Stone

TYPES OF WALL CONSTRUCTION

Ian Williams

Stone construction in traditional building can be initially divided into two types: rubble and ashlar. These two methods of construction are subject to further division. In the last century or so stone has also been used as cladding. Repairs must follow carefully previous methods of preparation and setting.

Подпись: Rubble

Rubble walls are either random, the stones being used more or less as they come to hand, or squared, with straightened edges. These two types further subdivide. Random rubble is either coursed, the stones roughly levelled up to form layers of varying thicknesses, or uncoursed, the larger stones being wedged by smaller stones, known as pinnings or spalls, with no attempt to form accurate vertical or horizontal joints. Broken residual rubble from dressed-down blocks and more thinly bedded stone was used as the infill between the inner and outer leaves of rubble walls. This infill was either consolidated by a semi-liquid sand : lime mortar to form a largely solid core, or left ungrouted.

Squared rubble may be laid uncoursed, coursed or regularly coursed. Uncoursed walls are usually formed of four stone sizes: large bonding stones (risers), two thinner stones (levellers) and small stones (snecks). Coursed walling is formed of larger stones of the same height, levelled off by thinner stones to form the courses. Regular coursed walls are formed of rows or courses of identical height stones, although the height of the courses can vary up the wall.

Caution must be exercised when cheaper means of repair are consid­ered. ‘Pitched-face’ stones sawn to bed heights are a convenient way to use offcuts from high-speed saws; it is considerably cheaper to install these for repair purposes than produce a traditional squared rubble block, but they bear little resemblance and are totally inappropriate when a proper match to the original stonework is required. Random rubble stone can be produced through extraction by means of a dragline or a JCB, when it will either be broken into manageable pieces as it is lifted or broken further

by a blow from the JCB’s bucket. Any further reduction can be achieved with a heavy hammer. Dressing off will be carried out with either a walling hammer or, more usually, a hydraulic guillotine.

Подпись: Materials & skillsAshlar

Ashlar masonry is formed of smooth squared stones with very thin mortar joints, usually laid in horizontal courses with stones of identical height, but each course may vary in height. ‘Random’ ashlar, often associated with later Victorian machine-cut stone, may be laid to a repeated pattern.

Ashlar can have various surface finishes. A polished finish to sandstone, achieved by rubbing the stone with a mixture of carborundum, sand and water, was advocated in 1883 by the quarry master and builder James Gowans, because ‘polishing removes the bruised material, and presents to wasting agents a surface more likely to prevent decay than any other kind of work’. Masonry may have rustication, usually to form a basement (that is, a ground floor storey) in a Palladian situation or quoins. The edges of the blocks are either rebated or chamfered (V-jointed ashlar), to all sides or to the top and bottom edges, to form channelled rustication. Other finishes include droved or boasted work, where a 2-inch chisel was worked over the surface to create parallel horizontal, vertical or diagonal lines (a technique also used on pennant stone paving to prevent slipping). A ‘tooled’ finish was similar to droved work except that it was carried out using a 4-inch chisel. A pointed chisel forms holes in the surface for a ‘stugged’ or ‘punched’ finish – ‘jabbed’ or ‘picked’ if using finer-pointed chisels. Often a droved margin was worked around both these punched finishes and around a ‘broached’ finish – horizontal or vertical lines formed with a gouge or toothed chisel. A rock-faced finish, as the name suggests, has a raised rough surface, sometimes set within a margin. Finally, ver – miculation is a pattern of irregular grooves suggestive of worm-eaten material.

OOLITIC LIMESTONE

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