Choice of stone
Characteristic walling stones are described in the following section, but a few general points are worth bearing in mind.
- Avoid highly fissile rock, good quality slate excepted. Fissile rock tends to split or fragment easily on weathering and so flakes away piece by piece. If it must be used, keep it in fairly thick blocks, rather than splitting it into thinner pieces which will quickly crumble.
- For standard double walling, avoid using very large stones, bigger than about 1′ (300mm) on a side or 8″ (200mm) high, for the face stones. Even foundation stones should be just high enough to come up to ground level from the bottom of the foundation trench, which is normally about 6″ (150mm). Although large stones speed the building process, they are hard to work around and reposition, and they may settle unequally relative to smaller stones in the wall.
Larger stones may be needed for foundations, throughs and coverbands. Where the available stone supply is mainly large stones, single walling and other styles have developed. The massive stones found in many old walls are there because of the need to clear adjacent fields or to incorporate immovable boulders, and not because they add strength to the wall. - Avoid using water-washed stones, if possible. One occasionally comes across very beautiful walls of rounded river-bottom stones which show the work of a master craftsman. However, rounded stones are hard to use, being too smooth to grip each other properly, and often with fine cracks which cause them to break apart in frost. Normally, large rounded stones which have to be used, should be split to give flat beds and faces. Alternatively they can be broken up for fillings.
Preventing decay of stonework
Stonework, whether dry or mortared, is attacked by a number of agents including:
- Wind-borne dust or sand which abrades the stone. This can be severe in coastal districts.
- Fluctuating temperatures, which cause the stone to flake at the surface or split due to differential expansion and contraction within the stone.
- Frost, which forces the stone apart along cracks as the water freezes and expands.
- Rain, which penetrates cracks in the stone and may lead to frost damage, and which also dissolves certain types of stone directly. Limestone, which is over 90% calcium carbonate, is easily affected by the dilute acids found in rain and ground water. Industrial air pollution, which can increase the acidity of rainfall, has been responsible for the rapid deterioration not only of limestone, but of sandstone as well.
Sedimentary rock
Sedimentary rocks in the ground contain ‘quarry sap’, a dilute acid having silica, lime and other chemicals in solution or suspension. If the stone is allowed to dry naturally for a year or more before use, the quarry sap forms a hard protective coating on the surface. Because of this, stone which is to be worked should be shaped before curing, when it is softest.
The weather resistance of cured sedimentary stone is reduced if the surface coating is removed in building. Naturally, stone used for dry walling is not given the careful treatment of masonry stone, but it is worth remembering that limestone, and to a lesser extent sandstone, may last longer if the protective skin in left intact. This is an argument against using the hammer too actively when building with these materials.
Cotswold stone is easily damaged by frost, and the most durable stone is left in the open to weather over winter, which ensures that any bad stone breaks up before use. The topmost layer in the quarry is always the hardest, since it has been exposed to frost over millennia. Blasting out of stone may leave minute stress fractures which only show up in the first hard frost. If the stone is purchased ‘green’, or still moist from the earth, it should be left to dry before use. This process is also used in other limestone areas.
An important consideration when walling with most types of stone is its bedding. As used in walling, the natural ‘bed’ of a sedimentary stone is the plane on which it was originally laid down. In the case of metamorphic rocks, the bed follows the planes of cleavage or foliation. Since layers of rock have often been tilted or contorted by geological processes, their bedding is not necessarily horizontal when found in the quarry. However, all stones with a laminate bedding should normally be placed with the bedding horizontal in walls and other structures, which gives the stone the greatest resistance to decay. The exception are upright copings, where the bedding is placed vertically.
Stones which are quarried in large blocks with no tendency to split in any particular direction are termed freestones, and can be given any orientation when placed in a structure. Freestones include granite and other igneous rocks and a few extremely fine grained sedimentary rocks such as Portland, Bath and some of the other famous building stones.
