Plastron

“ The plastron is large, but does not quite close the box. Besides the small nuchal there are twelve pairs {351}of marginal shields. The head is covered with smooth skin; the temporal arch is complete. The limbs are extensively webbed. The tail of the very young is nearly as long as the shell, but it becomes relatively shorter with age, being reduced in the males to about two-thirds, in the females to half the length of the shell. ”

“ The carapace of the young is keeled, each of the neural shields being raised in the middle line; but in full-grown specimens the shell becomes quite smooth and rounded. The horny shields of the plastron are relatively largest in the young, but they soon leave ever-increasing spaces between them, which are then filled with soft skin only, which thinly covers the underlying bone. The {344}fore- and hind-limbs, especially the latter, are extensively webbed, and are provided with five short claws. ”

“ The hinder surface of the thigh is quite smooth. The tip of the tail ends in a conical, horny spur. The coloration of the shell varies somewhat, but the ground-colour is yellow, each shield with a dark brown centre and irregular patches or confluent spots towards the margin. The plastron has an irregular, broad black border. The soft parts are grey-yellowish. Some specimens are rather pale, almost lemon yellow with little black; others incline towards orange with more or less black. The middle fields of the shields of young specimens are granular, although this area is rubbed smooth with age ”

“ The nine plastral bones include: an anterior pair of preplastra (= epiplastra, auct.) ; an unpaired, median bone, representing fused epiplastra (= entoplastron, auct.) , hereafter referred to as the epiplastron; a pair of hyoplastra; a pair of hypoplastra; and, posteriorly, a pair of xiphiplastra (Fig. 17) .
Siebenrock's (1902) synopsis of living trionychids was based entirely on plastral characters. ”

“ 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. ”

“ Plas′tering, the art of covering the internal faces of walls, the partitions and ceiling of a building, with plaster: a covering of plaster: the plasterwork of a building; Plas′ter-stone, gypsum.—adj. Plas′tery, like plaster, containing plaster.—Plaster cast, a copy of an object got by pouring a mixture of plaster of Paris and water into a mould formed from the object; Plaster of Paris, a kind of gypsum, originally found near Paris, used in building and in making casts of figures; Porous plaster, a plaster for application to the body, full of small holes, which prevent it from wrinkling. ”

“ Many plasters, as those of lead and resin, derive much of their whiteness and beauty from the treatment just referred to. White plasters are not, however, always the best; but they are those which are most admired, and the most sought after in trade.
Plasters are preserved by enveloping the rolls with paper, to exclude the air as much as possible, and by keeping them in a cool situation. A few, as those of belladonna and ammoniacum with mercury, are commonly placed in pots. When kept for any length of time, they are all more or less apt to become hard and brittle, and to lose their colour. ”

“ Be sure all is sound and firm, as also the nail on which the specimen hangs, otherwise your own, or your stag's, head may come to grief. Plaster heads being very heavy at first, before drying, it is as well to get them dried, if possible, in advance of the mounting, to obviate great weight, and also a tendency to cause mildew inside the skin. It is really astounding, however, to observe how very light plaster becomes when thoroughly dry; clay of the same sized model is, on the contrary, exceedingly heavy — more than twice the weight of plaster. ”

“ 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. ”

“ Plaster, or stucco, is sometimes regarded as an inferior material only to be used when nothing better can be obtained. It should not however be judged from the achievements of the domestic plasterer of to-day, who has to trust sometimes to the wall-paper to keep his stuff from crumbling away. Plaster as used by the ancients, and through a good part of the mediaeval and Renaissance periods up to the eighteenth century, is a fine material, susceptible of very varied and effective artistic treatment. ”

“ 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. ”

“ Scrape the plaster to a level surface, and cut two moderately large V-shaped nicks, one on each edge of the mould, build up around as before with wood, and fill in all interstices leading to the table below with clay. See that the mouth is properly shut, introducing a little clay if needed. Brush over with soft soap, not forgetting the top of the plaster mould, and mix some more plaster and pour over the lower jaw, on to the edges of the plaster mould, until stopped by the wall; build up thicker in the centre, as before, and suffer all to dry for about an hour. ”

“ 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. ”

