N and NDB.
A risk assessment of using nonylphenol ethoxylates is provided by Weeks et al. [180] and the
problems relating to the sorption of nonylphenol ethoxylates are discussed by Hayward and Allen
[181]. Swisher [182] provides an overview of the connection between the chemical structure and
surfactant biodegradation: one conclusion was that the linearity of the hydrophobic group is an
important factor. Linear surfactants are highly biodegradable, highly branched ones are not. The
effect of a single methyl branch in an otherwise linear molecule is barely noticeable; however,
increased resistance to biodegradability with increased branching is generally observed, particularly
by terminal quaternary branching. The nature of the hydrophilic group has only a minor influence
on the biodegradibility. The clearest examples of such influence are seen in LPAS, which undergoes
biodegradation significantly faster than other amonics and the polyethoxylate non-ionics, where
biodegradation is promoted by shorter EO chain length. Increased distance between the sulphonate
group and the far end of the hydrophobic group increases the speed of biodegradation. This is
known as the distance principle. According to Swisher [182], the problem with alkylphenol
ethoxylates (such as Synperonic N) is that the phenol is towards the centre of the molecule. APEs
readily undergo biodegradation when the phenol is linked to the hydrophobic chain at, or near, the
end. Ethoxylates of linear fatty acids and fatty amides are easily degraded even with ethoxylation as
high as EO=20. The use of a sugar as the hydrophilic group does not result in any spectacular
improvement in biodegradability, but falls in line with the usual principles.
Selected case histories using surfactants and other wash bath additives
Naithani and Kharbade [183] give a valuable overview on the aqueous cleaning methods of
historical textiles. Gentle and Muller [80] followed a repeated sequence of washing and rinsing
when carrying out washing of historical textiles for research purposes. Case histories on wet
cleaning of archaeological textiles have been published by Flury-Lemberg [37], Hillyer [184] and
Nagy [185]. Methods of wet cleaning of historical textiles are described by Rice [186], Finch and
Putnam [187, 188], Masschelein-Kleiner [189], Landi [129] and Pertegato [190, 1911. Landi [192]
may have been the first to introduce a modern washing table to textile conservation laboratories in a
paper published 1966. Results of thorough research into wet cleaning of historical textiles is
provided by Gunilla [193, 194]. Schneider [195] reports on removing 114g soiling (and possibly, in
the present author's opinion, deterioration products of fibres) from a sacred coat by wet cleaning.
The 'routine' wet cleaning methods of the Baltimore House Textile Conservation Workshop were
discussed by Wolf et al. [196]. Behar [197[ gives an overview and flood and bath washing is
described by Howard [198] for wet cleaning carpets. The use of foam from Hostapon T is common
in the Abegg-Stiftung, Swit7.erland, and in many other workshops. Pataki [199] reports using the
foam of a 0.5 g/litre Hostapon T anionic surfactant solution for cleaning a historical textile.
Washing of large textiles is a particularly difficult task, as reported by Fikioris [200], Davies [97],
the Textile Studio, Hampton Court [201], Keyserlingk [202], Haldane [203] and Marko et al.
[204]. Collins [205], for example claims that the soaking of the headquarters tent of George
Washington took 65 hours to loosen mud stains and other soil.
Conclusions
It is natural that a literature review also provides a historical overview. Publications on textile
conservation from the 1950s and 1970s show the strong influence of industry on the surfactants and
other ingredients used in wash baths for historical textiles. Working with conservators, scientists
entering conservation from an industrial background, sooner or later recognized the limits in the use
of the enormous number of different washing agents available and applicable to industry. Research
in the 1980s and 1990s resulted in wash bath recipes much more closely tailored to conservation.
However, there are several 'grey areas', such as the connection between critical micelle
concentration, cloud point and Krafft point; how to determine the remained adsorbed surfactant in
the washed historical textile (in September 2000 Howell and Carr [206] presented a promising new
method using X-ray Photoelectron Spectroscopy); or simply, what is the appropriate washing time.
After finding appropriate surfactant and washing solution compositions, many workshops started to
use predominantly one type of surfactant routinely (see the popularity of Orvus WA in the USA and
Synperonic N in the UK), despite there-being many varied surfactants available and recommended
in the conservation literature. The method of choosing particular surfactants and washing solutions
according to the specific need of the object to be treated, or using them in combination is still rare.
Fast dissemination of information is characteristic of recent years but not of the past: although Rice
published on the use of amomc detergent in the first, and non-ionic in the second wash bath in 1966
[95], it was only in 1995 that Gentle and Muller systematically researched the use of mixtures of
amonics and non-ionics in the same wash bath [80]. Dating from 1995, Walker's recommendation
to start with a lower, and end with a higher HLB surfactant in a two-step washing process seems to
have had little influence on the textile conservation field [43].
The environmental concerns, both relating to conservators' health and the biodegradability of
surfactants, necessitated new researches in this field, not for the purposes of the conservation of
historical textiles, but for the conservation of human beings and their environment. It is true that
there are negative remarks on the health effects and slow biodegradability of some popular
surfactants in specialist literature before Schick's work of 1966 [177]; however, the first warning
relating to the use of nonlyphenol ethoxylates in the field of textile conservation came from Potter
in 1992 [178], followed by Gentle and Müller's 1995 thesis [80] and ending with Daniels' dramatic
announcement in 1999 [179]. Now, as increasing numbers of conservators and scientists are
searching for a replacement for Synperonic N, it should be asked why it took such a long time to
start dealing seriously with the problem. Also, it may be questioned, as did
Howell [206], whether the very small quantities, which are used highly diluted in textile
conservation, are really the cause of such a serious problem or if this is much more a problem for
industry.
In conclusion, the author hopes that the above review shows the importance of studying the
'industrial' literature, of allowing enough time for a thorough conservation-related 'critical'
adaptation and of being sufficiently fast in following initiatives towards new research.
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Author
Agnes Timar-Balazsy qualified as a chemical engineer. She specialized in the study of textiles,
paper, leather and the synthetic polymer industry, carrying out research into textile conservation and
dye analysis. In 1985 she became a Technical Doctor and was awarded a Ph.D. in 1996. She has
been employed at the National Centre of Conservation / Hungarian National Museum since 1966.
Since 1974 she has been teaching material and conservation science and became Head of the
Faculty of Object Conservation in 1989 and a Professor in 1996. From 1991 she has also been
teaching the theory, ethics and history of conservation. She has lectured extensively outside
Hungary and has organized a number of international courses on the scientific principles of textile
conservation. Since 1999-2000 she has been Vice-chairperson of both the ICOM Committee for
Conservation and the ICCROM Council.