WEATHERING Weathering is a complex interaction of
physical, chemical and biological processes that
alters the stone in some general or specific way.
The physical properties of stone differs
widely between stone groups and even within
the same stone type. The mineral composition, textural differences, varying degrees of
hardness and pore/capillary structure are the main reasons why stone nor all the surface
of the same stone shows signs of alteration the same and evenly.
These minerals can be broken down, dissolved
or converted to new minerals by a variety
of processes which are grouped as Mechanical and Chemical.
MECHANICAL PROCESSES : This includes the processes of...frost
action...thermal
expansion...wetting and drying...salt decay.
Frost Action : Frost action or commonly called freeze/thaw cycles occur when water
within the pore structure or cracks freezes to ice. It has been estimated
when water freezes it expands between 8 to 11 percent, with a force of
2,000 pounds per square inch to 150 tons per square foot. This increase
of internal pressure combined with repeated freeze/thaw cycles produces
micro-fissures, cracks, flaking and spalling.
Thermal Expansion : Different minerals expand and
contract at different rates, this is
known as the coefficient of
heat expansion and contraction.
It is known that the temperature of stone can vary between 30
and 50 percent higher than the average air temperature. Some
of the darker minerals, absorb heat more readily, and also give
it up more quickly than some of the lighter ones. The daily and
seasonal heating and cooling of stone can cause stresses and
micro-fractures in and along mineral grains.Water in the pores
makes thermal stressing more effective. This can eventually
produce surface flaking. Marble is particularly affected by this.
Wetting & Drying : Coefficients of expansion and
contraction are also involved in
volume change. Stone expands when it absorbs water and
shrinks as it dries. This expanding and contracting produces
internal stresses at the grain boundaries. When the stone heats
up a "baking
effect" occurs, which
will eventually lead to
surface flaking. Marble is affected by this "baking effect."
Salt Decay : Salts are some of the most damaging
agents to stone. Salt manifest
themselves in a process commonly referred to as Efflorescence. There
are several different types. The most common form of efflorescence is the
appearance of salts at the surface in the form of whitish to gray powdery
fluffy blooms. This occurs when the stone, substrate or other sources of
soluble salts are in contact with moisture and move to the surface by
capillary action. As the moisture moves to the surface these soluble salts
are deposited at the surface to recrystallize into these blooms. This form
is generally harmless. Subflorescence is similar to efflorescence, however
instead of the salts being transported to the surface they crystallize and
buildup
within the pore/capillary structure beneath the surface. As the salts
accumulate
internal pressures develop generating spalling and flaking and
may eventually lead to deep deterioration of the stone. Numerous varieties
of salts have been identified in the efflorescence process. Some of these
varieties by themselves or when combined with others will form a "hard
and glassy skin" adhering
rather strongly to the surface. This type is
Crystalline efflorescence.
These mechanical processes generally lead to a weakening of
the stone, increasing its
permeability providing greater penetration of water and increasing the areas for the
chemical weathering processes
to take place.
CHEMICAL PROCESSES : Involves complex chemical reactions that alter the
internal
structure
of minerals by removing and or adding elements
through...dissolution...oxidation...hydration...hydrolysis.
Water, in all of its occurrences (rain, fog,
raising ground moisture, ocean, etc.) and its
chemical composition is the most important element in this process. It acts as a solvent
and also as a chemical reactant. Carbon dioxide (CO2) and oxygen are also important
agents along with climatic conditions.
Dissolution : This is a process whereby some minerals or part of
the chemical
composition of a mineral can be dissolved in water. When an acidic
element is added to the water it increases its ability to dissolve minerals
more readily, particularly those containing calcium, magnesium, sodium
and potassium. The atmosphere is a mixture of gases and one of these
natural occurring gases is carbon dioxide, CO2. CO2 is also produced by
the combustion of fuel and released into the atmosphere. When CO2 gas is
dissolved in water it is converted to carbonic acid, and a carbonated
solution
is formed. Just a small amount of acid increases the corrosive
effects
of water. Limestone and marble are particularly subject to this
process.
In exterior installations, they can develop a petted surface and if
they
have a polished
finish they will lose it. Sandstone that contains
calcite is also affected. The calcite can be
leached from the stone and may
result in a flaking or spalling of the surface and in some incidences may
cause the stone to crumble.
Rain is naturally acidic. This is the primary reason why
polished marble
and limestone in a exterior installation will not hold a polished finish.
Oxidation : Oxidation, also referred to as "rusting" occurs when oxygen assisted by
moisture combines with iron-bearing minerals. Oxidation is accelerated by
moisture and high temperatures. It is an important process in the alteration
of iron and magnesium rich minerals. Mineralogically iron occurs in three
states...metallic...ferrous (Iron II)...ferric
(Iron III). During the oxidation
process Iron II is converted to Iron III resulting in color changes and a
weakening of the mineral structure. Ferrousmagnesian silicate minerals that
undergo oxidation are responsible for the appearance of rust spots on some
granite-type stones. When marble or limestone contains sulfide minerals
such as pyrite, and it undergoes oxidation, the Iron II in this composition will
produce rust spots and the sulfur is converted to sulfuric acid which can
dissolve the calcium producing pitting. Iron
oxide minerals are common in
sandstone and over time may develop rust spots.During the oxidation
process an increase in volume of the mineral structure can occur, usually
making it softer and weaker and rendering it more vulnerable to other
elements of the chemical weathering processes.
