The Mode of Formation of Thecotrichite, a Widespread Calcium Acetate Chloride Nitrate
Efflorescence
Lorraine T. Gibson, Brian G. Cooksey, David Littlejohn, Kirsten Linnow, Michael Steiger and
Norman H.Tennent
The widespread occurrence of thecotrichite, in the museum environment is explained theoretically
by construction and examination of its phase diagram. Thecotrichite formation was simulated in the
laboratory to identify the key factors involved in its production. This efflorescence occurs on porous
limestone or calcareous artefacts such as pottery, stored in wooden cabinets that generate acetic
acid vapour. Salt production depends on the moisture content of the object and the concentration of
acetic acid in its surroundings. Furthermore, for thecotrichite to form the artefact must contain
soluble chloride and nitrate salts.
INTRODUCTION
Porous materials such as stone or brick absorb moisture at times of high humidity [1]. With
fluctuations in the ambient relative humidity (RH), soluble salts may be drawn in liquid form to the
surface of the object by capillary forces and evaporate to form white crystalline deposits. These
deposits, termed 'efflorescence', occur in a variety of crystalline habits, generally related to the
nature of the salt, most commonly prisms and acicular, whisker-like crystals [2, 3]. Whisker growth
may occur in any type of crystal, irrespective of whether the structural form is predicted to be
prismatic or cubic [3]. These efflorescent salts, most often inherent in the material of the objects,
can arise from a variety of sources [1, 2]: the burial environment of the object; absorption of saltladen groundwater into building materials such as stone, brick and tiles; or from previous cleaning
or conservation treatments. They are mainly chlorides, nitrates, sulphates and carbonates of the
alkali and alkaline earth metals [2].
In contrast, the particular type of efflorescence studied here involves the formation of crystalline
surface deposits by reaction of the material of the object, and impurities within it, with acetic acid
pollutant vapour. The formation of salt efflorescence on porous calcareous artefacts, as a result of
attack by organic acids, has been observed for many years. Efflorescence on shell collections,
known as Byne's disease, was first documented in 1896 [4] and investigated more fully m 1899 [5].
It took a further three decades, however, before it was recognized that acetic acid vapour, emitted
from oak wooden drawers was responsible for the formation of calcium acetate salts on the surface
of the shells [6]. Since then, a number of studies have related the presence of organic acid vapours,
caused by the materials used to construct museum cabinets, to pollution-induced deterioration (see,
for example [7—10]). In some cases these crystalline salts are novel chemical compounds whose
only known occurrence is as efflorescence on museum artefacts [8-10]. A wide range of porous
materials (for example fossils, pottery and limestone items) stored in wooden cabinets are adversely
affected by the production of acetate efflorescence. One of the most ubiquitous acetate
efflorescences, designated calclacite, Ca(CH3COO)Cl·5H2O [11], has been frequently observed and
reported, notably on a range of Greek and Roman objects where much of the pottery had been
previously treated with hydrochloric acid [12].
Another unusual form of acetate efflorescence has also been found to occur widely on a number of
museum objects [9, 10] and is the subject of the studies reported here. It was first referred to by
FitzHugh and Gettens [10J in an article that cites Van Tassel's description of this efflorescence as
'fibrous crystals showing polysyn-thetic twinning, negative elongation, monoclinic or tri-clinic
symmetry, oblique extinction about 20°, with refractive indices d = 1.504 and a = 1.492'. Chemical
analysis identified calcium, acetate and chloride ions in the salt, but full chemical characterization
was not achieved at this time and it was designated as efflorescence X [10]. It "was not until a
subsequent investigation using ion Chromatographie analysis that efflorescence X was identified as
Ca3(CH3COO) 3Cl(NO3) 2·7H2O [13]. Interestingly, the composition of the compound, which we
have named 'thecotrichite' — a hairy compound from a storage cabinet, from the Greek thêkê
(chest) and triches (hair) — was shown to be consistent on 11 limestone or ceramic items from
different collections around the world [14]. Comprehensive analytical, infrared and X-ray
diffraction data have been reported [13].