“ The painted plaster casts were so lifelike that they recalled the fish in his own Chinese waters. The Museum was to that Chinaman an object-lesson.
Now I am certain that if boys or girls knew how to make plaster-of-Paris fish look like the real ones, it would not alone be an interesting occupation, but an instructive one.
I have explained how the plaster is to be mixed. It is better to buy the plaster in small quantity, not more than a pound at a time. It should be kept in a well-closed glass preserving jar. ”

“ Cleaning Of Statuettes And Other Plaster Objects.—Nothing takes the dust more freely than plaster objects, more or less artistic, which are the modest ornaments of our dwellings. They rapidly contract a yellow-gray color, of unpleasant appearance. Here is a practical method for restoring the whiteness: Take finely powdered starch, quite white, and make a thick paste with hot water. Apply, when still hot, with a flexible spatula or a brush on the plaster object. The layer should be quite thick. Let it dry slowly. On drying, the starch will split and scale off. ”

“ 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. ”

“ PLASTER IS SENSITIVE to moisture, and should be kept in a dry place. If plaster does not set within a short time it probably is old and even if it does set in time it will have a tendency to pulverize. It is always safer to purchase an entire sack than to purchase a small lot from the hardware dealer, grocer, or druggist, for the chances are that his plaster has been exposed in a bin so long that its strength has vanished. This is mentioned because instances have been known where trouble in cast making was traced to just such conditions. ”

“ 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. ”

“ In this state, the plaster must be considered as perforated with little vacuities interspersed throughout its substance: and in consequence of this texture, when it afterward comes again into contact with water, or even with air loaded with humidity, these vacuities exert a capillary attraction, and imbibe the water afresh; and plaster of this sort can be used only in places that are dry, and sheltered from all accidental dampness. ”

“ 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. ”

“ It is manifestly of considerable practical importance to understand the conditions which determine the time of the setting up of plaster. According to Payen, the rapidity of setting, provided the plaster has dehydrated at a temperature sufficiently low, depends entirely on the structure of gypsum employed. Thus, according to him, the fibrous kinds gives a plaster setting almost instantaneously. The water, he says, penetrates the material freely, setting takes places almost simultaneously throughout the mass. ”

“ As the integrand molecules of gypsum, in their regeneration from plaster, can only crystallize, as has been said, in a very irregular and confused manner, the space occupied by the whole mass, after this reproduction, must be in proportion to this irregularity. Accordingly it is found that plaster acquires a very evident increase of bulk, a circumstance that is often very detrimental in the purposes for which it is employed ”

“ TO OIL THE RETAINING BARS, brush the surface to come in contact with the plaster and then hold the wood in position over the diagram below, using modeling wax or clay to keep it in position. The clay of course should always be in position on the outside of the wood and should never be in the space into which the plaster is to be poured.
PLASTER HARDENS SLOWLY IN COLD WEATHER and hardens rapidly in a warm temperature. Salt added to plaster will cause it to set more rapidly and to harden more firmly. ”

“ 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. ”

“ Plaster moulds were also used for moulding pottery lamps. The oiling of plaster was done on painted plaster statuettes, so as to make them waterproof. They can still be scrubbed in water without disturbing the colour.
The most artistic use of plaster was for the modelled heads, which were placed on mummy cases in Roman times. Though most such works were rather crude, some are found which show real ability of portraiture. In fig. 135 we have a sympathetic study of the face of a young man. The lips are beautifully true, the modelling of the cheek is quite natural, the nose and brow well formed ”

“ The action of plaster is often puzzling. From the fact that where land has been strongly limed, a small quantity of plaster applied shows such decided benefit, there would seem plausibility in Liebig's theory that its effects must be traceable not to the lime, but to the sulphuric acid. ”

“ Settle a board, cut to approximate body outline but much smaller, into the unset plaster and press the flap edges of the skin back together over the board, molding edge of back and belly to round back away from trimmed edge of mold. This must be all done with accuracy before the plaster sets, but you will find it gives enough time. Do not work in a strung-up, nervous way.
When the plaster is set hard, remove fish from mold, hold it upon palm of left hand and tack edges of skin to back-board. (For general details of this method see Fig. 21) . ”