Hydration : Hydration is the chemical addition of water molecules to a mineral. This
process frequently produce a new mineral compound. In addition, the size
of the mineral structure is increased causing stress and developing zones
of weakness. This can increase
the overall permeability of the stone
and lead to spalling, pitting
and flaking. Overall color
changes can take
place as well.
Hydrolysis : This is the dissolution and alteration process that affects silicate minerals in
granite-type stones. As with the dissolution process
the addition of a acidic
ingredient is an important element in this process. Feldspars are an important
silicate mineral group in granite and granite-type stones, with their general
formulas containing
potassium, sodium and calcium.
These soluble
elements
are leached out during
the chemical reaction with acidicly charged waters and
carried away. As the process proceeds permeability increases exposing
more mineral surface to the process, the mineral grains will eventually weaken
leading to spalling and
crumbling. Once this process
has started it is irreversible.
RATES OF WEATHERING
Intensity and duration are two key elements that govern to what extent
weathering
reactions will have on stone.
When the presence of pollutants like
nitrogen and sulfur-bearing gases are in the
atmosphere and combined with water (rain and fog) they form powerful acids.
Acid rain and fog have accelerated the weathering processes to several
times that in
unpolluted areas. The pH
of rain is naturally acidic, with a pH of approximately 5.6.
This is the result of the natural occurrence of carbon dioxide in the atmosphere being
dissolved in atmospheric moisture to form carbonic acid.
Combine this natural occurring
process with the addition of sulfur
dioxide and nitrogen oxides
which are transformed
into sulfuric and nitric
acids when combined with
water, and its corrosive
forces have
been dramatically intensified. The average pH of acid rain is about 4.4, ten times
stronger than normal rain. It also has been estimated that about
half of these two
pollutants are not mixed with atmospheric moisture and settle back to the earth as
aerosols, (dry gases
and dry particles). These acidic aerosols when
combined with
the next rain produce an even stronger corrosive solution.
The concentration (intensity) of acidic solutions have a direct effect on the rate at
which the weathering processes occurs. In addition to the intensity factor, is the
duration or length of time and frequency that a process is active. Prolonged and
frequent exposure accelerates the rates of weathering.
Several other factors influence the type and rates of
weathering, alterations and decay
of stone. The most important of these are...composition of the stone...physical
condition of the stone & environmental conditions.
Composition of the Stone : Its mineral and chemical composition
are important factors
as to the extent to which the stone will be affected and the
type of effects it may display. Generally...
Granite-type stones are more resistant to the mechanical
processes with the exception
of salt decay and more
susceptible to the chemical processes of hydrolysis and in
some cases oxidation.
Limestone and marble are vulnerable to
salt decay,
dissolution,
hydration and in some cases oxidation
Sandstone is susceptible to the processes of salt decay,
oxidation and if it is a calcareous variety of sandstone,
it is vulnerable to the dissolution process.
Clay slates are vulnerable to the chemical processes of
hydration, hydrolysis
and some varieties are affected by
the oxidation process.
Physical Conditions : This involves its physical features, such as its
pore/capillary
structure, visible and micro-fractures, bedding planes and the
finish of the stone, natural cleft, textured, honed and polished.
These physical features relate to water/moisture pathways into
the interior of the stone and mineral surfaces that will be exposed
to the chemical and mechanical processes.
Environmental Conditions : The environment that the stone is in can be said to be the
most significant factor upon the type and rates of weathering
alteration and decay. The chemical processes are more
pervasive and rapid in a warm and moist
environment. Salt
decay is also a prevalent process in this kind of environment
as well.
Some other contributing factors to the rates of weathering,
alteration and decay may
be the results of improper maintenance procedures, faulty restorations,
improper
preservation treatments or methods, etc. Any of these may accelerate the processes.
There is another aspect to the natural weathering / alteration process which is often
misunderstood. This involves understanding that some stones due to their natural
characteristics will naturally experience some sort of appearance change. One
example of this is...Some limestone's will develop a patina which actually gives the
stone more character and offers a certain amount of protection as well. However,
the prevailing mindset that has been developed by the consistent uniform appearance
of man made and synthetic building materials fosters the idea that something wrong is
happening with its appearance. It should be understood that what is taking place in this
particular situation is a natural occurring process.
It needs to be emphasized, that all stone will experience the natural weathering
process
to a certain degree. However, understanding
the natural characteristics of the stone
and the weathering
processes that affect
them, proper
maintenance and preservation
programs can be
employed to minimize and manage their effects.