The widespread occurrence of thecotrichite has since become evident. Samples from numerous
collections in Europe and North America have been identified. In particular, many samples
previously examined by X-ray diffraction at the Natural History Museum, London, but never
identified are now known to be thecotrichite [15]. As thecotrichite is now believed to be as
ubiquitous as calclacite in the museum environment, a full investigation into the mode of formation
and stability of this complex efflorescent salt was undertaken. It had been proposed that, for
thecotrichite formation to occur, a calcareous porous material must be contaminated with chloride
and nitrate salts and exposed to acetic acid vapour [13]. The acid vapour is responsible for dissolution of calcium carbonate from the matrix of the material, and the resultant solution containing
chloride, nitrate, acetate and calcium ions then forms thecotrichite. This study was initiated in an
attempt to confirm this supposition. Theoretical studies were also undertaken to elucidate the phase
diagram of thecotrichite.
METHODSAND MATERIALS
Impregnation of limestone blocks with chloride and nitrate salts
Oolitic limestone blocks (8x5x1 cm) were repeatedly steeped in distilled water to extract any
soluble salts from the body of the limestone. Ion Chromatographie analyses of the water confirmed
the absence of ions in the washings after five extractions over seven days. The blocks were then air
dried and weighed. To estimate the porosity of the limestone, the maximum mass of water held by
each block of limestone was determined. All blocks were found to be similar; each block absorbed
approximately 5% of water by weight of the limestone. After drying, four limestone blocks were
impregnated with a solution containing low (1:1) or high (1:2) molar ratios of calcium chloride and
calcium nitrate, with either low or high weights of impregnation (Table 1). Six sets of similarly
impregnated limestone blocks were prepared. Using these conditions, calcium ions from both the
salt solutions and the dissolution of calcium carbonate will contribute to the production of
thecotrichite. It should be noted that the addition of calcium salts to a limestone block would not
eliminate the corrosion effect (acetic acid reacting with calcium carbonate on the surface of the
object) observed in real situations. Moreover, the use of calcium salts enabled saturation of the
solution with calcium in order to produce thecotrichite in a reasonable time frame.
Table 1 Preparation of impregnated limestone blocks in an attempt to synthesize thecotrichite.
Generation of acetic acid environments
Two different volumes of glacial acetic acid were added to saturated solutions of sodium chloride
(75% RH) or 40% v/v calcium chloride (33% RH). The resulting acidic salt solutions were placed at
the bottom of glass desiccators to create environments contaminated with 150 or 10 mg·m-3,
equivalent to 61 or 4 parts per million (ppm), acetic acid at relative humidities of 75 or 33% RH
(Table 2). These are the theoretical relative humidities; the actual RH was not measured with
Table 2 Creation of four environments contaminated with acetic acid vapour.
hygrometers or dataloggers due to the high corrosiveness of the environments. Changes to the final
vapour pressure of the salt solutions, caused by the addition of small quantities of acetic acid, were
neglected as the intention was to provide high or low humidity environments as opposed to those
with exactly 75 or 33% RH. Although wooden cabinets in the museum environment have been
found to give rise to concentrations generally below 2.45 mg·m-3 (1 ppm) [9], higher concentrations
were used in this study to permit crystal formation in a relatively short period of time (a few
months). Over time, as the acetic acid vapour reacts with the calcareous materials inside the
desiccator, the acid concentration will decrease due to exhaustion of the acetic acid in the salt
solution. Despite this, solutions were not replenished during the experiment as it is thought that
vapour phase concentrations might also decrease in real situations.
Determination of the experimental acetic acid vapour concentration
The vapour phase concentrations, generated in the glass desiccators, were measured accurately
using passive diffusion tube samplers as described elsewhere [16], with one modification to the
method of tube preparation. Instead of glycerol, ethylene glycol dimethyl ether (Fluka, UK) was
used as a wetting agent in the 1 M potassium hydroxide trapping solution. This provides a more
stable solution for the trapping of acetic acid vapour.
Ion chromatography
Samples of the efflorescent salts were analysed using ion chromatography. A known weight of the
sample was dissolved m 10 mL of distilled water. Prior to injection into the ion Chromatograph, all
solutions were filtered using inorganic membrane filters (Anotop 10 C, 0.2 mm pore size, 10 mm
diameter, Whatman). Salt solutions were injected in quadruplicate and the mean results are
presented. The calibrant ion solutions were prepared by diluting 1000 mg-mL-1 stock solutions of
each analyte.
