CM 230updated

Yarmouk University
Department of Conservation and
Management of Cultural
Our cultural heritage is made with
almost all type of materials produced by
the nature and used by men to realize
several types of artefacts from very
simple mono-components to complex
structures integrating inorganic and
organic matters.
 These cultural heritage objects, even if
made with the more “resistant” stones
and metals, are influenced by the
environmental parameters, which can
modify their structure and composition
What Governs Deterioration of
Cultural materials
The deterioration and preservation of
materials depends on two things:
 The nature of the material
 Environment surrounding the
of the Material
What kinds of materials will I find in a
museum collection?
Museum objects are often divided
into three material-type categories:
organic, inorganic, and composite.
You must understand the properties
of each of the materials in each of
these categories.
Organic Objects:
Organic objects are
derived from things that
were once living —
plants or animals.
 Based on C. Contains O,
N, H
 Materials are processed
in a multitude of ways to
produce the objects that
come into your
Organic Objects:
Various material types
include wood, paper,
textiles, leather and
skins, horn, bone and
ivory, grasses and
bark, lacquers and
waxes, plastics, some
pigments, shell, and
biological natural
history specimens.
All organic materials share some
common characteristics
 Contain the element carbon
 Are combustible
 Are made of complicated molecular structures
that are susceptible to deterioration from
extremes and changes in relative humidity and
 Absorb water from and emit water to the
surrounding air in an ongoing attempt to reach
an equilibrium (hygroscopic)
 Are sensitive to light
 Are a source of food for mold, insects, and
Inorganic Objects
Inorganic objects
have a geological
origin. Just like
organic objects,
the materials are
processed in a
variety of ways to
produce objects
found in your
Inorganic Materials
Material types
include: metals,
ceramics, glass,
stone, minerals,
and some
All inorganic objects share some
common characteristics.
· have undergone extreme pressure or heat
· are usually not combustible at normal
· can react with the environment to change their
chemical structure (for example, corrosion or
dissolution of constituents)
· may be porous (ceramics and stone) and will
absorb contaminants (for example, water, salts,
pollution, and acids)
· are not sensitive to light, except for certain
types of glass and pigments
Composite Objects:
Composite or mixed
media objects are made
up of two or more
For example, a painting
may be made of a wood
frame and stretcher, a
canvas support, a variety
of pigments of organic
and inorganic origin, and
a coating over the paint.
A book is composed of
several materials such
as paper, ink, leather,
thread, and glue.
Composite Objects:
Depending on their
materials, composite
objects may have
characteristics of both
organic and inorganic
The individual materials
in the object will react
with the environment in
different ways.
Also, different materials
may react in opposition
to each other, setting up
physical stress and
causing chemical
interactions that cause
What is inherent vice?
In addition to deterioration caused by
the agents of deterioration, certain
types of objects will deteriorate because
of their internal characteristics. This
mechanism of deterioration is often
called inherent vice or inherent fault.
 It occurs either because of the
incompatibility of different materials or
because of poor quality or unstable
In nature, materials often possess
characteristics that protect them from
natural degradation. Their structure
and composition may include
features such as protective layers,
insect and mold resistant chemicals,
and photochemical protection.
 Processing during object
manufacture can remove these
natural safeguards
Short-lived materials:
Short-lived materials are often the result of
manufacturing processes that do not
consider the long-term stability of the items
that were produced.
Examples of impermanent materials with
inherent vice include:
· cellulose nitrate and cellulose ester film
· wood pulp paper
· many 20th century plastics
· magnetic media, including electronic
Structural nature:
Inherent vice can also be
related to the structure
of an object. Poor
design, poor
construction, or poor
application of materials
may cause structural
failure. Examples of
such damage include:
· drying cracks in paint
improperly applied
· broken or lost
· loose joints
The way an object was used or where it was
stored or deposited before it comes into your
collection may lead to inherent vice.
Here, damage and deterioration is caused by the
original function of the object, its maintenance,
or its environment.
Examples of inherent vice caused by history
· accumulation of dissimilar paint layers, such
as oil and latex
· saturation in a wooden bowl that had been
used as a container for oil
· deposits of soluble salts in an archeological
ceramic during burial
You may have trouble identifying
deterioration caused by inherent vice
because often there is little or no
information on the selection and
processing of materials, manufacturing
details, and previous use of an object.
 Train your critical eye by reviewing
similar objects and by developing
knowledge of object technology. Over
time, you will become more proficient at
identifying inherent vice.
What makes archeological objects different from other
materials commonly found in museum collections?
The condition of these objects depends
entirely on their reaction with the
environmental conditions to which they have
been exposed through time.
Underground the object reaches a kind of
equilibrium with the surrounding soil. Then,
when the object is excavated, it must adjust to
a new and radically different environment.
Reactions can involve both physical and
chemical changes.
Regardless of the condition of the
object before excavation, the moment it
becomes exposed it is vulnerable to
rapid deterioration. Figure I.1 illustrates
the deterioration rate of archeological
 Deterioration
of Museum Objects
What is deterioration?
 Deterioration
is any physical or
chemical change in the condition
of an object.
 Deterioration is inevitable. It is a
natural process by which an object
reaches a state of physical and
chemical equilibrium with its
immediate environment.
Types of Deterioration
 The
types of deterioration can be
divided into two broad categories:
physical deterioration and
chemical deterioration. Both types
often occur simultaneously.
What is chemical deterioration?
Chemical deterioration is any change
in an object that involves an alteration
of its chemical composition.
 It is a change at the atomic and
molecular level.
 Chemical change usually occurs
because of reaction with another
chemical substance (pollution, water,
pest waste) or radiation (light and heat).
Examples of chemical change include:
· oxidation of metals
· corrosion of metals and
stone caused by air
· damage to pigments by
air pollution or reaction
with other pigments
· staining of paper
documents by adjacent
acidic materials
· fading of dyes and
Examples of chemical change include:
darkening of resins
darkening and
embrittlement of pulp
burning or scorching of
material in a fire
embrittlement of textile
bleaching of many
organic materials
(development of
additional chemical
bonds) of plastics
rotting of wood by
growing fungus
What is physical deterioration?
Physical deterioration is a change in
the physical structure of an object.
 It is any change in an object that
does not involve a change in the
chemical composition.
 Physical deterioration is often
caused by variation in improper
levels of temperature and relative
humidity or interaction with some
mechanical force.
Examples of physical deterioration include:
melting or softening of
plastics, waxes, and
resins caused by high
cracking or buckling of
wood caused by
fluctuations in relative
warping of organic
materials caused by high
relative humidity
warping or checking of
organic materials caused
by low relative humidity
shattering, cracking, or
chipping caused by impact
stone cracking and
structural failure (for
example, metal fatigue,
tears in paper, rips in
loss of organic material
due to feeding by insects
and/or their larvae
·staining of textiles and
paper by mold
Physical deterioration and chemical
deterioration are interrelated. For
example, chemical changes in
textiles caused by interaction with
light also weaken the fabric so that
physical damage such as rips and
tears may occur.
Why is it important to understand the
environmental agents of deterioration and how
to monitor them?
If you understand basic information about the
chemistry and physics of temperature, relative
humidity, light, and pollution, you will be better
able to interpret how they are affecting your
museum collections.
This chapter gives you a basic overview of
these agents and describes how to monitor
them. You will be able to tell how good or bad
the conditions in your museum are and
whether or not the decisions you make to
improve the environment are working the way
you expect.
In the past, simplified standards such
as 50% RH and 65°F were promoted.
With research and experience, it is
now understood that different
materials require different
environments. You must understand
the needs of your collection for the
long-term in order to make thoughtful
decisions about proper care.
