Adhesive plaster

“ Where our object is simple support, we should of course select a plaster which is the most adhesive and the least irritating. Many plasters which have gained great celebrity for their curative virtues will be found to owe all their powers to their adhesiveness, such is the Emplastrum Oxidi Ferri Rubri of the Edinburgh Pharmacopœia, for it is impossible that the iron should communicate any tonic effect. ”

“ See Mortar.
Plaster of Paris. Calcined sulphate of lime. See Alabaster, Gypsum, Lime, &c.
PLASTER. (In pharmacy.) Syn. Emplastrum, L. Plasters (emplastra) are external applications that possess sufficient consistence not to adhere to the fingers when cold, but which become soft and adhesive at the temperature of the human body.
Plasters are chiefly composed of unctuous substances united to metallic oxides, or to powders, wax, or resin. They are usually formed, whilst warm, into 1⁄2-lb. ”

“ If the hole is up out of reach, and too large for you to fill, cement the edges with plaster, stretch a piece of cloth, or extra thickness of paper over it, and it will look all right, because the paper will shrink tight when it dries. If you find places where the clinches are broken, and the plaster is loose, press the plaster back to its place if you can, and cut small holes through the plaster and turn small broad headed screws into the lath even with the plaster and cement around the screws with plaster paris. ”

“ Plaster dries rapidly and will dry in the sun more rapidly. When plaster is damp it can be scraped or carved easily. Temperature and the age of the plaster affect its drying periods.
LARGE TILE MOLDS should be strengthened by having burlap strips or wire imbedded into the back while the plaster is soft. This creates a stronger layer than where plaster alone is used. Wood strips should not be used for backing plaster unless thoroughly dried as otherwise it causes cracking by its shrinkage, and it is best not to use it for reinforcing. ”

“ Painted plaster-work, so long as water can be kept from percolating through the cracks or faults, or gaining entrance from above by filtration, or from below by capillary action, is a highly durable material, but the moment water gains a footing the lime in some degree is dissolved, and, upon being removed and redeposited, undergoes the process of recrystallization, a powdery substance is thus formed that comes as a stranger between the paint and the plaster, in which respect it bears a strong resemblance to rust, the result of the oxidization of iron. ”

“ A PLASTER TILE IS MADE MORE DURABLE if it is dried and given a coat or two of shellac before it is used for molding plaster or cement. Otherwise repeated brushing of damp plaster with the oil brush will gradually wear the edges and details of the mold away.
TO MAKE A CAST FROM THE PLASTER MOLD, it is surrounded with the retaining bars after they have been oiled and held in position with the modeling wax or clay; this time they are placed firmly against the sides of the plaster mold. ”

“ When very large objects that would require a great deal of plaster are cast, they are generally made hollow in the following manner:
Obtain the glue mould by the process described, and into it pour a quantity of thin plaster, having first oiled the surfaces that come in contact with it. Turn the mould about and upside down, so the plaster will enter every part and adhere to the glue form. ”

“ Such a matrix is required for rubber type, but paper is rather too susceptible to heat although good work can be done with it. It also does not enter as deeply into the cavities of the type as is desirable. As a rule a fine quality of plaster of paris is to be recommended. What is sold as dental plaster is the best, but common plaster can be used. It is mixed with water or with a solution of gum arabic or dextrine in water. For the latter enough gum should be added to make the mixing solution as thick as thin syrup. ”

“ In the erection of new buildings, the plasterer’s pit takes up much room, and is often looked upon as a necessary evil. In putting on the common three coats of plaster, the second and third can only be laid on when that before it is sufficiently dry. Owing to the unequal shrinkage of the different materials, it is often an uncertain method of doing good work. When using the cement we speak of, the plasterers can be put into a room with the requisite quantities of sand and cement, and work straight away. ”

“ If you find one edge of a crack higher than the other, gently press against it, and if it goes back to place, cement it with plaster paris wet up in clear water, and it will set in three minutes hard enough to hold the plaster in place. If the loose edge will not go back by gentle pressure, lay a piece of board over it and push hard as you dare to and not crush the plaster. ”

“ Neutral Substances, or Binding Materials.—Almost any substance not easily soluble in water, and many which are, from common dust or earth, or clay, sand, chalk, powdered egg-shells, sawdust, 6 shell-powder, &c., when combined with certain adhesives, form cements. This is sometimes due to chemical combination, but more frequently to mechanical union. In the latter case the adhesive clinging to every separate grain has the more points of adhesion, just as a man by clinging with both hands to two posts is harder to remove than if he held by one. ”

“ When the pieces are not exactly round, and cannot be thrown or pressed on jiggers, it is the custom to have them made in plaster moulds, which have been cast on models prepared for the purpose. As long as the clay keeps soft, it takes the shape of any hard substance against which it is pressed, and for that reason, plaster, which has the property of absorbing moisture readily, is preferred. ”

“ This cement is applicable to glass, wood, or metal. Like the following, it has the advantage of being always ready to use, and requires no boiling. If it becomes too hard to use freely, let so much of it as is required steep for a time in water. Many think, from merely dampening it in the mouth when it is hard, and using it immediately, that it is a very weak adhesive, which is a mistake. A great deal of that sold by the stationers is, however, of very inferior quality, and made with very common glue. ”

“ Run this back and forth to trim off the surplus, so that the plaster will have an even thickness when removed from its bed.
TO REMOVE THE PLASTER first remove the wooden strips and then tap the layer of plaster lightly on the edge with a hammer. This will loosen it. Then with a T square and sharp nail mark off{37} bars five-eighths inch wide and also bars one and one-quarter inches wide. At least twelve bars of each width should be made. After drying for three days the bars may be sawed apart with any sharp saw, care being taken to hold the saw within the groove made by the nail. ”