A Dionex 4000i ion Chromatograph was used with an AG4A guard and AS4 columns for analysis
of acetate ions, an AG5 and AS5 set of columns for analysis of chloride and nitrate ions, and CS12
guard and separator columns for cations. The eluents for anion determination, at a flow rate of 2 mL
per minute, were 6 mM disodium tetraborate decahydrate (Merck, UK) or a solution of 0.75 mM
sodium carbonate and 2.2 mM sodium hydrogencarbonate (Dionex HiPerSolv vial, Merck, Poole).
For cations, an 18 mM solution of methane sulphonic acid (Fluka, UK) eluent was used at 1 mL per
minute. All eluents were purged with helium for 20 minutes prior to use. Analytes were detected
after chemical suppression by a Dionex CDII conductivity cell set to a detection range of 30 μS.
The relative standard deviation (R.SD) values of signals obtained by replicate injections were
normally less than 2%. Recoveries of analytes, measured by analysing solutions of efflorescence
spiked with 5, 5, 4 or 8 mg-mL-1 of calcium, acetate, chloride or nitrate ions, respectively, ranged
from 96 to 104%.
Calculation of stoichiometric formulae from ion Chromatographie analyses
A full explanation of the method used to determine the stoichiometry of efflorescent salts by ion
chromatography has been published elsewhere [13]. The calculation of the stoichiometric ratios of
anions (nia) and cations (nic) can be summarized by the following equations:
where mia and mic are the molar masses of the ith anion and cation, zia and zic are the charge on the
ith anion and cation, Σviazia and Σviczic represent the theoretical total number of anion and cation
equivalencies, and EeTa and EeTc represent the total experimental number of anion and cation
equivalencies, respectively, in a solution as determined by ion Chromatographic analysis. The
concentrations of sodium, potassium and calcium ions were measured in every solution by ion
chromatography. However, in each case the concentration of calcium ions was > 99%, hence for
simplicity the molar ratio of the minor ions, sodium and potassium, are not reported here.
RESULTS AND DISCUSSION
Production of thecotrichite using impregnated limestone blocks in acetic-acid-contaminated
environments
Using the controlled acetic acid environments, attempts were made to produce thecotrichite on
blocks of oolitic limestone. Four acetic-acid-contaminated environments of approximately 150 or
10 mg·m-3, with either a low (33%) or high (75%) RH were prepared (Table 2). Six sets of four
blocks were impregnated with chloride and nitrate as outlined in Table 1 and placed in desiccators
A to F, where A to D contained acetic acid as indicated in Table 2. Desiccators E and F were
control desiccators with no acetic acid, but a RH of 75% or 33%, respectively.
The limestone blocks were monitored visually. After only three weeks, a small amount of crystals
was clearly visible on one of the limestone blocks stored in desiccator A, which had an acetic acid
environment of approximately 150 mg·m-3 at 75% RH (see Table 2). After 18 months, all four
limestone blocks in this environment had produced crystals (Figure 1), which grew up to
approximately 2 cm in length (Figure 2). The highest amount of crystals was produced on the block
impregnated with a 1:1 molar ratio of calcium chloride: calcium nitrate and a weight of
impregnation of 0.5% w/w. Different crystal habits were observed on the blocks, despite the
relatively stable RH over the period of the experiment. Acicular and fibrous crystals, and compact
encrustations were observed on the four limestone blocks in desiccator A.
Sub-samples of the salts were carefully removed under a microscope, weighed and dissolved in
distilled water to provide salt solutions for ion Chromatographic analysis (fibrous crystals and
encrustations were removed separately from block 4). The stoichiometries of the salts were
determined as described previously. The results (Table 3) show that, despite differences in the
molar ratio of chloride and nitrate salts, and the weights of impregnation, all of the synthetic salt
samples had a composition similar to thecotrichite found in the museum environment. Interestingly,
block 4 had a higher concentration of nitrate and chloride (compared to acetate), which may suggest
the presence of the double salt CaCl(NO3)·2H2O in combination with thecotrichite. However, a
solution of this composition is extremely hygroscopic and it is expected that no solids would
precipitate at humidities greater than 21% RH. Therefore it is suggested that the salt was
contaminated with a small amount of the mother liquor. At the end of the experiment, after 18
months, salt formation was not observed on any of the similarly impregnated limestone blocks
stored in desiccators B, C or D, nor was salt formation observed on any of the limestone samples
held in the control environments in desiccators E or F.