You will want to develop microenvironments for storage
of particularly fragile objects. A microenvironment
(microclimate) is a smaller area (box, cabinet, or
separate room) where temperature and/or humidity are
controlled to a different level than the general storage
Common microenvironments include:
· freezer storage for cellulose nitrate film
· dry environments for archeological metals
· humidity-buffered exhibit cases for fragile organic
· temperature-controlled vaults for manuscript
 Agents
of Deterioration
What is temperature?
 Temperature is a measure of the motion of
molecules in a material. Molecules are the
basic building blocks of everything. When the
temperature increases, molecules in an object
move faster and spread out; the material then
expands. When the temperature decreases,
molecules slow down and come closer
together; materials then contract.
 Temperature and temperature variations can
directly affect the preservation of collections in
several ways.
How does temperature affect
museum collections?
Temperature affects museum collections in a variety of ways.
 · At higher temperatures, chemical reactions increase.
For example, high temperature leads to the increased
deterioration of cellulose nitrate film. If this deterioration
is not detected, it can lead to a fire. As a rule of thumb,
most chemical reactions double in rate with each
increase of 10°C (18°F).
 · Biological activity also increases at warmer
temperatures. Insects will eat more and breed faster, and
mold will grow faster within certain temperature ranges.
 ·
How does temperature affect
museum collections?
At high temperatures materials can
soften. Wax may sag or collect dust
more easily on soft surfaces, adhesives
can fail, lacquers and magnetic tape
may become sticky.
 In exhibit, storage and research
spaces, where comfort of people is a
factor, the recommended temperature
level is 18-20° C (64-68° F). Temperature
should not exceed 24° C (75° F). Try to
keep temperatures as level as possible.
Avoid Fluctuating Temperatures
Avoid abrupt changes in temperature. It is
often quick variations that cause more
problems than the specific level. Fluctuating
temperatures can cause materials to expand
and contract rapidly, setting up destructive
stresses in the object. If objects are stored
outside, repeated freezing and thawing can
cause damage.
 Temperature is also a primary factor in
determining relative humidity levels. When
temperature varies, RH will vary.
Relative Humidity
What is relative humidity (RH)?
Relative humidity is a relationship between the volume
of air and the amount of water vapor it holds at a given
temperature. Relative humidity is important because
water plays a role in various chemical and physical
forms of deterioration.
There are many sources for excess water in a museum:
exterior humidity levels, rain, nearby bodies of water,
wet ground, broken gutters, leaking pipes, moisture in
walls, human respiration and perspiration, wet mopping,
flooding, and cycles of condensation and evaporation.
Relative Humidity
All organic materials and some inorganic
materials absorb and give off water depending
on the relative humidity of the surrounding air.
Metal objects will corrode faster at higher
relative humidity.
Pests are more active at higher relative
humidities. We use relative humidity to
describe how saturated the air is with water
vapor. “50% RH” means that the air being
measured has 50% of the total amount of water
vapor it could hold at a specific temperature.
Relative Humidity
It is important to understand that the
temperature of the air determines how much
moisture the air can hold. Warmer air can hold
more water vapor. This is because an increase
in the temperature causes the air molecules to
move faster and spread out, creating space for
more water molecules.
For example, warm air at 25°C (77°F) can hold a
maximum of about 24 grams/cubic meter
(g/m3), whereas cooler air at 10°C (50°F) can
hold only about 9 g/m3.
Relative Humidity
Relative humidity is directly related to temperature. In a
closed volume of air (such as a storage cabinet or
exhibit case) where the amount of moisture is constant,
a rise in temperature results in a decrease in relative
humidity and a drop in temperature results in an
increase in relative humidity.
For example, turning up the heat when you come into
work in the morning will decrease the RH; turning it
down at night will increase the RH.
Relative humidity is inversely related to temperature. In
a closed system, when the temperature goes up, the RH
goes down; when temperature goes down, the RH goes
What is the psychrometric chart?
The relationships
between relative
temperature, and
other factors such
as absolute humidity
and dew point can
be graphically
displayed on a
The following definitions will help you understand the factors
displayed on the chart and how they affect the environment in your museum.
Absolute humidity (AH) is the quantity of moisture
present in a given volume of air. It is not temperature
dependent. It can be expressed as grams of water per
cubic meter of air (g/m3). A cubic meter of air in a
storage case might hold 10 g of water. The AH would be
10 g/m3.
· Dew point (or saturation temperature) is the
temperature at which the water vapor present saturates
the air. If the temperature is lowered the water will begin
to condense forming dew. In a building, the water vapor
may condense on colder surfaces in a room, for
example, walls or window panes. If a shipping crate is
allowed to stand outside on a hot day, the air inside the
box will heat up, and water will and condense on the
cooler objects.
Relative humidity relates the moisture content of the air
you are measuring (AH) to the amount of water vapor
the air could hold at saturation at a certain temperature.
Relative humidity is expressed as a percentage at a
certain temperature. This can be expressed as the
RH = Absolute Humidity of Sampled Air/ Absolute
Humidity of Saturated Air at Same Temperature x 100
In many buildings it is common to turn the temperature
down in the evenings when people are not present.
If you do this in your storage space, you will be causing
daily swings in the RH. Suppose you keep the air at 20°C
(68°F) while people are working in the building. A cubic
meter of air in a closed space at 20°C (68°F) can hold a
maximum of 17 grams of water vapor. If there are only
8.5 grams of water in this air, you can calculate the
relative humidity.
The AH of the air = 8.5 grams
The AH of saturated air at 20°C = 17.0 grams
Using the equation above
RH = 8.5 x 100%/17 = 50%
50% RH may be a reasonable RH for your storage areas.
But, if you turn down the heat when you leave the
building at night, the RH of the air in the building will
rise rapidly. You can figure out how much by using the
same equation. If the temperature is decreased to 15°C
(59°F), the same cubic meter of air can hold only about
13 grams of water vapor. Using the same equation The
AH of the air = 8.5 grams
The AH of saturated air at 15°C (59°F) = 13.0 grams
RH = 8.5 x 100%/13 = 65%
By turning down the heat each night and turning it up in
the morning you will cause a 15% daily rise and fall in
How do organic objects react with
relative humidity?
Organic materials are hygroscopic.
Hygroscopic materials absorb and
release moisture to the air. The RH of
the surrounding air determines the
amount of water in organic materials.
When RH increases they absorb more
water; when it decreases they release
moisture to reach an equilibrium with
the surrounding environment. The
amount of moisture in a material at a
certain RH is called the Equilibrium
Moisture Content (EMC).
What deterioration is caused by
relative humidity?
Deterioration can occur when RH is too high, variable, or too low.
Too high: When relative humidity is high, chemical
reactions may increase, just as when temperature is
elevated. Many chemical reactions require water; if there
is lots of it available, then chemical deterioration can
proceed more quickly.
Examples include metal corrosion or fading of dyes.
High RH levels cause swelling and warping of wood and
ivory. High RH can make adhesives or sizing softer or
sticky. Paper may cockle, or buckle; stretched canvas
paintings may become too slack. High humidity also
supports biological activity. Mold growth is more likely
as RH rises above 65%. Insect activity may increase.
What deterioration is caused by
relative humidity?
 Too
low: Very low RH levels cause
shrinkage, warping, and cracking
of wood and ivory; shrinkage,
stiffening, cracking, and flaking of
photographic emulsions and
leather; desiccation of paper and
adhesives; and dessication of
basketry fibers.
What deterioration is caused by
relative humidity?
Variable: Changes in the surrounding RH can affect the
water content of objects, which can result in
dimensional changes in hygroscopic materials. They
swell or contract, constantly adjusting to the
environment until the rate or magnitude of change is too
great and deterioration occurs. Deterioration may occur
in imperceptible increments, and therefore go unnoticed
for a long time (for example, cracking paint layers). The
damage may also occur suddenly (for example, cracking
of wood). Materials particularly at high risk due to
fluctuations are laminate and composite materials such
as photographs, magnetic media, veneered furniture,
paintings, and other similar objects.
