This means that moisture inside the stone cannot escape and salts may crystallise behind this hard layer and eventually cause spalling, which leaves a weak exposed surface, more vulnerable to natural weathering.
The owners of the building notice the problem but choose the wrong type of treatment, perhaps a water repellent treatment that does not breathe.
Some characteristics are, however, more important than others.
The most important of these are strength, porosity, permeability, thermal dilation and colour.
There are three major causes of deterioration in natural stone; pollutants, frost and crystallisation of soluble salts.Gradually more of the exposed surfaces begin to erode a little. A decade of extremely wet weather interspersed with unusually low temperatures heightens the natural weathering.The atmospheric pollutant, sulphur dioxide, combines with the calcium carbonate in the stone and creates a hard gypsum layer covering the surface of the stone.It doesn't penetrate any further than the surface and the result is a hard, impervious outer surface like the gypsum.Ultimately the same problem arises as before; spalling occurs and weathering is accelerated. If it is not possible to shelter or isolate the material from the weather in some way, and continual erosion will lead to the loss of the original, then conservation through consolidation is the answer.