The above experiments suggest that the combination of high RH and high acetic acid concentration
has a profound efFect on the rate oí thecotrichite formation on contaminated limestones. Where the
acetic acid concentration was high, but the RH low (desiccator B) no efflorescence formation was
observed. Equally, when the acetic acid concentration was low, regardless of high or low humidity
conditions (desiccators C and D, respectively), salts were not observed on the surface of the
limestone, despite having been impregnated with calcium chloride and calcium nitrate. When the
acid concentration and RH were high enough to produce efflorescence, the stoichiometric
compositions of the salt produced were similar, at least for the 1:1 and 1:2
Figure 1 The four limestone blocks that were placed in desiccator A for a period of 18 months.
Figure 2 Close-up of crystals growing on the side of one of the limestone blocks.
Table 3 Stoichiometric formulae of synthetic samples of thecotrichite grown in dessicator A.
molar ratios at different weights of impregnation studied here.
Analysis of objects that have promoted thecotrichite formation
The above experiments demonstrate that thecotrichite is a stable salt and will commonly crystallize
on limestone or porous materials such as pottery containing calcium carbonate when environmental
conditions permit. However, in practice, it is often the case that a number of such materials are held
in the same contaminated storage or display environment, but are affected to very different extents.
Many instances have occurred where a range of shells [4—6, 8], birds' eggs [7] and earthenware
objects [9] have been damaged by salt formation, whereas neighbouring objects have remained in a
pristine condition. Obviously, in addition to the surrounding atmospheric environment, the nature of
the material must also be considered.
Material was carefully removed from two previously studied objects that supported thecotrichite
formation; an Egyptian limestone relief and a Dutch tile [14]. Limestone scrapings were obtained
for a series of depths up to 20 mm from the edge of the limestone relief and up to 7 mm from the
back of the Dutch tile. The samples were weighed, submersed in water and the extracted solutions
were analysed using ion chromato-graphy. The results for both artefacts (Figures 3a—3c) indicate a
decrease in all three ions inwards from the surface. The compact nature of the fired clay body of the
tile enabled a greater number of discreet scrapings to be taken, thereby demonstrating the sharp
decrease in anion concentration from the surface to a depth of about 7 mm. For the limestone relief,
it was impossible to obtain a sample of the friable surface layer that was not intermingled with
crystals of efflorescence and so these results are not reported in Figure 3. These observations
suggest an external source of acetate and that chloride and nitrate ions exist deep into the body of
the artefact.
Figure 3 Concentration profiles of (a) acetate ions, (b) nitrate ions and (c) chloride ions extracted
from limestone scrapings of an Egyptian limestone and a Dutch tile.
Enrichment of chloride and nitrate close to the surface of the tiles may be explained by capillary
transport and evaporation, consistent with their removal from relatively damp walls to a more
controlled museum environment. Further evidence was obtained by monitoring the environment
surrounding the Egyptian limestone relief. Passive samplers were deployed in the wooden cabinets
for 14 days. The concentration of acetic acid measured was approximately 11 mg·m-3, but less than
0.005 mg·m-3 of nitrogen dioxide was measured. It is also interesting to note that formaldehyde and
formic acid were present at appreciable concentrations of 0.4 mg·m-3 and 0.3 mg·m-3, respectively,
yet no formate was present in the efflorescent salt.
A sample of the fired clay matrix removed from the Dutch tile was also examined by X-ray
diffraction analysis. A second tile was also sampled for comparison. This tile was located in the
same area as the first but no thecotrichite efflorescence had formed. Although both tiles contained
significant quantities of quartz, only the tile that supported thecotrichite formation contained calcite.
These observations suggest that more highly fired (and consequently more vitreous) wares, which
do not contain calcite, are resistant to formation of acetate efflorescence. Therefore to induce
thecotrichite formation porous materials not only need to be contaminated with soluble salts prior to
acid exposure, but must also be comprised of or contain calcium carbonate.
The phase diagram of thecotrichite
There are a large number of possible solid phases that can precipitate from mixed solutions
containing calcium acetate, calcium chloride and calcium nitrate ions. All possible solid phases
(including the single, double and triple salts) suggest a very complex system for thecotrichite.
