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PLASTER OF PARIS

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Originally appearing in Volume V21, Page 786 of the 1911 Encyclopedia Britannica.
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PLASTER OF PARIS, a variety of calcined gypsum (calcium sulphate) which forms a hard cement when treated with water (see CEMENT). The substance obtained its name in consequence of being largely manufactured in the neighbourhood of Paris. PLASTER-WORK, in building. Plastering is one of the most ancient of handicrafts employed in connexion 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 plaster-work 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 the " 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. The temple of Apollo at Bassae, built of yellow sandstone about 470 B.C., 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 B.C., gives detailed information concerning the methods of making plaster and the manner of using it. " The lime used for stucco," he writes, " should beof the best quality and tempered a long time before it is wanted for use. The Greeks, besides making their stucco work 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 formerly plastered above the wainscoting and coloured, while the ornamented plaster ceilings of the time of Henry VIII., Elizabeth and James I., are still the admiration of lovers of the art. Still earlier specimens of the plasterer's skill are extant in the pargeted and ornamented fronts of half-timbered houses. With regard to the smaller buildings, comprising small dwelling-houses and cottages, the general application of plaster is of comparatively late date; for wainscoted walls and boarded ceilings or naked joists alone are frequently found in houses of not more than a century old both in England and on the Continent. 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 colouring washes, &c. 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 " (e to A in. thick), " lath and a half " (; in. thick), and " double " (e to * in. 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 e in. 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 fibres 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 fire-proof 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. The lime principally used for internal plastering is that calcined from chalk or other nearly pure limestone, and is known as fat, pure, chalk or rich lime. Hydraulic limes, which are Limes. referred to in the articles BRICKWORK and MORTAR, 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 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, Hair. 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 fibre 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 fibre broke at 195 lb, plaster mixed with Sisal hemp at 15o lb, jute at 145 lb, and goats' hair at 144i 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 fibre. 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 fibre 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. Some remarks are made on the ordinary sands for building in the articles on BRICKWORK and MORTAR. For fine plasterer's Sand. work special sands, not hitherto referred to, are used, such as silver sand, which is used when a light colour 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 i to External h in. 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 138 in. thick, and is worked with a hand float on the surface of the rendering, which must first be well wetted. Stucco is a term loosely applied to nearly all kinds of external plastering, whether composed of lime or of cement. At the present Stucco. time it has fallen into disfavour, but in the early part of the 19th century a great deal of this work was done. The principal varieties of stucco are common, rough, trowelled and bastard. Cement has largely superseded lime for this work. Common stucco for external work is usually composed of one part hydraulic lime and three parts sand. The wall should be sufficiently rough to form a key and well wetted to prevent the moisture being absorbed from the plaster. Rough stucco is used to imitate stonework. It is worked with a hand float covered with rough felt, which forms a sand surface on the plaster. Lines are ruled before the stuff is set to represent the joints of stonework. Trowelled stucco, the finishing coat of this work, consists of three parts sand to two parts fine stuff. A very fine smooth surface is produced by means of the hand float. Bastard stucco is of similar composition, but less laboar is expended on it. It is laid on in two coats with a skimming float, scoured off at once, and then trowelled. Coloured stucco: lime stucco may be executed in colours, the desired tints being obtained by mixing with the lime various oxides. Black and greys are obtained by using forge ashes in 1arying proportions, greens by green enamel, reds by using litharge or red lead, and blues by mixing oxide or carbonate of copper with the other materials. Rough-cast or Pebble-dash plastering is a rough form of external plastering in much use for country houses. In Scotland it is termed " harling." It is one of the oldest forms of external plastering. In Tudor times it was employed to fill in between the woodwork of half-timbered framing. When well executed with good material this kind of plastering is very durable. Rough-casting is performed by first rendering 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. S,graito (Italian for " scratched ") is scratched ornament in plaster. Scratched ornament is the oldest form of surface decoration, and at the present day it is much used on the continent of Europe, especially in Germany and Italy, in both external and internal situations. Properly treated, the work is durable, effective and inexpensive. The process is carried out in this way: A first coat or rendering of Portland cement and sand, in the proportion of one to three, is laid on about a in. thick; then follows the colour coat, 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, i to in. 