Rough-cast or Pebble-dash plastering is a rough form of exter nal plastering in much use for country houses. In Scotland it is termed "harling." It is one of the oldest forms of external plaster ing. In Tudor times it was employed to fill in between the wood work of half-timbered framing. When well executed with good material this kind of plastering is very durable. Rough-casting is performed by first covering the wall or laths with a coat of well haired coarse stuff composed either of good hydraulic lime or of Portland cement. This layer is well scratched to give a key for the next coat, which is also composed of coarse stuff knocked up to a smooth and uniform consistency. While this coat is still soft, gravel, shingle or other small stones are evenly thrown on with a small scoop and then brushed over with thin lime mortar to give a uniform surface. The shingle is often dipped in hot lime paste, well stirred up, and used as required.
Sgraffito (Italian for "scratched") is scratched ornament in plaster. Scratched ornament is the oldest form of surface decora tion, and at the present day it is much used on the Continent, especially in Germany and Italy, in both external and internal situations. Properly treated, the work is durable, beautiful and inexpensive. The process is carried out by applying a first coat or rendering of Portland cement and sand, in the proportion of one to three, laid on about thick; then following with the colour coat, which is sometimes put on in patches of different tints as required for the finished design. When this coat is nearly dry, it is finished with a smooth-skimming, to tin. thick, of Parian, selenitic or other fine cement or lime, only as much as can be finished in one day being laid on. Then by pouncing through the pricked cartoon, the design is transferred to the plastered surface. Broad spaces of background are now exposed by removing the finishing coat, thus revealing the coloured plaster beneath, and following this the outlines of the rest of the design are scratched with an iron knife through the outer skimming to the underlying tinted surface. Sometimes the coats are in three different colours, such as brown for the first, red for the second and white or grey for the final coat. The pigments used for this work include In dian red, Turkey red, Antwerp blue, German blue, umber, ochre, purple brown, bone black or oxide of manganese for black. Com binations of these colours are made to produce any desired tone.
mixed and skilfully applied, are essential to a perfect result. When brickwork is to receive plaster, it is all-important that its surface should be rough enough to form a key, or, alternatively, that the joints should be well raked out. Plaster is applied in successive coats or layers on walls or lathing, and gains its name from the number of these coats. "One coat" work is the coarsest and cheapest class of plastering, and is limited to inferior buildings, such as outhouses, where merely a rough coating is required to keep out the weather and draughts. This is described as "render" on brickwork, and "lath and lay" or "lath and plaster one coat" on studding. "Two coat" work is often used for factories or warehouses and the less important rooms of residences. The first coat is of coarse stuff finished fair with the darby float and scoured. A thin coat of setting stuff is then laid on, and trowelled and brushed smooth. "Two coat" work is described as "render and set" on walls, and "lath, plaster and set," or "lath, lay and set" on laths. "Three coat" work is usually specified for all good work. It consists, as its name implies, of three layers of material, and is described as "render, float and set" on walls and "lath, plaster, float and set," or "lath, lay, float and set," on lathwork. This makes a strong, straight, sanitary coating for walls and ceilings. The process for "three coat" work is as follows: For the first coat a layer of well-haired coarse stuff, about fin. thick, is put on with the laying trowel. This is termed "pricking up" in London, and in America "scratch coating!' It should be laid on diagonally, each trowelful overlapping the previous one. When on laths the stuff should be plastic enough to be worked through the spaces between the laths to form a key, yet so firm as not to drop off. The surface while still soft is scratched to give a key for the next coat, which is known as the second or "floating coat," and is 4 to gin. thick. In Scotland this part of the process is termed "straightening" and in America "browning," and is performed when the first coat is dry, so as to form a straight surface to re ceive the finishing coat. Four operations are involved in laying the second coat, namely, forming the screeds; filling in the spaces between the screeds ; scouring the surface; keying the face for finishing. Wall and ceiling screeds are plumbed and levelled.