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 omposition 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 tates…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.

When a stone begins to show signs of decay…pitting, spalling, flaking, crumbling, swelling, surface crusts and discoloration, one or more of these processes are at work.

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.

Its the sum of these factors that determines the type and rates at which dimensional stone will be affected by the chemical weathering processes.

There is another aspect to the natural weathering / alteration process which is often isunderstood. 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 nderstood 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.