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PLASTER OF PARIS
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See also:PLASTER OF See also:PARIS , a variety of calcined See also:gypsum (See also:calcium sulphate) which forms a hard See also:cement when treated with See also:water (see CEMENT). The substance obtained its name in consequence of being largely manufactured in the neighbourhood of Paris.
PLASTER-See also:WORK, in See also:building. Plastering is one of the most See also:ancient of handicrafts employed in connexion with building operations, the earliest See also:evidence showing that the dwellings of See also:primitive See also:man were erected in a See also:simple See also:fashion with sticks and plastered with mud. Soon a more lasting and sightly material was found and employed to take the See also:place of mud or slime, and that perfection in the compounding of plastering materials was approached at a very remote See also:period is made evident by the fact that some of the earliest plastering which has remained undisturbed excels in its scientific See also:composition that which we use at the See also:present See also:day. The pyramids in See also:Egypt contain plaster-work executed at least four thousand years ago (probably much earlier) and yet existing, hard and durable, at the present See also:time. From See also:recent discoveries it has been ascertained that the See also:principal tools of the plasterer of that time were practically identical in See also: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 " See also:lath, plaster, See also:float and set " work. See also:Hair was introduced to strengthen the " stuff," and the whole finished somewhat under an See also:inch thick. Very See also:early in the See also:history of See also:Greek See also:architecture we find the use of plaster of a See also:fine See also: The See also:temple of See also:Apollo at Bassae, built of yellow See also:sandstone about 470 B.C., is an excellent example. Pavements of thick, hard plaster, stained with various See also:pigments, were commonly laid in Greek temples. The See also:Roman architect See also:Vitruvius, in his See also:book on architecture written about 16 B.C., gives detailed See also: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 See also: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 See also:chalk or marble, caused the plaster when being mixed to be beaten with wooden staves by a See also:great number of men. Some persons cutting slabs of such plaster from ancient walls use them for tables and mirrors." See also:Pliny the See also: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 See also:beat it with pestles of See also:wood." In See also: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 See also: The thicker laths should be used in ceilings, to stand the extra See also:strain, and the thinner variety in See also:vertical work such as partitions, except where the latter will be subjected to rough usage, in which See also:case thicker laths become necessary. Laths are usually nailed with a space of about e in. between them to form a See also: There are very many kinds of this material Metal made in different designs under various See also:patents, the best-known in England being the Jhilmil, the Bostwick, Lathing. and the See also:Expanded Metal lathing. The two last-named are also widely used in See also:America. Lathing nails are usually of See also:iron, cut, wrought or See also:cast—and in the better class of work they are galvanized to prevent rusting. See also:Zinc nails are sometimes used, but are costly. The lime principally used for See also:internal plastering is that calcined from chalk or other nearly pure See also:limestone, and is known as See also:fat, pure, chalk or See also:rich lime. See also:Hydraulic limes, which are Limes. referred to in the articles See also:BRICKWORK and See also:MORTAR, are also used by the plasterer, chiefly for See also:external work. Perfect slaking of the calcined lime before being used is very important as, if used in a partially slaked See also:condition, it will " See also:blow " when in position and See also:blister the work. Lime should therefore be run as soon as the building is begun, and at least three See also:weeks should elapse between the operation of See also:running the lime and its use. Hair is used in plaster as a binding See also:medium, and gives tenacity to the material. Ox-hair, which is sold in three qualities, is the See also:kind usually specified; but horsehair, which is shorter, Hair. is sometimes substituted in its See also:stead or mixed with the ox-hair in the See also:lower qualities. Good hair should be long, strong, and See also:free from grease and dirt, and before use must be well beaten to See also: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 See also:pound of hair to two or three cubic feet of coarse stuff. See also:Manila See also: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 See also: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 See also: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 See also:frost and rough See also:weather. It is useful sometimes for heavy cornices and similar work, as it renders the material See also:light and strong. The sawdust should be used dry. Some remarks are made on the See also:ordinary sands for building in the articles on BRICKWORK and MORTAR. For fine plasterer's Sand. work See also:special sands, not hitherto referred to, are used, such as See also:silver sand, which is used when a light See also:colour and fine texture are required. In England this fine white sand is procured chiefly from See also:Leighton See also:Buzzard. For external work See also: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 See also:part of cement to two of sand to one part to five of sand. The See also:finishing or setting coat is about 138 in. thick, and is worked with a hand float on the See also:surface