Plasterer
From Wikipedia, the free encyclopedia
A Plasterer is a tradesman who works with plaster, such as forming a layer of plaster on an interior wall or plaster decorative moldings on ceilings or walls. The process of creating plasterwork, called plastering, has been used in building construction for centuries.[1]
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[edit] History
Plasterwork is one of the most ancient of handicrafts employed in connection with building operations, the earliest evidence showing that the dwellings of primitive man were erected in a simple fashion with sticks and plastered with mud. Soon a more lasting and sightly material was found and employed to take the place of mud or slime, and that perfection in the compounding of plastering materials was approached at a very remote period is made evident by the fact that some of the earliest plastering which has remained undisturbed excels in its scientific composition that which we use at the present day. The pyramids in Egypt contain plasterwork executed at least four thousand years ago, probably much earlier, and yet existing, hard and durable, at the present time. From recent discoveries it has been ascertained that the principal tools of the plasterer of that time were practically identical in design, shape and purpose with those used to day. 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 stuff, and the whole finished somewhat under an inch thick. Very early in the history of Greek architecture we find the use of plaster of a fine white lime stucco, 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, which at this period of Grecian history had reached a very high degree of beauty.
[edit] Tools and Materials
In the more common operations of plastering, comparatively few tools and few materials are required, but the workman efficient in all branches of the craft will possess a very large variety of implements. The materials of the workman are laths, lath nails, lime, sand, hair, plaster of Paris, and a variety of cements, together with various ingredients to form coloring washes, et cetera.
[edit] Laths
Wood laths are narrow strips of some straight-grained wood, generally Baltic or American fir, 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 Lathing-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, 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. A large quantity, however, 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 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 of metal, either of wire or in the form of perforated sheets, is now extensively used on account of its fireproof and lasting quality. There are very many kinds of this material Metal made in different designs under various patents, the best known in England being the Jhilmil, the Bostwick, Lathing, and the 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.
[edit] 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.
[edit] Hair
Hair is used in plaster as a binding medium, and gives tenacity to the material. Ox-hair, which is sold in three qualities, is the kind usually specified; but horsehair, which is shorter, is sometimes substituted in its stead 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. As a result of experiments to ascertain its strength as compared with Substitutes that of other materials, it was found that plaster for hair slabs made with Manila hemp fiber broke at 195 lb, plaster mixed with Sisal hemp at 150 lb, jute at 145 lb, and goats' hair at 144 lb. 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 apparently quite uninjured. Sawdust has been used as a substitute for hair and also instead of sand as an aggregate. It 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.
[edit] Methods
For fine plasterer's sand-work, special sands, not hitherto referred to, 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 properties. The first coat or rendering is from 1/2 to 3/4 inches thick, and is mixed in the proportions of from one part of cement to two of sand to one part to five of sand. The finishing or setting coat is about 3/16 inches thick, and is worked with a hand float on the surface of the rendering, which must first be well wetted.