Moreover, it is difficult to know the conditions that are necessary to precipitate thecotrichite from
such a mixed solution, rather than say calcium acetate. The most convenient way to represent the
crystallization behaviour of mixed salt systems is the use of a phase diagram. The thecotrichite
system, consisting of three anions and one common cation, is a quaternary system. The phase
diagram for this type of system can be constructed in the form of a tetrahedron with water at the
apex and the three solid solutes, A, B and C, on the base triangle [17]. For clarity, a twodimensional projection, the Jänecke projection, is frequently employed. Figure 4 shows the
tetrahedron and the triangular Jänecke projection of the simplest type of quaternary systems. In the
triangular diagram the solvent water is
Figure 4 Isothermal representation of a quaternary system of three salts in water: (a) tetrahedral
space model; (b) Jänecke projection.
excluded and all possible solution compositions are expressed as mole fractions of the solutes A, B
and C in the base triangle of the tetrahedron. The corners of the triangle represent the pure salts, and
the mole fractions decrease from the corner to the opposite side (a concentration of zero). The lines
in the interior of the phase diagram give the compositions of saturated solutions coexisting with two
solid phases. The intersection of the lines represents the composition of the saturated solution with
respect to three solid solutes, this is an invariant point of the system. The stability fields 1, 2 and 3
between the lines represent compositions of solutions saturated with respect to only one solid phase
A, B or C, respectively. In other words, in a particular stability field, a solid of known composition
will deposit from solutions containing the range of mole fractions that outline the stability field.
The thecotrichite system is considerably more complex, because chemical reactions take place
between the components leading to the formation of a number of double and triple salts.
Experimental studies revealed a total of ten different solid phases that can exist in stable
equilibrium with saturated solutions. Both, calcium chloride and calcium nitrate can exist in two
different hydrated forms, hence, there are five different solids representing the pure salts, i.e.
CaCl2·6H2O, CaCl2·4H2O, Ca(NO3) 2 ·4H2O, Ca(NO3) 2 ·3H2O, and Ca(CH3COO),2·H2O. In
addition, at room temperature, four double salts can crystallize from mixed solutions containing at
least two of the calcium salts, i.e. Ca(CH3COO)(NO3) ·3H2O, Ca2(CH3COO)3(NO3) ·2H2O,
CaCl(NO3) ·2H2O and Ca(CH3COO)Cl·5H2O. The triple salt, thecotrichite, Ca3(CH3COO)
3Cl(NO3)7 •7H2O, can crystallize from solutions containing all four ions. In order to determine
which of these solid phases would precipitate out from a mixed solution of any given composition,
the phase diagram of the system has been constructed. However, the extent of experimental data for
the Ca(CH3COO) 2-CaCl2-Ca(NO3) 2-H2O system was insufficient to construct the complete phase
diagram. Therefore, a chemical equilibrium model, details of which may be found elsewhere [18],
had to be used. By application of this model, together with the available data, construction of the
phase diagram was achieved.
The phase diagram of the system at 25°C is depicted in Figure 5. All possible solution compositions
are expressed as mole fractions of Ca(CH3COO) 2 , CaCl2, and Ca(NO3)2 in the triangular diagram.
As outlined above, the corners represent the pure salts, the sides represent compositions of salt
mixtures of two salts and the interior represents compositions of salt mixtures containing all three
salts. The stability fields 1-10 between the lines represent the compositions of solutions saturated
with respect to just one of the solid phases 1-10.
According to the phase rule a maximum of three solid phases can coexist with a saturated solution
in a quaternary system at constant temperature. Hence, only one composition of a saturated solution
exists for each possible combination of three solid phases. These solution compositions are given as
the intersections of the respective stability fields. These are the isothermal invariant points of the
system. The junctions between two stability fields in the interior give the compositions of saturated
solutions coexisting with two solid phases.
Figure 5 Stability fields in the Ca(CH3COO)2-CaCI2-Ca(NO3)2 system at 25°C, solid phases
corresponding to the stability fields are:
(1) CaCl2-6H2O, (2) CaCI2·4H2O, (3) CaCI(NO3)·2H20,
(4) Ca(NO3) 2·3H2O, (5) Ca(NO3)2·4H2O, (6) Ca(CH3COO)(NO3)·3H2O.