of the rendering, which must first be well wetted. Stucco is a See also: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 See also:deal of this work was done. The principal varieties of stucco are common, rough, trowelled and See also: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 See also: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 See also:colours, the desired tints being obtained by mixing with the lime various oxides. See also:Black and greys are obtained by using forge ashes in 1arying proportions, greens by See also:green See also:enamel, reds by using litharge or red See also:lead, and blues by mixing See also:oxide or carbonate of See also:copper with the other materials. Rough-cast or Pebble-dash plastering is a rough form of external plastering in much use for See also:country houses. In See also:Scotland it is termed " harling." It is one of the See also: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 See also:uniform consistency. While this coat is still soft, See also:gravel, See also:shingle or other small stones are evenly thrown on with a small See also:scoop and then brushed over with thin lime mortar to give a uniform surface. The shingle is often dipped in hot lime See also:paste, well stirred up, and used as required. S,graito (See also:Italian for " scratched ") is scratched See also: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 See also:Europe, especially in See also:Germany and See also: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 See also: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 See also:rest of the design are scratched with an iron See also:knife through the See also:outer skimming to the underlying tinted surface. Sometimes the coats are in three different colours, such as See also: " 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 See also:trowel. This is termed " pricking up " in See also: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 See also: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 " See also: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 See also:face for finishing. Wall screeds are plumbed and See also:ceiling screeds levelled. Screeds are narrow strips of plastering, carefully plumbed and levelled, so as to form a See also:guide upon which the floating See also:rule is run, thus securing a perfectly See also:horizontal or vertical surface, or, in the case of circular work, a uniform See also: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 See also:motion, at the same time sprinkling the work with water from a stock See also: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 See also:interval between each operation of from six to twenty-four See also:hours. This process leaves the plaster with a See also: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 See also: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 See also: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 " See also: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 See also: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 See also: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 See also: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 See also: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, See also:Keene's, and See also: See also:Plain, or unenriched, See also:mouldings are formed with a running See also:mould of zinc cut to the required See also:profile. Enrichments may be Mouldings. added after the See also: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 See also:lays on the plaster to the shape of the moulding. The mitres at the angles are finished off with joint rules made of See also:sheet See also:steel of various lengths, three or four inches wide, and about one-eighth inch thick, with one end cut to an See also:angle of about 3o°. In some cases the steel See also:plate is let into a " stock " or handle of hardwood. Cracks in plastering may be caused by See also:settlement of the building, and by the use of inferior materials or by See also: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 See also:heat or the heat of the See also: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 See also:soffit of the See also: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 See also: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 See also:weight, and when finished See also: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 See also:Gaol during the rebuilding of the new sessions See also:house in the Old See also: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 See also: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 See also:shop and then nailed or otherwise fixed in position, saves the delay often attendant upon the working of ornament in position. Desachy, a See also:French modeller, took out in 1856 a patent for " producing architectural mouldings, ornaments and other See also:works of art, with surfaces of plaster," with the aid of plaster, See also:glue, wood, wire, and canvas or other See also:woven fabric. The See also: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 See also:Christian era; for ancient coffins and mummies still preserved prove that See also:linen stiffened with plaster was used for decorating coffins and making masks. Cennino Cennini, See also: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- See also:Britain up to the See also:middle of the last century. This work is also much used for temporary work, such as See also:exhibition buildings. The principal books of reference on the subject are: W. See also:Millar. Plastering, Plain and Decorative; G. R. See also:Burnell, Limes, Cements, Mortars and Mastics; See also:Rivington, Notes on Building Construction, Part III. " Building Materials "; the works on architecture of See also:Robert and James See also:Adam. (J. Additional information and CommentsThere are no comments yet for this article.
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