Light causes fading, darkening, yellowing,
embrittlement, stiffening, and a host of other chemical
and physical changes.
Be aware of the types of objects that are particularly
sensitive to light damage including: book covers, inks,
feathers, furs, leather and skins, paper, photographs,
textiles, watercolors, and wooden furniture.
What is light?
Light is a form of energy that stimulates our sense of
vision. This energy has both electrical and magnetic
properties, so it known as electromagnetic radiation.
To help visualize this energy, imagine a stone dropped
in a pond. The energy from that stone causes the water
to flow out in waves. Light acts the same way. We can
measure the “wavelength” (the length from the top of
each wave to the next) to measure the energy of the
The energy in light reacts with the molecules in
objects causing physical and chemical
Because humans only need the visible portion
of the spectrum to see, you can limit the
amount of energy that contacts objects by
excluding UV and IR radiation that reaches
objects from light sources.
All types of lighting in museums (daylight,
fluorescent lamps, incandescent (tungsten),
and tungsten-halogen lamps) emit varying
degrees of UV radiation.
The unit of measurement is
the nanometer (1 nanometer
(nm) equals 1 thousand
millionth of a meter). We can
divide the spectrum of
electromagnetic radiation into
parts based on the wavelength.
The ultraviolet (UV) has very
short wavelengths (300-400
nm) and high energy. We
cannot perceive UV light. The
visible portion of the spectrum
has longer wavelengths (400760 nm) and our eyes can see
this light. Infrared (IR)
wavelengths start at about 760
nm. We perceive IR as heat.
The energy in light reacts with the molecules in
objects causing physical and chemical
Because humans only need the visible portion
of the spectrum to see, you can limit the
amount of energy that contacts objects by
excluding UV and IR radiation that reaches
objects from light sources.
All types of lighting in museums (daylight,
fluorescent lamps, incandescent (tungsten),
and tungsten-halogen lamps) emit varying
degrees of UV radiation.
This radiation (which has the most energy) is
the most damaging to museum objects.
Equipment, materials, and techniques now
exist to block all UV. No UV should be allowed
in museum exhibit and storage spaces.
The strength of visible light is referred to as
the illumination level or illuminance. You
measure illuminance in lux, the amount of light
flowing out from a source that reaches and
falls on one square meter.
Reciprocity law
When considering light levels in your
museum you should keep in mind the
 “reciprocity law.” The reciprocity law
states, “Low light levels for extended
periods cause as much damage as
high light levels for brief periods.”
The rate of damage is directly proportional to the
illumination level multiplied by the time of exposure.
A 200-watt light bulb causes twice as much damage as a
100-watt bulb in the same amount of time. A dyed textile
on exhibit for six months will fade about half as much as
it would if left on exhibit for one year.
So if you want to limit damage from light
you have two options:
 · reduce the amount of light
 · reduce the exposure time
What are the standards for visible
light levels?
You can protect your exhibits from damage caused by
lighting by keeping the artificial light levels low.
The human eye can adapt to a wide variety of lighting
levels, so a low light level should pose no visibility
problems. However, the eye requires time to adjust when
moving from a bright area to a more dimly lighted space.
This is particularly apparent when moving from daylight
into a darker exhibit area. When developing exhibit
spaces, gradually decrease lighting from the entrance
so visitors’ eyes have time to adjust.
Do not display objects that are sensitive to light near
windows or outside doors.
Basic standards5 for exhibit light levels are:
50 lux maximum for especially light-sensitive materials including:
dyed organic materials
photographs and blueprints
prints and drawings
biological specimens
· 200 lux maximum for less light-sensitive objects including:
- undyed organic materials
- oil and tempera paintings
- finished wooden surfaces
· 300 lux for other materials that are not light-sensitive
- metals
- stone
- ceramics
- some glass
In general don't use levels above 300
lux in your exhibit space so that light
level variation between exhibit
spaces is not too great.
In order for collections to be seen and used in
various ways (for example, long-term exhibit,
short-term exhibit, research, teaching) you should
take into account a variety of factors:
 · light sensitivity of the object
 · time of exposure
 · light level
 · type of use
 · color and contrast of object
Dust and Gaseous Air Pollution
Air pollution comes from contaminants produced
outside and inside museums.
Common pollutants include: dirt, which includes sharp
silica crystals; grease, ash, and soot from industrial
smoke; sulfur dioxide, hydrogen sulfide, and nitrogen
dioxide from industrial pollution; formaldehyde, and
formic and acetic acid from a wide variety of
construction materials; ozone from photocopy machines
and printers; and a wide variety of other materials that
can damage museum collections.
pollutants are divided into two types:
particulate pollutants (for
example, dirt, dust, soot, ash, molds,
and fibers)
 · gaseous pollutants (for example,
sulphur dioxide, hydrogen sulphide,
nitrogen dioxide, formaldehyde,
ozone, formic and acetic acids)
What are particulate air pollutants?
Particulate pollutants are solid particles suspended in
the air.
Particulate matter comes both from outdoor and indoor
sources. These particles are mainly dirt, dust, mold,
pollen, and skin cells, though a variety of other materials
are mixed in smaller amounts.
The diameter of these pollutants is measured in microns
(1/1,000,000 of a meter). Knowing the particulate size is
important when you are determining the size of air filters
to use in a building.
Some particles, such as silica, are abrasive. Pollen, mold
and skin cells can be attractive to pests. Particulates are
particularly dangerous because they can attract
moisture and gaseous pollutants.
Three forms of Damage Mechansims
A source for sulfates and nitrates
(These particles readily become
acidic on contact with moisture.)
 · A catalyst for chemical formation
of acids from gases
 · An attractant for moisture and
gaseous pollutants
What are gaseous air pollutants?
 Gaseous
pollutants are
reactive chemicals that can
attack museum objects.
 These pollutants come from
both indoor and outdoor
Outdoor pollutants
Outdoor pollutants are brought indoors through a
structure’s HVAC system or open windows.
There are three main types of outdoor pollution:
 · sulfur dioxide (SO2), and hydrogen sulphide (H2SO)
produced by burning fossil fuels, sulfur bearing coal,
and other organic materials
 · nitrogen oxide (NO) and nitrogen dioxide (NO2),
produced by any kind of combustion, such as car
exhaust as well as deteriorating nitrocellulose film,
negatives, and objects
 · ozone (O3), produced by sunlight reacting with
pollutants in the upper atmosphere and indoors by
electric or light equipment, such as photocopy
machines, printers, some air filtering equipment
 When
sulfur and nitrogen
compounds combine with moisture
and other contaminants in the air,
sulfuric acid or nitric acid is
 This acid then causes
deterioration in a wide variety of
objects. Ozone reacts directly with
the objects causing deterioration.
The main sources of indoor air pollution
· wood, which can release acids
· plywood and particle board, which give off acids from wood and
formaldehyde and acids from glues
· unsealed concrete, which releases minute alkaline particles
· some paints and varnishes, which release organic acids, peroxides,
and organic solvents
· fabrics and carpeting with finishes, such as urea-formaldehyde, and
wool fabrics that release sulfur compounds.
· glues, used to attach carpets, that can release formaldehyde
· plastics that release plasticizers and harmful degradation products
such as phthalates and acids
Museum objects themselves may also
contribute to indoor air pollution.
Examples of sources of pollutants from
museum objects include:
· celluloid and other unstable plastics used to
produce many 20th-century objects
· cellulose nitrate and diacetate plastic, used
for film
· pyroxylin impregnated cloth used for book
· residual fumigants, such as ethylene oxide
Object Materials Deterioration
Primary by Air Pollutants
sulfur oxides
stone surface erosion
and discoloration
Soluble Salts and Deterioration of
Archeological Materials
Porous archeological artifacts such as
ceramics, stone, bone, and ivory often contain
soluble salts. Ground water and seawater can
carry these salts into the pores of the artifact
during burial leaving them behind when the
water evaporates.