(7) Ca2(CH3COO)3(NO3)·2H2O, (8) Ga(CH3COO)2·H2O,
(9) Ca(CH3COO)Cl·5H2O, (10) Ca3(CH3COO)3CI(NO3)2·7H2O.
For the construction of the phase diagram an equilibrium model calculated the invariant points. For
simplicity, the two salt coexistence curves are simply straight lines interconnecting the invariant
points. Stability field 10 represents solution compositions saturated with respect to thecotrichite.
This field comprises a wide range of chloride to nitrate mixing ratios and extends close to the
CaCl2—Ca(NO3) 2 side of the diagram. Hence, adding only a small amount of acetate to a mixed
solution of chloride and nitrate is sufficient to reach the stability field of thecotrichite. Most likely,
this is the main reason why thecotrichite is often found in artefact efflorescence.
A lot of information on the crystallization properties of salt mixtures can be readily derived from
the phase diagram. As an example, consider a mixed solution of composition A in Figure 5. Point A
lies in stability field 7, thus, if water is removed from that solution by isothermal evaporation,
Ca3(CH3COO)3(NO3)·2H2O is deposited and the solution composition changes due to the
precipitation of acetate and nitrate. The stoichio-metric composition of the double salt is represented
by point B. Upon further evaporation of water, the composition of the salts in the solution moves
from A to C along the straight line drawn from B through A. Reaching point C, the solution is now
saturated with respect to both the double salt Ca2(CH3COO)3(NO3)·2H2O and thecotrichite. Any
further removal of water causes the precipitation of thecotrichite.
Similar crystallization pathways can be derived for other solution compositions in the quaternary
system. Usually one solid is precipitated first but when a sufficient quantity of water has been
removed, saturation with respect to a second solid is reached and two salts will be in equilibrium
with the solution. Hence the presence of two different phases in salt efflorescence is not surprising.
However, there are restrictions. For example, according to the phase diagram there are solutions
coexisting with thecotrichite and either one of the two double salts Ca2 (CH3COO)3(NO3)·2H2O and
Ca(CH3COO)Cl·5H2O (calclacite) but not with Ca(CH3COO)2·H2O (calcium acetate hydrate).
Hence, the occurrence of both thecotrichite and calcium acetate hydrate as simultaneous
efflorescence products is not possible, while the presence of thecotrichite and calclacite may be
expected, and has been observed [19], depending on the composition of the salt mixture.
Thecotrichite is an incongruently soluble salt. It cannot coexist in stable equilibrium with a solution
of the same composition, i.e. containing the four ions in the same stoichiometric ratio. It is,
therefore, impossible to define a unique value of equilibrium humidity unambiguously, although
there is a definite RH at which thecotnchite will absorb water to form a solution. However, this
solution is not saturated with respect to thecotrichite, but rather with respect to the acetate/ nitrate
double salt (see Figure 5, point A). The exact deliquescence RH of thecotrichite is not known, but
by placing small amounts of crystals in a series of desiccators conditioned by saturated salt
solutions over the range 56-100% RH, thecotrichite was found to deliquesce at approximately 80—
90% RH. Although there is a unique value of the deliquescence humidity at > 80% RH,
crystallization/dissolution can take place across a range of relative humidities. In the case of
thecotrichite, the RH range extends from approximately 80% (at the acetate-rich side of the
thecotrichite field) down to about 20% (at the acetate-deficient side). In the museum environment,
the ubiquity of thecotrichite suggests that at 55% RH thecotrichite can crystallize out, while
allowing enough water, as a result of the hygroscopic properties of the mixed calcium nitrate—
chloride solution, to promote further dissolution of acetic acid and the associated deterioration of
the limestone.
CONCLUSIONS
These results confirm that objects are prone to thecotrichite production when they are comprised of
or contain calcium carbonate contaminated with chloride and nitrate ions and, furthermore, are
located in an environment containing acetic acid vapour. In addition, it has been demonstrated that
the ambient RH is also an important secondary issue in the formation of thecotrichite; low values of
RH inhibit efflorescence growth. While chloride and nitrate salt residues are an essential
prerequisite for the formation of thecotrichite, salt contaminants in general predispose artefacts to
acetate efflorescence formation. Other studies have demonstrated a positive correlation between
efflorescence formation and salts present in the affected item [20, 21]. These salts occur as residues
from previous conditions or have been introduced by conservation or cleaning treatments [12, 20—
23]. The reasons for the deterioration of some items held in the same environment as others that
remain in pristine condition warrant further study but the presence of salt residues is undoubtedly an
important factor.
