The most common building stones are sedimentary rocks which basicaly, can be broken down into sandstones and limestones. Both were originally formed as deposits of material, which accumulated in layers on the floors of seas and lakes, and on old land surfaces. It is these layers (beds), the material binding the particles together, and the pore space between them, which dictates the quality of the stone and its colour. There are huge variations in both. A good analogy for the horizontal beds, is a book laid flat, in which the pages are the beds, and the covers are the horizontal planes of weakness, usually formed by clay deposits, that occur in all sedimentary rocks, particularly sandstones. The distance between these planes of weakness is referred to as the "depth of stone on bed" and is a major factor in deciding on the suitability of the stone for building. Limestone is generally paler than sandstone, but the surest way to tell them apart is by the fact that water running down the stone will cause white streaks on limestone, and black streaks on sandstone. A freestone, sometimes referred to as liver rock is a stone in which the beds are strongly bonded, making it relatively easy to work in either direction, and durable, suitable for balusters etc. Tufa is a very light sponge-like limestone formed in aireated springs, and used decoratively eg in grottos or where lightness is important eg at the top of domes. (A cubic metre of sandstone or limestone weighs approx 1.5 tons)
The traditional manner, evolved over centuries, of building in stone, is of two leaves, tied together with bonding stones, with the cavity between them filled with stone pieces, lime and sand. The inner leaf carries the structure of the building, floors and roof, while the outer leaf offers weather protection. It is the surface of the outer leaf which is the usual cause of concern, and which can present a variety of difficult and searching conservation problems. Of paramount importance is the relationship between the stones and the mortar used in the construction. Consider, for example parts of the New town of Edinburgh where they obviously got it right. Here there are hundreds of yards of continuous building can be found, with little or no signs of distress, and not an expansion joint in sight!
The weathering of stone is natural and to an extent, unavoidable, it will in time revert to the material from which it was formed. There are various factors which hasten this decomposition which can basically be broken down into man-made and natural.
The main cause of failure is the improper use of the stone, a result of lack of understanding of how the material was formed and how it should be used in building. Generaly, the natural layers of the stone should be laid flat. Failure is also caused by lack of maintenance usually to gutters and downpipes which leads to saturation and rusting of fixings etc. Lack of protection and loss of projections designed not just for visual effect but also to throw water from the building is a problem, as is use of innapropriate mortars used for pointing and repairs, cleaning, and ill-advised coatings such as paint or so called preservatives.
Salt crystallisation, where salts within the stone are drawn to the surface by the process of wetting and drying, where they crystallise can be very damaging. If the pore structure of the stone is unable to accommodate the resulting expansion, the surface of the stone can disintigrate. Frost action is probably the next most damaging occurance, followed by the action of organisms such as algaes and lichens and plants such as ivy. Air pollutants, are no longer such a problem following the clean air acts.
Case hardening - when stone is first taken from the ground, it is relatively soft. As it dries out, and salts are drawn to the surface, a patina or harder outer skin known as the case hardening, is formed.
Contour scaling - the term used to describe the process where salt crystallisation detaches the case hardening.
Edge bedding - occurs where the layers of the stone are vertical and run at 90 degrees to the plane of the wall. Projections such as cornices and pediments should be edge bedded. Increasingly, the French term en delit is used to describe this technique, which neatly avoids the confusion which can arise when the natural layers of the stone are described as beds, and the stones themselves are usually laid in the horizontal mortar joints, universally referred to as beds.
Face bedding - occurs where the layers of the stone are vertical and parallel to the plane of the wall. Usually leads to extensive powdering and scaling of the stone.
Quarry sap - the moisture found in most newly quarried stone which quickly dries out forming the case hardening.
Spalling - a general term applied to stonework on which the outer face is peeling off. A spall is a small piece of stone. (Illustration).