After excavation, these salts can crystallize at
or just below the surface of the artifact causing
A variety of descriptive terms are used for this
damage including spalling , flaking, powdering,
and sugaring.
The force of growing crystals can break apart
the surface of bone, stone, ceramics and other
porous materials so that detail is lost.
In bad cases it can remove the entire surface
of an artifact.
In the worst cases, it can destroy an artifact.
Soluble Salts and Insoluble Salts
Conservators divide the salts that are
deposited in and on an artifact during
burial into two groups:
 insoluble salts and soluble salts.
Soluble salts will dissolve in moisture in the air. This
property is known as deliquescence.
The salts can move through the porous structure of an
artifact as moisture is drawn out through evaporation.
As the salts reach the surface of the artifact they may
crystallize as white, often furry growths on the surface.
If the surface is less porous than the underlying
structure they can crystallize just below the surface.
These crystals exert immense pressure and may cause
the surface layer to spall off.
Salt Damage to Porous Materials
Salt damage is largely attributable to two mechanisms:
crystallization of salts from solution
 hydration of salts, that can exist in more than one hydration
The growth of salt crystals within pores can cause stresses,
which are sufficient to overcome the stone's tensile strength
 When the migration of the salt to the surface of the stone is
faster than the rate of drying, the crystals deposit on the top of
the external surface and form visible efflorescences, which do
not damage the stone. When the migration is slower than the
drying rate, the solute crystallizes within the pores, at varying
depth, causing crumbling and powdering of the stone.
Why does salt speed up corrosion?
Water is required for corrosion and salt speeds
up the process.
Corrosion is the transfer of electrons from one
substance to the other so salt present in water
improves the capability of water to carry
electron through redox reactions.
Rusting in metals is the oxidizing of metal to
metal oxide. Water acts as the medium to
transfer the electrons and salt helps the
corrosion process to speed up the process.
Insoluble salts
“Insoluble” salts are not
truly insoluble but will take
days or weeks to dissolve
in water. They are not
deliquescent and so will not
cause further damage after
Insoluble salts can,
however, be quite
disfiguring, and may
require removal for
identification or
reconstruction of an
 Carbonates
 Nitrates
 Sulphides
 Sulfates
 Phosphates
Identifying Salts
In order to identify the salts conservators use analytical
methods such as spot tests or x-ray diffraction.
Soluble salts are visible as a white growth on the
surface of an artifact. In newly excavated material, they
often form first along cracks or abraded areas of a
surface. Often they can look like a white bloom or haze
on the surface. As the crystals continue to grow and
form they will extend further from the surface and
appear as a white powder or even look somewhat like
table salt. They may have a soft, fuzzy feel if touched.
Deterioration of Archaeological
Deterioration of Ceramics, Glass, and
Physical Forces
The agents of deterioration that can have the most profound
effect on ceramics, glass, and stone in museum collections are
direct physical forces.
If ceramic or glass objects are dropped, they usually break.
Most stone will chip, crack, or break if dropped. Cumulative
damage can occur with improper handling––pieces can be
chipped off and residues left from handling. Some ceramic,
glass, and stone objects also have flaws, either inherent or
from their previous use, that make them vulnerable to heat or
Deterioration of Pottery and Ceramics
How ceramics were made?
Ceramic objects are made up of a mixture of
natural materials that are combined, formed
into shape by a variety of processes, and
transformed by heat to create a solid, brittle
substance not found in nature.
Different firing temperatures produce objects
with a vast range of hardness and porosity.
Most clay objects are a mixture of materials:
Clay is a fine-grained mineral--the smallest particles
produced by the weathering of certain rocks.
When heated to a high temperature it chemically and
physically changes to a hard, brittle material.
• Adding fluxes such as soda, mica, potash, magnesia,
or lime lowers the firing temperature of clay.
These fluxes may also be found in natural clay deposits.
Non-plastic additives (temper) are added to clay to
reduce shrinkage and cracking during firing and drying.
Temper also increases porosity in the finished object.
These basic materials are mixed together by the potter
to produce a heterogeneous plastic mass that is then
formed into the ceramic object.
Ceramics are loosely divided into four groups. These groups are
based on
their firing temperature, clay type, and physical characteristics:
• Adobe or mudbrick
is an unfired clay
mixture. This
material is often
used for building, but
mudbrick objects,
such as cuneiform
tablets and sculpture,
are often found in
museum collections.
• Earthenware is a low-fired clay mixture.
These objects are fired between about
950-1100ºC. At this low temperature
sintering occurs but not vitrification.
Earthenware is generally soft and
scratches easily. It is often red in color
from naturally occurring iron in the clay;
brown, black, and yellow are also
common colors.
Earthenware has the following
− It is porous and will readily absorb
water unless glazed.
− The structure is often granular in
appearance with numerous coarse
− There is a clear distinction between the
ceramic body and any glaze
• Stoneware is fired between
1100-1350ºC. Stoneware
objects are partially vitrified.
Common colors for stoneware
are buff, brown, and gray.
Stoneware has the following
− It is partially vitrified and less
porous than earthenware.
− It is harder and denser than
earthenware and does not
scratch easily.
− If tapped lightly, the body will
give a distinctive ring.
− The glaze and body are
tightly adhered.
• Porcelain is fired at very high
temperatures, usually above 1300ºC.
Porcelain is made of a special clay called
kaolin. This clay is difficult to work and
must be fired under precise conditions.
Porcelain can be formed into objects
with thin, complex structures.
Porcelain has the following
− The body is completely vitrified and
impervious to water (nonporous).
− The clay body is white and translucent
and extremely hard and
− When tapped lightly, the object rings
with a higher tone than
− In cross-section, glaze and body are
nearly indistinguishable.
A general rule of thumb is that lower-fired
ceramics will easily absorb water, while higherfired ceramics will absorb little or no water.
 To test this, you can use a small paintbrush to
apply a little water to an unglazed area of
ceramic, and watch to see if it is drawn in.
Because high-fired ceramics are less likely to
absorb water, they have fewer salt problems
Ceramics may have different surface finishes,
coloration, or impressed designs.
A glaze is a thin layer of clear or colored glass
on the ceramic surface. A slip is usually more
like a thin layer of clay and has a matte
appearance and is a different color than the
clay body. Ceramics may be coated with other
materials as well, including paints and inks.
Ceramics are decorated most commonly with a slip or
glaze that is fired on, or melted onto the surface when it
is fired. Ceramics with a fired-on overall glaze or other
decorations are impervious to normal variations in
temperature below several hundred degrees.
These fired-on decorations also help protect ceramics
from humidity.
In recent decades, some ceramics have been initially
fired but later decorated with paint or some other
decoration that is never fired. These unfired decorations
are very fragile and are easily damaged by exposure to
water, heat, or light.
Repaired ceramics may
suffer damage from
temperature and humidity
Broken ceramics
reassembled with adhesive
have weaknesses. Most
adhesives soften and give
way at elevated
temperatures. Ceramic pots
with repairs may sag,
collapse, or fall apart if they
are stored in a hot area,
such as an attic or a
building that does not have
air conditioning.
Salts can damage or
destroy ceramics. The
clay may have originally
contained a significant
amount of salt, and other
types of earth added to
adjust the properties of
the clay may include
Water or foods stored in
ceramic vessels often
leave salts behind.
Contact with seawater or
burial below ground can
also introduce salts.
Fluctuating humidity levels
aggravate the harmful effects
of salts in ceramics.