Thecotrichite efflorescence most usually takes the form of whiskery crystals. This is in line with
previous observations that the formation of efflorescence whiskers occurs on porous substrates
without any liquid water
supply from the interior (see for example [24]). Thus, the appearance of thecotrichite on artefacts is
consistent with whisker growth from a small amount of solution in the zone of efflorescence. When
further solution is supplied and conditions for whisker growth are maintained, the process continues
and a dense carpet of thecotrichite is formed.
The question remains: how can acetate efflorescence formation be prevented on porous museum
artefacts? The standard treatment method for obviating salt efflorescence is desalination. If this
approach is to be followed to prevent the formation of thecotrichite, care must be taken to ensure
that salts are removed completely from the core of the object otherwise they will be drawn to the
surface by capillary action and become available for reaction with acetic acid vapour. Perhaps the
most straightforward suggestion would be to remove susceptible items from acetic acid
contaminated storage or display areas. However, if whole collections are affected, this solution
becomes impractical. Testing of materials to ensure suitability for showcases has been advocated
since the 1970s [25] and later publications demonstrate the vast range of materials tested for
emission of acids and aldehydes (see, for example, [26-28]).
As demonstrated by the phase diagram, even with small quantities of acetate present in the solution
phase, the stability field of thecotrichite will be reached. Since the concentration of acetate ions
present in the solution phase is determined by the acetic acid concentration and the humidity of the
surrounding environment, consideration can usefully be given to the reduction of both these factors.
In this 18-month study, no thecotrichite formed on limestone blocks held at an acetic acid vapour
concentration of 10 mg-m-3 at 75 or 33% RH. Similar studies have shown that lowering the RH to
below 30% retards acetic-acid-induced degradation of shells [29] and lead [30].
It is important, however, not to misinterpret the benefit of low acetic acid concentrations as
'threshold' values. If the limestone blocks had been left in the low acid-contaminated environments
for a longer period of time, it is highly probable that thecotrichite would have been formed. In
practice, acetate efflorescences have been observed for many items over a wide range of
contaminated environments with varying acid concentration [9, 31]. Unfortunately there is no easy,
global, solution. Each situation must be assessed on its own merits to determine the best practical
answer to the problem. Careful consideration must be undertaken of the artefact's composition, the
environmental conditions (pollutant concentration and RH) under which the object is stored and
possible soluble salt contamination.
Finally, the authors realize that in real situations ions other than calcium, chloride, nitrate and
acetate will also be present in solutions found in the pores of contaminated objects. Although this
study focused solely on the thecotrichite system, it should be noted that the far more complex
quinary system, including sodium in the phase diagram, is currently under consideration. Although
the phase diagram is not yet complete it has been observed that the main conclusions are also valid
in the quinary system. Addition of a relatively small amount of acetate to a solution containing
calcium, sodium, chloride and nitrate ions will lead to the formation of thecotrichite.
ACKNOWLEDGEMENTS
The authors wish to acknowledge financial support from the Natural Environment Research
Council, and the Scottish Conservation Bureau of Historic Scotland (for LTG). We are grateful to
Jane Porter, formerly of the Burrell Collection, Glasgow, for sourcing the limestone used in the
experimental work.
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AUTHORS
LORRAINE T. GIBSON gained her BSc (Hons) in chemistry with computing applications at the
University of Glasgow in 1991, and PhD in analytical chemistry at the University of Strathclyde in
1995. She undertook postdoctoral research directed towards the implementation of analytical
science in preventive conservation at the University of Strathclyde and the Netherlands Institute for
Cultural Heritage. Currently she is a lecturer in analytical chemistry at the University of Strathclyde. Address: WESTChem, Department of Pure and Applied Chemistry, University of Strathclyde,
295 Cathedral Street, Glasgow Gl 1XL, UK. Email: lorraine.gibson@strath. ac.uk
BRIAN G. COOKSEY became a graduate of the Royal Institute of Chemistry in 1965 after 15 years in
industry as a laboratory technician and a chemical analyst. He was awarded a PhD in chemistry at
the University of Aston in Birmingham in 1970. After a postdoctoral study at the
University of Exeter he joined the University of Strathclyde in Glasgow as a lecturer in chemistry.
