Stone deteriorates and gets old in a natural manner, but the existence of environmental pollution certainly speeds the deterioration process.
The natural stone is a rather changing material, more than a static one, and not all kinds of stones are solid and tough, some stones are fragile, especially when liquid chemicals are applied to the surface, or when being in an atmosphere of polluted gazes. A fine example for this kind of stones would be the limestone, for being porous and easily carbonated, particularly when the stone itself has been previously polluted, one very evident example for this can be noticed in all the stone buildings in the Paris region, it can easily be noticed that, the period at which these buildings deteriorated was considerably short in the past fifty years, as compared to their state in the past three centuries.

One thing that is so significant of the French Land, is the abundance of the lime stones, which as solid as it is, can be easily affected by acid solutions, following the surrounding atmospheric conditions.
To fully understand how weather and climatic changes affect the state of a limestone, one must understand its composition.

The composition of limestone:
Limestone is a compound of two major components Calcite and Sulfin, the Calcite is shaped into a solid anti porous surface, originally resulting from the crystallizing of different types of salt that already exist in the rocks, and that is why most of it is wasted while it is being quarried, due to the water used while cutting the blocks. The natural components in Calcite which are "Calcium Carbonates" provide natural protection for the stone in its surface.

The Sulfin on the other hand, which is formed in a black solid layer, is mostly composed of Sulfuric calcium, because originally it is the outcome of acids, especially sulfur, which is why it expands over the surface of the stone at its own expense, which in turn activities the deterioration process.

On a more general view of the nature of deterioration, water is the major reason that causes defects to the stone, especially limestones, with its solvent effect, and the fact that it acts as the perfect medium for the existence of chemical, physical, mechanical, and biological elements which form the direct cause for the deterioration of natural stone.

These kinds of deterioration and defects take a long time to develop on the stone surface, because they are directly effected by the presence of solvent salts and humidity in most cases, and of Chlorophyl elements in some cases.

The small cracks are first seen on the surface of the stone, most evidently in damp areas, where the parts of the surface start peeling off the body of the stone, and develop to reach a stage where big chunks of stone fall off the body, containing huge amounts of solvent salts, deposited in different areas of the surface, following the structure of the stone itself.

In this context, and as far as cleaning is concerned, they all start with "Scurb treatment", which removes the pieces and shattered parts of the surface, without jeopardizing the body of the stone, and in any case, a careful study of the stone in terms of nature, condition, type of pollution, and the degree of deterioration should always be conducted before deciding on any method of restoration.

Another cleaning technique is mechanical scrubbing using sand blasting, and it can only be applied in extremely special cases, because using this particular type of cleaning can cause direct damage to the stone, in addition to the fact that the presence of chemicals with the water being used is sure to increase the solvent feature of the water, ultimately causing the stone to lose big portions of its body. Also it should be mentioned that all types of acids are prohibited to be applied to lime stones.

Of course, there remains some physical operational treatments that are conducted over small areas of the stone, and they are usually done in labs, some of which fall under the category of biological bandages, low temperature generating, or even laser.

There are also some conditions where the deterioration degree would be too deep for any cleaning treatment, that's when reinforcing, replacing, and renovations become necessary. When replacing is at hand, the new stone should be identical with the original one, in terms of toughness, nature of structure, core and even the color, and if the new stone is defected, and full of poisoned salts, it might very much effect the surrounding stones, and if it was from a different type of stone, it might cause a different kind of deterioration due to the difference in porousity.

There are four major techniques for stone installation, ranging from cutting, trimming, dressing, and positioning the stone forward for sculpture.
As for renovation, it is used to fill up the holes caused by deterioration, and attempting to restore the stone to its previous condition. This is done through a special kind of mortar, which behaves in a similar manner vis-à-vis the stone which is being restored, then small holes are made in the stone body, which are filled with mortar, and consequently the stone is installed over it.

The reinforcing technique, props the fusion between surface and the internal part of the stone, and inorder for this method to be successful, it must reach the original solid surface of the stone, and the material being used should all be uni component, especially in the liquid state.

There might be some cases where the stone may change its ability to absorb water, it becomes necessary then to prevent any kind of contact with water, by avoiding the solvent effect of the rain, and it is done by forming a protective layer to replace the Calcite element, hence protecting the stone from dampness, and blustering many layers on the internal sides of the body of the stone, without blocking the porous parts with an anti water shield. This method prevents the water from penetrating the body of the stone, and simultaneously allows specific amounts of air to go through in a gradual manner.

Article from Arab Stone Magazine