Above 60 percent relative
humidity, the salts dissolve
and move around inside the
ceramics. When the ceramics
dry, the salts migrate to the
surface and are left behind
when the water evaporates.
This is called salt
efflorescence. Efflorescence
generates tremendous forces,
pushing off areas of glaze or
decoration and even breaking
up entire ceramics.
Generally, mold will not
grow on ceramics, and
insects will not attack them.
 In very wet conditions,
however, mold or lichens
may grow on ceramic
surfaces, although the mold
will not digest the ceramic
itself. Insects will eat food
residues left on ceramics
and will eat materials
applied after firing.
However, proper
environmental conditions
prevent mold, lichens, and
Light is not harmful to
ceramics as such, but
pigments used in
surface decoration could
be damaged by over
Some old repair methods
have caused damage in the
long term. Very strong
adhesives were used in the
past, but in ageing they
have been found to
discolour and shrink, and in
shrinking a layer of the
ceramic can be pulled away
from the body of the pot.
 Today's conservators have
a wide range of adhesives
from which to choose.
Those used with ceramics
will usually be weaker than
the ceramic body to prevent
too strong a join from
causing further damage.
What flaws might I find in ceramic objects?
It is important to recognize the flaws
that may occur during the
manufacturing process so you can
separate flaws from damage or active
Deterioration of glass
The Nature of Glass Objects
Glass has been used for personal
adornment, containers, construction
materials, and a host of other
purposes throughout the last four
 In order to understand how to
preserve glass objects, you must
understand how they are produced.
What materials make up the
structure of glass objects?
The basic materials of glass are silica and
alkaline oxide (also known as flux).
Silica generally comes from sand or crushed
flint. The flux interacts with the silica and
lowers the melting temperature.
Typical fluxes include lead, calcium,
potassium, and sodium oxides.
Other oxides (iron, copper, cobalt, manganese,
chromium and nickel) are added as colorants.
When melted, this mix of materials flows
readily to form various shapes.
Glass component Relative amount (%)
Silica (glass former) 60–75
Soda (modifier) 30–15
Lime (stabilizer) 15– 8
Iron oxides (generally added
unintentionally) 10–less than 1
The basic process for making glass, although not the actual
technology, has changed little since antiquity. Over the
centuries the technology has advanced, being continuously
improved and refined beyond recognition
Six main manufacturing stages are involved in the
glassmaking process:
1. Selecting the raw materials
2. Comminuting and mixing the raw materials
3. Heating and melting the mixture
4. Fabricating, that is, forming and shaping objects
5. Annealing the objects
6. Finishing
Glass is a unique material––a rigid liquid.
A liquid is an amorphous material that does not
have an organized, crystalline structure. Most
materials, such as metals, form a crystalline lattice
as they cool from a liquid to a solid state.
Molten glass, however, cools too quickly for
this structure to form. The structure is "frozen"
into a random network of molecules.
Glass is rigid and brittle at room temperature.
Depending on the materials included in the
mix, it can be transparent, translucent, or
Lattice VS Amorphous
Glazes and Enamels
Glazes and enamels are
also glasses with small
differences in
composition from bulk
Glazes are applied to
ceramics; enamels are
usually applied to a
metal support.
Glazes and enamels are
generally opaque and
fired at lower
temperatures than glass.
What flaws might I find in glass objects?
 Flaws
can be introduced during the
manufacturing process. Learn to
distinguish these flaws from active
deterioration problems. Look for:
They may also be added
intentionally for
decorative effect. A few
isolated bubbles will not
weaken a glass object,
however, a cluster of
bubbles might. The
shape of the bubbles
gives clues to the
direction that the object
was worked in the
molten state.
Inclusions or foreign
bodies: These are more
noticeable in translucent
glass. Often these flecks
come from
contamination in the
crucible or impurities in
the raw materials. Small
inclusions may disrupt
the surface and look of
an object, but they will
not affect its strength.
Compositional flaws
flaws: Sometimes
these are not
apparent for many
How does glass deteriorate?
Most damage to glass is
mechanical. It is easily
broken and chipped.
Water is the major
chemical agent of
deterioration for glass
and the susceptibility of
glass to deterioration
depends greatly on its
original chemical
Glass, a supercooled liquid, is in a metastable
state, that is, an apparently stable condition
that may be perturbed by external conditions
and undergo unpredictable changes, so that
the supercooled liquid may be converted to a
When glass is made from a well-balanced
mixture of former, modifier, and stabilizer, it is
remarkably stable.
changes may,
however, cause
the glass to
crystallize, or, as
the condition is
known, to
devitrify, that is,
to lose its
vitreous (glassy)
Glass exposed to the environment or buried in
the soil under dry conditions, even for long
periods of time, is usually stable and
undergoes very little devitrification or decay.
The more humid the environment or burial
site, the more easily glass decays and the
more extensively it devitrifies.
Extended periods of alternating dry and wet
conditions may result in periodic decay effects
and the formation of devitrified layers, first on
the outer faces and then throughout the bulk of
the glass
The chemical decay of
glass often starts when
its alkaline components,
soda, potash, or lime,
are leached by water
from the surrounding of
the glass (leaching is the
process of extraction of
the soluble components
of a solid by their
dissolution, usually in
water but also in mild
Decay of Glass
The tendency to, and the extent of decay of
glass are determined mainly by its
composition, the environmental temperature
and humidity, and/or the surrounding water
conditions at the location sit of the glass. Salts
in, and the pH of groundwater, and even
microorganisms with which glass is in contact,
alter its rate of decay.
Stages of Glass Decay
Various stages in the
decay of glass have
been defined:
dulling, which entails the
loss of clarity and
transparency, is the
simplest; frosting, the
formation of a network of
small cracks on the
surface follows;
Stages of Glass Decay
Strain cracking, the
occurrence of small
cracks running in all
directions, is a more
advanced form of
decay that may
result in the partial
or total
disintegration of
Stages of Glass Decay
Frosting and strain
Take place particularly
when water is abundant:
the water leaches from the
glass most of the soda and
potash and part of the lime,
leaving behind only thin
layers of hydrated silica.
Stages of Glass Decay
The final stages of such
a decay process may
result in the glass
becoming just a residue
of generally separate,
flaky, highly porous,
layers of hydrated silica
displaying a sugarlike
appearance that
eventually totally
disintegrates .
Crusts of weathered glass. The
deterioration of glass and the formation
of weathered layers under humid or wet
conditions is a rather complex process.
Seasonal variations of temperature and
of the amount of environmental
moisture may provide the trigger that
initiates the glass weathering process.
Still, the final product of the process are
opaque layers composed mainly of
hydrated silica (silica is the main
component of glass).
Partial leaching and
, Although basically damaging, often enhance the appearance of old glass; they
give origin to iridescence, the display of rainbow-like variegated colors when
illuminated old glass is moved or turned. Iridescence generally occurs on ancient
glass that has been recurrently exposed to seasonal variations, in some climates
in yearly cycles, of temperature and humidity.
Such cyclic processes result in the formation, on the outer, exposed faces of the
glass, of very thin layers of decay products (composed mainly of hydrated
silica); the thickness of single layers has been measured and found to vary in the
range 0.3–15 microns. Light incident on these layers causes interference
between beams reflected from their front and back surfaces and gives rise to the
variety of colors often seen on ancient glass objects
Crizzling Glass
• Crizzling is a fine
network of surface
cracks that turn glass
Moisture in the air reacts
with unstable glass
containing too little lime
(calcium oxide). The
moisture causes
potassium and sodium
in the glass structure to
leach out. As the
structure weakens, small
cracks appear.
Weeping Glass
• Weeping is caused by
leaching sodium or potassium
absorbing water on the surface
of deteriorating glass to form
sodium or potassium hydroxide.
These compounds accumulate on
the surface of the glass and may
give it a greasy feeling. The
hydroxides may also react with
carbon dioxide in the atmosphere
to form carbonates, which can
absorb even more water.