Address: as for Gibson. Email: b.g.cooksey@strath.ac.uk
DAVID LITTLEJOHN has been Professor of Analytical Chemistry at the University of Strathclyde
since 1988. He was elected Fellow of the Royal Society of Chemistry in 1991 and Fellow of the
Royal Society of Edinburgh (FRSE) in 1988. He is a past winner of the Royal Society of Chemistry
(RSC) SAC Silver Medal and was recently awarded the Theophilus Redwood Lectureship by the
RSC. Address: as for Gibson. Email: d. littlejohn @strath .ac.uk
KIRSTEN LINNOW gained her diploma in chemistry at the University of Hamburg, and has
undertaken postgraduate research in analytical chemistry. Address: Institut für Anorganische und
Angewandte Chemie, Universität Hamburg, Martin-Euther-King-Platz 6, 20146 Hamburg,
Germany. Email: kirsten.linnow@chemie.uni-hamburg.de
MICHAEL STEIGER gained a diploma in chemistry and PhD in analytical chemistry at the University
of Hamburg. His postdoctoral research was concerned with all aspects of salt damage in porous
materials. Currently he is a lecturer in inorganic and analytical chemistry at the University of
Hamburg. Address as for Einnow. Email: michael.steiger@chemie. uni-hamburg.de
NORMAN H. TENNENT studied chemistry at Glasgow University and as a postdoctoral researcher at
Ohio State University. He was recently appointed to the special Chair for Chemistry of
Conservation and Restoration at the University of Amsterdam, a position which he combines with
work as a freelance conservation scientist in Scotland. Research specialities include preventive
conservation, polymer degradation and technical examination and conservation of ceramics and
glass. Address: van 't Hoff Institute for Molecular Sciences, Faculty of Science, University of
Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands. Email:
bmckenna@directqfficesolutions.fsnet.co.uk
Résumé — Ea fréquente formation de thécotrichite, Ca3(CH3COO)3Cl(NO3)2·7H2O, dans
l'environnement muséal est expliquée théoriquement grâce à la construction et à l'examen de son
diagramme de phase. Ea formation de thécotrichite a été simulée en laboratoire afin d'identifier les
facteurs dés impliqués dans sa production. Cette efflorescence apparaît sur des objets calcaires
poreux comme de la poterie entreposés dans des armoires en bois produisant des vapeurs d'acide
acétique. Ea production de sel dépend du degré d'humidité de l'objet et de la concentration
environnante en acide acétique. En outre, pour que la thécotrichite se forme, l'objet doit contenir
des chlorures et des nitrates solubles.
Zusammenfassung — Das verbreitete Vorkommen von Thecotrichit,
Ca3(CH3COO)3Cl(NO3)2·7H2O in Museen kann anhand der Konstruktion und einer Auswertung
des Phasendiagramms verstanden werden. Die Bildung von Tliecotrichit wurde im Labor simuliert,
um die Faktoren der Bildung zu bestimmen. Die Ausblühungen entstehen auf porösem Kalkstein
oder kalkhaltigen Materialien wie Keramik, wenn sie in hölzernen Behältern aufbewahrt werden,
die Essigsäure ausgasen. Die Salzbildung hängt vom Feuchtegehalt der Objekte und der
Konzentration der Essigsäure ab. Darüber hinaus müssen die Artefakte Chlorid und Nitrat in Form
löslicher Salzen enthalten.
Resumen — La existencia abundante de tecotriquita, Ca3(CH3COO)3Cl(NO3)2·7H2O, en los
ambientes de los museos puede explicarse teóricamente por la construcción y examen de sus
diagramas de fase. La formación de tecotriquita se simuló en el laboratorio con el fin de identificar
los factores clave relacionados con su desarrollo. Esta eflorescencia se produce en calizas porosas
u objetos calcáreos, como la cerámica, cuando se almacenan en armarios de madera que generan
vapores de ácido achico. El surgimiento de estas sales depende del contenido de humedad del
objeto y de la concentración de ácido acético en el entorno. Adicionalmente hay que considerar que
para la formación de tecotriquita los objetos deben contener sales solubles en forma de cloruros y
nitratos.