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 spacesof 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 Indian red, Turkey red, Antwerp blue, German blue, umber, ochre, purple brown, bone black or oxide of manganese for black. Combinations of these colours are made to produce any desired tone. Lime plastering is composed of lime, sand, hair and water in proportions varying according.to the nature of the work to be done. In all cases good materials, well mixed and skilfully applied, are essential to a perfect result. Plaster is Internal applied in successive coats or layers on walls or lathing, Work. 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 1 in. 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 with a lath to give a key for the next coat, which is known as the second or " floating coat," and is s to s in. 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 receive 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 screeds are plumbed and ceiling screeds levelled. Screeds are narrow strips of plastering, carefully plumbed and levelled, so as to form a guide upon which the floating rule is run, thus securing a perfectly horizontal or vertical surface, or, in the case of circular work, a uniform curve. The " filling in," or " flanking," consists of laying the spaces between the screeds with coarse stuff, which is brought flush with the level of the screeds with the floating rule. The " scouring " of the floating coat is of great importance, for it consolidates the material, and, besides hardening it, prevents it from cracking. It is done by the plasterer with a hand float which he applies vigorously with a rapid circular motion, at the same time sprinkling the work with water from a stock brush in the other hand. Any small holes or inequalities are filled up as he proceeds. The whole surface should be uniformly scoured two or three times, with an interval between each operation of from six to twenty-four hours. This process leaves the plaster with a close-grained and fairly smooth surface, offering little or no key to the coat which is to follow. To obtain proper cohesion, howevet, a roughened face is necessary, and this is obtained by " keying " the surface with a wire brush or nail float, that is, a hand float with the point of a nail sticking through and projecting about * in.; sometimes a point is put at each corner of the float. After the floating is finished to the walls and ceiling, the next part of internal plastering is the running of the cornice, followed by the finishing of the ceiling and walls. The third and final coat is the " setting coat," which should be about e in. thick. In Scotland it is termed the " finishing," and in America the " hard finish " or " putty coat." Considerable skill is required at this juncture to bring the work to a perfectly true finish, uniform in colour and texture. Setting stuff should not be applied until the floating is quite firm and nearly dry, but it must not be too dry or the moisture will be drawn from the setting stuff. The coarse stuff applied as the first coat is composed of sand and lime, usually in proportions approximating to two to one, with hair mixed into it in quantities of about a pound to two or three cubic feet of mortar. It should be mixed with clean water to such a consistency that a quantity picked up on the point of a trowel holds well together and does not drop. Floating stuff is of finer texture than that used for " pricking up," and is used in a softer state, enabling it to be worked well into the keying of the first coat. A smaller proportion of hair is also used. ' Fine stuff mixed with sand is used for the setting coat. Fine stuff, or lime putty, is pure lime which has been slaked and then mixed with water to a semi-fluid consistency, and allowed to stand until it has developed into a soft paste. For use in setting it is mixed with fine washed sand in the ratio of one to three. For cornices and for setting when the second coat is not allowed time to dry properly, a special compound must be used. This is often " gauged " stuff, composed of three or four parts of lime putty and one part of plaster of Paris, mixed up in small quantities immediately before use. The plaster in the material causes it to set rapidly, but if it is present in too large a proportion the work will crack in setting. The hard cements used for plastering, such as Parian, Keene's, and Martin's, are laid generally in two coats, the first of cement and sand z to ; in. in thickness, the second or setting coat of neat cement