Crusty or waxy deposits
on the surface, which
may have a white
crystalline appearance, are
typically seen on
ethnographic beadwork
and may be a reaction of
the glass deterioration
products to oils in adjacent
• Iridescence is a
rainbow-like effect on the
glass surface and is an
indication of
deterioration. The colors
are visible when light is
diffracted between the
air-filled layers of
deteriorated glass.
• Devitrification is the
production of small areas of
crystal growth in the otherwise
amorphous glass structure.
These crystals may be
intentionally produced during
production as they give glass
good thermal shock resistance.
Unintentional devitrification is
caused by unstable glass with
too much alumina or too much
The factors considered to be among the
leading causes of building stone deterioration
salt crystallization
aqueous dissolution
frost damage,
microbiological growth
human contact
original construction
Salt Crystallization
Crystallization of salts
within the pores of
stones can generate
sufficient stresses to
cause the cracking of
stone, often into
powder fragments. This
process is considered
to be the major cause
of stone deterioration
Closely related to the
crystallization of salt is
damage caused by salt
hydration and by
differential thermal
expansion of salts
The resistance of stone to salt
damage is dependent on the
pore size distribution and
decreases as the proportion of
fine pores increases
 Crystallization damage caused
by highly soluble salts, such as
sodium chloride and sodium
sulfate, is usually manifested
by powdering and crumbling of
the stone's surface
Less soluble salts such as
calcium sulfate form glassy,
adherent films which cause
spalling of a stone's surface
A major source of salts in
urban environments is the
reaction between air
pollutants and stone. For
example, limestone can
react with sulfur dioxide to
ultimately produce calcium
sulfate. Other sources of
salts include ground water
airborne salts sea spray and
chemical cleaners
Aqueous Dissolution (Pollution)
Carbonate sedimentary
stones e.g., limestone),
sandstone, and marbles are
types of stone that are
susceptible to dissolution
by water acidified with
dissolved carbon dioxide,
sulfur dioxide, and nitrogen
oxides problem
It has been reported that the
rainwaters in many urban
areas in the United States
and Europe are sufficiently
acidic to accelerate the
weathering of exposed
building stone.
In areas where the
rainwater is relatively free
from pollutants, the
dissolution of most
common building stones is
usually not a serious
Frost Damage
Certain stones which
are exposed to freezing
temperatures and wet
conditions may
undergo frost damage.
The frost susceptibility
of a stone is largely
controlled by its
porosity and pore size
Of stones with a given
porosity, those with the
smallest mean pore size will
generally be the most
susceptible to frost
damage. Frost resistance
also generally decreases
with increased available
porosity pore volume which
is accessible to water.
Some European stone
conservators believe that in their
countries frost damage is not an
important process in the
deterioration of stone. They regard
frost damage as a secondary
process, e.g., frost damage may be
responsible for the final
fragmentation of stone damaged
by other processes, such as salt
crystallization. However, because
of the use of possibly more frostsusceptible stone and more severe
climates, frost damage may be an
important factor in the southern
and eastern part of Jordan.
Microbiological Growth
The attack of stone by a
variety of plants and
animals has been reported
including roots of plants,
ivy vines, microorganisms,
boring animals, and birds.
Of these, microorganisms
appear to be the most
Some types of bacteria, fungi,
algae, and lichens produce
acids and other chemicals
which can attack carbonate
and silicate minerals
 . It appears that under certain
environmental conditions
attack by microorganisms can
be a serious problem [].
However, it seems that many
conservators feel that such
instances are uncommon and
that microorganism growth
usually takes place in stone
which had been partially
deteriorated by other
Human Contact
Because of an increasing interest
by the public in historic structures,
the effects of human contact upon
the condition of stone, as well as
all other building materials, is of
growing concern.
For example, stone floors are
gradually worn by foot traffic,
stones are damaged by people
either collecting souvenirs or
poking into soft stone , and graffiti
removal has become an important
maintenance problem . It is
conceivable that human contact
may become a major problem
challenging the ingenuity of both
stone conservators and
maintenance specialists.
Tourism and Urban development
Original Construction
The durability of stone structures also depends on factors
encountered during their original construction including
proper design, good construction practices, and proper
selection of materials. Unfortunately, these are factors over
which the preservation scientist has no control.
However, the same mistakes should not be repeated in
repairing or restoring historic structures. For example, normal
steel and cast iron anchors, dowels, reinforcing rods, etc.,
were often used in the construction or repair of stone
structures. Certain ferrous metals are susceptible to corrosion
which can lead to the cracking and spalling of stonework [].
Therefore, noncorroding material should be selected, e.g.,
epoxy-coated steel ], certain types of stainless steel ], or noncorroding non-ferrous alloys .
A large portion of stone durability problems are the consequence of
using poor quality stone in the original construction.
Corrosion of Metals
3.1-What happens to ancient metal as it
Metals in nature, the way they are found in the ground,
are generally fairly stable. Malachite, the gemstone, is
for instance, a stable form of copper found in nature. It
has naturally combined with things in the environment
to create a substance that looks almost nothing like the
metal copper, yet it is made of more than 70% copper
and can be refined to create metallic copper. When the
metal copper, which is not stable, is returned to the
earth, it will unrefine itself, slowly, recombining naturally
with elements in the soil, and the result, within a few
hundred years will be a layer of malachite and other
related minerals on the surface of the metal. This is the
type of deterioration known as verdigris.
3.1-What happens to ancient metal as it
Another example of this is the black
or gray tarnish that you see on silver
items. This is silver combining with
sulfur in the environment, and copper
alloyed into the silver, combining
with oxygen, both returning to a
stable natural condition, and at the
same time, becoming less attractive
and useful.
3.1-What happens to ancient metal as it
So, to clarify, metal ores are, through heat,
refined and purified into pure metals that must
eventually, at normal temperatures, combine
with elements in their environment and return
to their more stable natural states. This
process can take hundreds or even thousands
of years, and is what we know as patination,
verdigris, corrosion, and the other properties
of aged metal.
3.1-What happens to ancient metal as it
The second important thing that happens as metals age
is that, those which are alloyed, or made of a
combination of two or more metals, may separate slowly
into their components. An example of this is ancient
silver coins which become brittle. Silver used in coins is
almost always a combination of silver with about 1.5 to
15% copper. Adding a little bit of copper to silverlowers
it's melting point and makes the normally soft silver
harder, and more resistant to wearing down. Silver and
copper don't really mix all that well, however, and over
time (300-500 years or more), at normal temperatures,
the copper will sometimes begin to separate itself from
the silver.
3.1-What happens to ancient metal as it
The technical name for this is the precipitation
of copper at the grain boundaries, which
means copper coming out of the alloy at the
edges of the natural crystals of the metal. This
is known as crystallization of the metal, to coin
collectors, all though it is really just the
crystals of the metal becoming visible as the
copper comes out of the alloy and begins to
corrode, thus weakening the metal.
To clarify this point, some alloys are not stable,
and, over hundreds or thousands of years,
they will begin to separate back into more
stable natural states.
3.2-Corrosion of metals
The overall driving forces of nature
work to return metals to their stable
oxidised states, that is, combined
with oxygen, sulphates, carbonates,
sulphides and chlorides. Unoxidised
or native metallic element is
produced when metals are unbound
from their compounds with oxygen,
sulphate, carbonate, sulphide and
3.2-Corrosion of metals
For this to happen there must be a
sufficient driving force available
through a high energy intervention.
This intervention can be a carbon
reduction or smelting. When metal
ores are processed to produce
metals, they start to corrode.
