Plasterwork refers to construction or ornamentation done with plaster, such as a layer of plaster on an interior wall or plaster decorative moldings on ceilings or walls. This is also sometimes called pargeting. The process of creating plasterwork, called plastering , has been used in building construction for centuries.
History
Plasterwork is one of the most ancient of building techniques. Evidence shows that the dwellings of primitive man were erected in a simple fashion with sticks and plastered with mud. The pyramids in Egypt contain plasterwork executed at least four thousand years ago, probably much earlier, and yet hard and durable, at the present time. From recent discoveries it has been ascertained that the tools of the plasterer of that time were practically identical in design, shape and purpose with those used today. For their finest work, the Egyptians used a plaster made from calcined gypsum just like plaster of Paris of the present time, and their methods of plastering on reeds resemble in every way our lath, plaster, float and set work. Hair was introduced to strengthen the material, and the whole finished somewhat under an inch thick.
Very early in the history of Greek architecture plaster of a fine white lime stucco was used, such has been found at Mycenae. The art had reached perfection in Greece more than five centuries before Christ, and plaster was frequently used to cover temples externally and internally, in some cases even where the building was of marble. It formed a splendid ground for decorative painting.
The temple of Apollo at Bassae, built of yellow sandstone about 470 BC, is an excellent example. Pavements of thick, hard plaster, stained with various pigments, were commonly laid in Greek temples. The Roman architect Vitruvius, in his book on architecture written about 16 BC, gives detailed information concerning the methods of making plaster and the manner of using it. "The lime used for stucco," he writes, "should be of the best quality and tempered a long time before it is wanted for use. The Greeks, besides making their stuccowork hard with thin coats of marble-dust plaster polished with chalk or marble, caused the plaster when being mixed to be beaten with wooden staves by a great number of men. Some persons cutting slabs of such plaster from ancient walls use them for tables and mirrors." Pliny the Elder tells us that, "No builder should employ lime which had not been slaked at least three years," and that, "The Greeks used to grind their lime very fine and beat it with pestles of wood."
In England the walls of large houses and mansions were plastered above the wainscoting and colored, while the ornamented plaster ceilings of the time of Henry VIII, Elizabeth and James I, are still to be seen. Still earlier specimens of the plasterer's skill are visible in the pargeted and ornamented fronts of half-timbered houses.
Tools and materials
Tools and materials include trowels, floats, hammers, screeds, a hawk, scratching tools, utility knives, laths, lath nails, lime, sand, hair, plaster of Paris, a variety of cements, and various ingredients to form color washes.
While most tools have remained unchanged over the centuries, developments in modern materials have led to some changes. Trowels, originally constructed from steel, are now available in a polycarbonate material that allows the application of certain new, acrylic-based materials without staining the finish. Floats, traditionally made of timber (ideally straight-grained, knot-free, yellow pine), are often finished with a layer of sponge or expanded polystyrene.
Laths
Traditionally, plaster was laid onto laths, rather than plasterboard as is more commonplace nowadays. Wooden laths are narrow strips of straight-grained wood, generally pine, in lengths of from two to four or five feet to suit the distances at which the timbers of a floor or partition are set. Laths are about an inch wide, and are made in three thicknesses; single (1/8 to 3/16 inch thick), lath and a half (1/4 inch thick), and double (3/8-1/2 inch thick). The thicker laths should be used in ceilings, to stand the extra strain (sometimes they were doubled for extra strength), and the thinner variety in vertical work such as partitions, except where the latter will be subjected to rough usage, in which case thicker laths become necessary. Laths are usually nailed with a space of about 3/8 of an inch between them to form a key for the plaster. Laths were formerly all made by hand. Most are now made by machinery and are known as sawn laths, those made by hand being called rent or riven laths. Rent laths give the best results, as they split in a line with the grain of the wood, and are stronger and not so liable to twist as machine-made laths, some of the fibers of which are usually cut in the process of sawing.
Laths must be nailed so as to break joint in bays three or four feet wide with ends butted one against the other. By breaking the joints of the lathing in this way, the tendency for the plaster to crack along the line of joints is diminished and a better key is obtained. Every lath should be nailed at each end and wherever it crosses a joist or stud. All timbers over three inches (76 mm) wide should be counter-lathed, that is, have a fillet or double lath nailed along the centre upon which the laths are then nailed. This is done to preserve a good key for the plaster. Walls liable to damp are sometimes battened and lathed in order to form an air cavity between the damp wall and the plastering.
Lathing in metal, either in wire or in the form of perforated galvanised sheets, is now extensively used on account of its fireproof and lasting quality. There are many kinds of this material in different designs, the best known in England being the Jhilmil, the Bostwick, Lathing, and Expanded Metal lathing. The two last-named are also widely used in America. Lathing nails are usually of iron, cut, wrought or cast, and in the better class of work they are galvanized to prevent rusting. Zinc nails are sometimes used, but are costly.
Lime
The lime principally used for internal plastering is that calcined from chalk, oyster shells or other nearly pure limestone, and is known as fat, pure, chalk or rich lime. Hydraulic limes are also used by the plasterer, chiefly for external work. Perfect slaking of the calcined lime before being used is very important as, if used in a partially slaked condition, it will "blow" when in position and blister the work. Lime should therefore be run as soon as the building is begun, and at least three weeks should elapse between the operation of running the lime and its use.
Hair
Hair is used in plaster as a binding medium, and gives tenacity to the material. Traditionally horsehair was the most commonly-used binder, as it was easily available before the development of the motor-car. Hair functions in much the same way as the strands in fiberglass resin, by controlling and containing any small cracks within the mortar while it dries or when it is subject to flexing. Ox-hair, which is sold in three qualities, is now the kind usually specified; but horsehair, which is shorter, is sometimes substituted or mixed with the ox-hair in the lower qualities. Good hair should be long, strong, and free from grease and dirt, and before use must be well beaten to separate the lumps. In America, goats' hair is frequently used, though it is not so strong as ox-hair. The quantity used in good work is one pound of hair to two or three cubic feet of coarse stuff.
Manila hemp fiber has been used as a substitute for hair. Plaster for hair slabs made with manila hemp fiber broke at 195 lb (88 kg), plaster mixed with sisal hemp at 150 lb (68 kg), jute at 145 lb (66 kg), and goats' hair at 144 lb (65 kg). Another test was made in the following manner. Two barrels of mortar were made up of equal proportions of lime and sand, one containing the usual quantity of goats' hair, and the other Manila fiber. After remaining in a dry cellar for nine months the barrels were opened. It was found that the hair had been almost entirely eaten away by the action of the lime, and the mortar consequently broke up and crumbled quite easily. The mortar containing the Manila hemp, on the other hand, showed great cohesion, and required some effort to pull it apart, the hemp fiber being undamaged.
Sawdust has been used as a substitute for hair and also instead of sand as an aggregate. Sawdust will enable mortar to stand the effects of frost and rough weather. It is useful sometimes for heavy cornices and similar work, as it renders the material light and strong. The sawdust should be used dry. The sawdust is used to bind the mix together sometimes to make it go further.
Methods
For fine plasterer's sand-work, special sands are used, such as silver sand, which is used when a light color and fine texture are required. In England this fine white sand is procured chiefly from Leighton Buzzard. For external work Portland cement is undoubtedly the best material on account of its strength, durability, and weather resisting external prop
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