about s in. thick. These and similar cements have gypsum as a base, to which a certain proportion of another substance, such as alum, borax or carbonate of soda, is added, and the whole baked or calcined at a low temperature. The plaster they contain causes them to set quickly with a very hard smooth surface, which may be painted or papered within a few hours of its being finished. The by-laws made by the London County Council under § 31 of the London Council (General Powers) Act 1890 set forth the description and quality of the substances of which plastering is to be made for use in buildings erected under its jurisdiction. Plain, or unenriched, mouldings are formed with a running mould of zinc cut to the required profile. Enrichments may be Mouldings. added after the main outline moulding is set, and are cast in moulds made of gelatine or plaster of paris. For a cornice moulding two running rules are usual, one on the wall, the other on the ceiling, upon which the mould is worked to and fro by one workman, while another man roughly lays on the plaster to the shape of the moulding. The mitres at the angles are finished off with joint rules made of sheet steel of various lengths, three or four inches wide, and about one-eighth inch thick, with one end cut to an angle of about 3o°. In some cases the steel plate is let into a " stock " or handle of hardwood. Cracks in plastering may be caused by settlement of the building, and by the use of inferior materials or by bad workmanship, but Cracks. apart from these causes, and taking the materials and labour as being of the best, cracks may yet ensue by the too fast drying of the work, caused through the laying of plaster on dry walls which suck from the composition the moisture required to enable it to set, by the application of external heat or the heat of the sun, by the laying of a coat upon one which has hot properly set, the cracking in this case being caused by unequal contraction, or by the use of too small a proportion of sand. For partitions and ceilings, plaster slabs are now in very general use when work has to be finished quickly. For ceilings they require Slabs. simply to be nailed to the joists, the joints being made with plaster, and the whole finished with a thin setting coat. In some cases, with fire-proof floors, for instance, the slabs are hung up with wire hangers so as to allow a space of several inches between the soffit of the concrete floor and the ceiling. For partitions the slabs frequently have the edges tongued and grooved to form a better connexion; often, too, they are holed through vertically, so that, when grouted in with semi-fluid plaster, the whole partition is bound together, as it were, with plaster dowels. Where very great strength is required the work may be reinforced by small iron rods through the slabs. This forms a very strong and rigid partition which is at the same time fire-resisting and of light weight, and when finished measures only from two to four inches thick. The slabs may be obtained either with a keyed surface, which requires finishing with a setting coat when the partition or ceiling is in position, or a smooth finished face, which may be papered or painted immediately the joints have been carefully made. Partitions are also formed with one or other of the forms of metal lathing previously referred to, fixed to iron uprights and plastered on both sides. So strong is the result that partitions of this class only two or three inches thick were used for temporary cells for prisoners at Newgate Gaol during the rebuilding of the new sessions house in the Old Bailey, London. Fibrous plaster is given by plasterers the suggestive name " stick and rag," and this is a rough description of the material, for it is Fibrous composed of plaster laid upon a backing of canvas Piaster. stretched on wood. It is much used for mouldings, circular and enriched casings to columns and girders and ornamental work, which, being worked in the shop and then nailed or otherwise fixed in position, saves the delay often attendant upon the working of ornament in position. Desachy, a French modeller, took out in 1856 a patent for " producing architectural mouldings, ornaments and other works of art, with surfaces of plaster," with the aid of plaster, glue, wood, wire, and canvas or other woven fabric. The modern use of this material may be said to have started then, but the use of fibrous plaster was known and practised by the Egyptians long before the Christian era; for ancient coffins and mummies still preserved prove that linen stiffened with plaster was used for decorating coffins and making masks. Cennino Cennini, writing in 1437, says that fine linen soaked in glue and plaster and laid on wood was used for forming gmunds for painting. Canvas and mortar were in generaluse in Great- Britain up to the middle of the last century. This work is also much used for temporary work, such as exhibition buildings. The principal books of reference on the subject are: W. Millar. Plastering, Plain and Decorative; G. R. Burnell, Limes, Cements, Mortars and Mastics; Rivington, Notes on Building Construction, Part III. " Building Materials "; the works on architecture of Robert and James Adam. (J. BT.)
End of Article: PLASTER OF PARIS
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