3.3-A simple overview of corrosion
The corrosion of metals consists of
two separate reactions:
an oxidation reaction; and a reduction
To explain these reactions, it is necessary to give a
simple overview of the structure of atoms. Atoms are
made up of a nucleus which contains neutral particles
called neutrons and positively charged particles called
protons. Electrons, which are negatively charged
particles, orbit around the nucleus of the atom. The
number and activity of the electrons will determine how
readily the atoms will react with other atoms. Many
metals, because of the way their molecules are
structured, can readily lose electrons. When they do
this, they are no longer atoms. They are positively
charged and are called ions. Because of the charge, ions
are not stable and combine readily to achieve a stable,
electrically neutral state.
An oxidation reaction is one in which an atom
loses electrons. This can be represented very
simply by the equation:
M-------Mn+ +newhere 'n' represents the number of electrons lost
For example, copper—Cu—can be put into this
equation. In an oxidation reaction:
 Cu -------- Cu++ eIt can be oxidised further:
 Cu+---------- Cu2+ + e
Copper is described as polyvalent, that is, it
has different combining powers: a Cu+ ion
needs one negative ion to achieve a stable
state, while a Cu2+ ion needs two negative
ions to form neutral compounds.
Once these ions combine with other
substances, they produce cuprous and cupric
compounds respectively. For example, Cu2O is
cuprous oxide or copper (I) oxide and CuO is
cupric oxide or copper (II) oxide.
These electrolytic reactions are used to
produce solid metals from their ionic
solutions. The negative ions can be supplied
by a range of materials. For example, if the
metal object is in a seaside location, chloride
ions—Cl-—will combine readily with the metal
 They will also combine with:
NO32-—from atmospheric pollutants; and
If the metal combines with oxygen, it
forms a metal oxide on the surface of
the metal. If this metal oxide is
continuous, then the overall
corrosion rate of the underlying metal
will slow down and it will become
passivated or protected.
3.4-Corrosion cells
Corrosion cells are small areas on
metal objects where electrical
differences are set up. Electrons flow
between the charged areas, just as an
electrical current flows between the
positively and negatively charged
electrodes of a battery.
A corrosion cell is an electrochemical
cell which acts very much like a
battery. The corrosion of metals
consists of two separate reactions:
oxidation. The oxidation reactions are
called anodic reactions; and
reduction. The reduction reactions
are called cathodic reactions.
In an electrochemical cell the anodic,
oxidation, half of the cell produces electrons
as the metal is oxidized, while at the cathodic
half of the cell, reduction occurs. The electrons
are taken and held by the oxidising agent,
which in aerated environments is oxygen.
In a corrosion cell, these reactions can
continue in a cycle. The localised corrosion
activity causes pitting in the metal.
The rate at which the electrons move out of the
metal and across into the oxygen molecules is
the principal factor controlling the overall
corrosion rate.
3.5-Fats, oils and sweat
Organic acids—formed by the oxidation of oils
and fats—are capable of attacking metals
which rely on a protective oxide coating to
produce a good corrosion resistance. To
prevent this type of damage, avoid direct
contact between the object and the source of
the organic material. Some examples of this
type of damage are leather objects with copper
fittings. The gradual deterioration of old candle
wax in leather-lubricating oils leads to organic
acids penetrating the protective copper oxide
film, and reacting with the underlying metal—
to form outgrowths of bright green organic
copper compounds.
Human sweat on metal objects causes
corrosion. Bacterial reactions with sweat can
produce sulphides as metabolic by-products,
and convert inherently inert sulphate ions into
reactive sulphide ions.
Uneven coatings of oil—from sweaty hands for
instance—can alter the ease of access of
oxygen to metal surfaces. This has two major
effects. It hinders the formation of passivating
layers of corrosion. It also alters the relative
reactivities of areas of the metals; and so it
causes one part of the metal to corrode at the
expense of another.
Inorganic acids such as hydrochloric
acid—derived from the decay of
plastics like polyvinyl chloride—and
nitric and sulphuric acids—derived
from air pollution—will attack metals
which are either in the same storage
environment as the plastic or in the
open air.
Anything that prevents direct contact
between the metal surface and acidic
solutions helps to prolong the life of
the object. Therefore, vapour phase
inhibitors, lacquers, waxes and other
coatings minimize the damage from
air pollution. The filtering of external
air also greatly helps to minimize
corrosion damage.
Normally unreactive metals such as copper
and silver can suffer significant corrosion in
the presence of sulphide ions. Common
sources of sulphide ions are:
hydrogen sulphide—H2S—from the anaerobic
decay of plant material; and
carbonyl sulphide—COS—from the
degradation of sulphur-containing proteins,
such as those found in wool.
3.7-Forms of corrosion
There are eight forms
of corrosion (based on
visual characteristics.
These are:
1) Uniform corrosion:
most common form of
characterized by a
reaction over the entire
exposed surface;
Galvanic Corrosion
2) Galvanic
corrosion or twometal corrosion:
driving force for
current flow and
metal corrosion is
the potential
developed between
the two metals;
Crevice Corrosion
3) Crevice
corrosion: intense
localized corrosion
that occurs
frequently within
crevices on metal
surfaces exposed
to corrosives;
Pit corrosion
4) Pit corrosion:
localized attack that
results in holes in
the metal. This is
one of the most
destructive forms of
Intergranular corrosion
5) Intergranular
corrosion: localized
attack at and adjacent
to grain boundaries,
with relatively little
corrosion of the grains.
It can be caused by
impurities at the grain
boundaries causing the
alloy to disintegrate
and/or lose its
Selective leaching
6) Selective
leaching: removal
of one element from
a solid alloy by
Errosion corrosion
7) Erosion
acceleration or
increase in rate of
deterioration or
attack on a metal
because of relative
movement between
a corrosive fluid
and the metal
Stress corrosion
8) Stress corrosion:
caused by the
presence of tensile
stress and a
specific corrosive
Recent studies have expanded the
corrosion categories and redefined
them by mechanisms rather then by
visual appearance. Overlap between
the mechanisms may exist. Factors
affecting Corrosion can be
accelerated by differential
temperature cells or by the presence
of mechanical forces in conjunction
with chemical forces.
There are six major factors that affect the rate of
corrosion of alloys in an aqueous environment:
1) acidity;
 2) presence or absence of oxidizing agents;
 3) presence or absence of films on the alloy;
 4) temperature;
 5) velocity of moving aqueous solution;
 6) heterogeneity both in the solution and in the
3.8-Effect of environment
on metal corrosion:
3.8.1-Atmospheric Environments
Specific factors influencing the corrosivity of
atmospheres are dust content, gases in the
atmosphere, and moisture (critical humidity).
Atmospheres are often classified as rural,
industrial, or marine in nature, but this is an
over simplification. There are locations along
the seacoast that have heavy industrial
pollution in the atmosphere and so are both
marine and industrial. Two decidedly rural
environments can differ widely in average
yearly rainfall and temperature and therefore
can have considerably different corrosive
Atmospheric corrosion
Industrial expansion into formerly rural areas can easily
change the aggressiveness of a particular location.
Finally, long-term trends in the environment, such as
changes in rainfall pattern, mean temperature, and
acidity of the rainfall, can make extrapolations from past
behavior much less reliable. Other factors that limit the
usefulness of atmospheric exposure data are the
general nonlinearity of weight loss due to corrosion over
time and the fact that most atmospheric corrosion data
are presented as an average over the entire test panel
surface. Most atmospheric exposure data for steels
show a decrease in the rate of attack with duration of
exposure so that extrapolations of such data to times
longer than those covered by the exposure data can
lead to significant errors.
Atmospheric corrosion proceeds in a relatively
complicated system consisting of surface
electrolyte, atmosphere, metal, and corrosion
products. Analyses of the corrosion products
give the following general characteristics.
Nearly all nitrates and acetates are soluble and
these anions are not found in corrosion films.