“ Rough places in plaster take more color than a smooth wall, hence they are liable to show spots; so it stands you in hand to make such places smooth as possible; to do this take off the rough sand with sandpaper and knife or trowel on a thin coat of plaster paris, or give the rough places an extra coat or two or size. Fill all cracks and holes, and give the filling time to dry before putting on the size, because otherwise it will take more color than the balance of the wall and your work will look spotted. ”

“ Indeed, this commodity is very frequently adulterated. A kind of plaster, with a very thick and brittle covering, is often sold for it. The manufacturers of this, instead of isinglass, use common glue, which is much cheaper; and cover the whole with spirit varnish, instead of balsam of Peru. This plaster cracks, and has none of the balsamic smell by which the genuine court plaster is distinguished. Another method of detecting the adulteration is to moisten it with your tongue on the side opposite to that which is varnished; and, if the plaster be genuine, it will adhere exceedingly well. ”

“ Plaster-casting in General
It is not a difficult matter to reproduce in plaster almost any object that has no undercut or parts that will not easily detach from a mould. When making them, the first thing to do is to get a piece of board about a foot square; that is to work on, so as not to soil the table. Then you want a lump of clay about as big as a football, five or six pounds of plaster of Paris—it only costs three cents a pound—and a half-dozen wooden pegs. ”

“ Lime plastering is composed of lime, sand, hair and water in proportions varying according to the nature of the work to be done. Internal Work.In all cases good materials, well mixed and skilfully applied, are essential to a perfect result. 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. ”

“ When the painter has to paint over holes and cracks in walls recently filled by the plasterer, he will be likely to have to deal with plaster made in part of fresh lime. In such cases, it is always best to soak the newly plastered places with strong vinegar, to kill as much as possible the caustic properties of the lime. Put on the vinegar plentifully and let it soak in; when dry, give the new plaster a coat of size made of linseed oil, japan and turpentine; when dry, put on a coat of white shellac before painting. ”

“ A very important point is the thorough slaking and tempering of the plaster, which continues to improve the longer it is kept. The glue that was used was made from hides, size being made from the fine kinds [Pg 216] of skin, vellum and parchment, as the finest kind is made now. Fish glue was also used from very early times.
Very useful gesso is made with whiting (calcined chalk) instead of the plaster. The latter is said to be tougher, but whiting is certainly easier in working. ”

“ IT IS THEN POURED PROMPTLY into the center of the space on the glass prepared for the plaster tile so that the poured plaster gradually spreads from the center toward the corners and edges filling up the spaces and coming up the sides to about one-half inch height. Jarring the table or a slight shaking of the glass will cause the plaster to settle very level as well as causing any bubbles in the plaster to rise to the surface and break. Care should be taken that the plaster is not too thick before being poured. ”

“ The most perfect adhesion between glass and plaster is attained by spreading the paste of plaster on the up-turned surface of the slab of glass and lowering the iron bed-plate of the grinding table down upon it, the bed-plate with the adhering slab of glass being afterwards turned over and brought into position in the grinding machine. ”

“ If the plaster sticks to the end of the rammer, it shows, either that you have used the plaster too wet, or have not turned the rammer round a sufficient number of times.
No more plaster must be mixed at a time than will suffice for one case. When plaster has once set, it cannot be mixed up a second time; therefore take a fresh piece of paper, and let the knife be cleaned every time. It is advisable to have two dessert-knives, then one can be used with which to scrape the other. As much plaster should be used as will fill the case up about 1⁄2 an inch. ”

“ The durability of plaster casts, exposed to the weather, is greatly increased by saturating them with linseed oil combined with wax or rosin. They are made to resemble bronze by the application of a soap composed of linseed oil and soda, and colored with the sulphate of copper and iron.
13. Moulds are, also, formed of a warm solution of glue, which hardens upon cooling, and such are called elastic moulds. This material is sometimes preferred on account of its more easy separation from irregular surfaces. For small and delicate impressions in bas-relief, melted sulphur is sometimes employed ”

“ The success of the plasterer depends in no small degree upon the way the carpenter does this work. If corners are not firmly constructed, cracks will be sure to appear in the plastering.
In lathing, joints must be broken upon different studding every dozen lath, and joints are not to be allowed about a door or window opening where their presence would weaken the wall; such as short lath nailed at one end only. Neither are lath to be placed at right angles to the usual run of lath on the wall because uneven shrinkage would cause the plaster to crack. ”

“ To a certain extent the size stops the absorbent powers of the wood or plaster, but it prevents the proper adhesion of the oil paint, which soon cracks or peels off. It may, however, be used with advantage in old work, where the grease would prevent the proper drying of the oil paint, but even in such cases it is best, when possible, to scrape the wood or plaster until a new surface is reached, on which the oil paint may be successfully applied. ”

“ Plaster mixed with weak glue-size—which prevents its setting too rapidly—is the best material for stopping walls preparatory to painting, and each coat of paint should be carefully rubbed with worn sand-paper, before the succeeding coat is put on. For preparing walls a small pocket-trowel will be found a most serviceable tool, or a trowel-shaped putty-knife, which article has come into general use. ”

“ The edges of the wood facing inward should always be oiled, the wood strips are held in position by small lumps of modeling wax pressed firmly into the outer edge of the wood so that it attaches it to the surface of the glass. This will leave a channel between the two wooden strips into which the plaster is poured. The two ends of the channel may be stopped with blocks of wood or blocked with wax or clay. As soon as the plaster is poured into the mold it should be evened by running a straight strip of wood or metal over it spanning the width of plaster. ”