An exception to this is with copper where
basic nitrates have been detected. Simple
chlorides and sulfates are soluble and
generally are not found in corrosion films.
All hydroxides are insoluble as are many
mixed salts that include hydroxide as one of
the constituents. Both hydroxides and mixed
salts are common corrosion products. Normal
and hydroxy carbonates are common
constituents of corrosion films. Atmospheric
corrosion is an electrochemical process with
the electrolyte being a thin layer of moisture on
the metal surface. The composition of the
electrolyte depends primarily on the deposition
rates of the air pollutants and varies with the
wetting conditions.
Environmental factors can cause the median
thickness loss to vary by as much as 50% or
more in a few extreme cases. Those
environmental factors that tend to accelerate
metal loss include high humidity, high
temperature (either ambient or due to solar
radiation), proximity to the ocean, extended
periods of wetness, and the presence of
pollutants in the atmosphere, such as sulfur
oxides (SOx), nitrogen oxides (NOx), hydrogen
sulfide, ammonia, and carbonyl sulfide (COS).
The most important corrosive constituent of
industrial atmospheres is sulfur dioxide, which
originates predominately from the burning of
coal, oil, and gasoline. The small amount of
carbon dioxide normally present in the air,
neither initiates nor accelerates corrosion.
 Metallurgical factors can also affect metal loss.
Within a given alloy family, those with a higher
alloy content tend to corrode at a lower rate.
Surface finish also plays a role in that a highly
polished metal will corrode slower than one
with a rougher surface.
The atmospheric contaminants most often
responsible for the rusting of structural
stainless steels are chlorides and metallic iron
dust. Chloride contamination may originate
from the calcium chloride used to make
concrete, from exposure in marine or industrial
locations, or from the use of road salts. Rural
atmospheres, uncontaminated by industrial
fumes or coastal salt, are extremely mild in
terms of corrosivity for stainless steel, even in
areas of high humidity. Industrial or marine
environments can be considerably more
Copper and copper alloys are suitable for
atmospheric exposure. They resist corrosion
by industrial, marine, and rural atmospheres,
except atmospheres containing ammonia,
sulfur dioxide, or oxides of nitrogen. The
severity of the corrosion attack in marine
atmospheres is somewhat less than that in
industrial atmospheres, but greater than that in
rural atmospheres. However, these rates
decrease with time due mainly to the formation
of a protective film (e.g., copper chloride or
copper sulfate) that develops on the surface of
the copper or copper alloy.
3.8.2-Soil and Groundwater
Soils are defined as unconsolidated rock
material over bedrock and/or freely divided
rock-derived material containing a mixture of
organic matter and capable of supporting
vegetation. Worldwide, corrosion of metals in
soil is responsible for a large percentage of
corrosion and corrosion failures. While
several individual characteristics of soils have
been used to indicate the corrosivity of soils,
currently no method describes the synergistic
effects of these characteristics.
In particular, the corrosivity of soil is based
largely upon the interaction of electrical
resistivity, dissolved salts, moisture content,
total acidity, bacterial activity, and
concentration of oxygen. Other secondary
factors are also important but are more difficult
to define. Thus, simply testing metals and
alloys in a variable pH solution or in aerated or
deaerated solutions will not accurately
describe the conditions in soil. In addition, soil
environments are generally stationary
electrolyte exposure conditions. Therefore,
depleting and/or concentrating effects can
occur at the surface of alloys.
Soil corrosion
Factors controlling soil
However, some factors associated with the soil
environment which can have an impact on the
corrosion rates of metal alloys include:
soil texture, internal drainage, resistivity,
redox potential, moisture content, permeability,
chloride ion content, sulfide and sulfate ion
content, presence of corrosion-activating
bacteria, oxygen content, pH, total hardness
and hardness as calcium carbonate of soil
moisture, and stray direct currents (dc).
Soil texture
Soil texture is determined by the proportions
of sand, silt, and clay that make up a soil. Clay,
having the finest particle size and minimum
pore volume between particles, tends to
reduce the movement of air and water and can
develop conditions of poor aeration when wet.
Sand has the largest particle size and
promotes increased aeration and distribution
of moisture. Soil texture thus has as important
influence on the diffusivity of soluble salts and
Internal drainage
Internal drainage is that property of soil that
describes the water retention properties of a
soil and is related to soil texture. Internal
drainage is also affected by the height of the
water table. Thus, a sandy soil which would
normally have good permeation to moisture is
considered to have poor internal drainage if
the water table is high and keeps the soil in a
saturated condition.
Soil resistivity
Soil resistivity is a measure of how easily a soil
will allow an electric current to flow through it.
This is also a measure of how effective the soil
is as an electrolyte. The lower the resistivity of
a soil, the better it will behave as an electrolyte
and the more likely it is to promote corrosion.
A soil with a resistivity below 500 ohm-cm is
considered to be corrosive. Above 2000 ohmcm the relation of soil resistivity to soil
corrosivity is less reliable.
The temperature of the soil is an important
factor in the corrosion process. The resistivity
of soil is inversely proportional to temperature
and therefore an increase in soil temperature
would be expected to increase the rate of the
corrosion reaction. However, an increase in
temperature also reduces the solubility of
oxygen, which tends to reduce the rate of
reaction. The net result is that soil
temperature does not have as large an effect
on underground corrosion as would be
Soil pH is the acidity or alkalinity of the soil
media. Most soils and all loams are fairly well
buffered, resulting in a soil pH that is not
affected by rainfall. Sand, because of its high
moisture diffusivity, can have its soluble salts
leached out or diluted to the point that its pH
will change during a heavy rain. This can
cause the corrosion rate to either increase or
Redox potential
The redox potential or oxidationreduction potential of a soil gives an
indication of the proportions of
oxidized and reduced species in that
soil. Very high corrosion rates can
occur in poorly aerated (reducing)
soils where anerobic bacteria often
The presence of increasing concentrations of
chloride ions lowers the resistivity of soil and
water and will cause an increase in the
corrosion rate. The presence of sulfides and
sulfates is often an indicator of sulfate
reducing bacteria (SRB’s). These bacteria can
shift the pH in the acidic direction, causing an
increase in corrosion. The higher the water
hardness, i.e., the higher the concentration of
calcium carbonate in the soil, the lower the
corrosion rate will be.
3.8.3- Seawater or Marine Environments
Seawater is a
biologically active
medium that contains a
large number of
microscopic and
organisms. Many of
these organisms are
commonly observed in
association with solid
surfaces in seawater,
where they form
biofouling films.
Immersion of any solid surface in seawater
initiates a continuous and dynamic process,
beginning with adsorption of nonliving,
dissolved organic material and continuing
through the formation of bacterial and algae
slime films and the settlement and growth of
various macroscopic plants and animals. This
process, by which the surfaces of all structural
materials immersed in seawater become
colonized, adds to the variability of the ocean
environment in which corrosion occurs.
The amount of oxygen and other gases dissolved in
seawater depends on the temperature and the salinity of
the seawater and the depth. In some seawater
compositions, hydrogen sulfide is also present.
Hydrogen sulfide is formed in seawater by the action of
sulfate-reducing bacteria (SRB), usually under deposits
where oxygen is depleted or when the seawater is
stagnant or polluted and becomes anaerobic, even in
large volumes. Silt deposits in estuarial waters are also
contributory. Mineral and organic materials are also
carried in suspension by the seawater, particularly near
the mouths of rivers.
Since seawater is a complex, delicately
balanced solution of many salts containing
living matter, suspended silt, dissolved gases,
and decaying organic material, the individual
effect of each of the factors affecting the
corrosion behavior is not readily separated.
Because of the interrelation between many of
the variables in the seawater environment, an
alteration in one variable may affect the
relative magnitude of the other variables. The
factors which effect the amount or rate of
corrosion may be divided into chemical,
physical, and biological.