The practical method of carrying this out is as follows: - A large zinc vessel is required with a tight-fitting cover. In each vessel is a grating made of strips of zinc, resting on feet 1 1/4 in. high. This vessel is 2/3 tilled with soft water at 54° to 77° F. (12° to 25° C), and to every 25 gal. of water is added 8 lb. fused or 14 lb. crystallised pure hy-drated barium oxide, also 0.6 lb. lime previously slaked in water. The solution stands about 4° Beck (1.0241 sp. gr.). As soon as the baryta water gets clear, it is ready to receive the casts. They are wrapped in suitable places with cords, and after removing the scum from the baryta bath, are dipped in as rapidly as possible, face first, and then allowed to rest upon the grating.
Hollow casts are first saturated by rapid motions, then filled with the solution and suspended in the bath with the open part upwards. After the cords are all secured above the surface of the liquid, the zinc vessel is covered. The casts are left in it for 1 to 10 or more days, according to the thickness of the waterproof strata required. After taking off the cover and removing the scum, the plaster casts are drawn up by the strings, rinsed off with lime-water, allowed to drain, carefully wiped with white cotton or linen rags, and left to dry, without being touched by the hands, in a warm place free from dust. The same solution which has been used once can be employed again by adding a little more baryta and lime.
Of course this process can only be applied to casts free from dust, smoke, dirt, coloured particles of water, rosin, varnish, soap, animal glue from the moulds, or sweat from the hands. To prevent the casts getting dust upon them, they should be wrapped in paper when taken from the mould, and dried by artificial heat below 212° F. (100° C). If, in spite of every precaution, the casts when finished show single yellow spots, the latter can be removed in this manner: - The perfectly dry, baryta ted casts, saturated with carbonic acid, are painted over with water and oil of turpentine, then put in a glass case and exposed to the direct rays of the sun.
(6) Process With Silicate Of Potash Solution
This depends upon the conversion of the lime sulphate into lime silicate, an extremely hard, durable, insoluble compound, and is accomplished by the use of a dilute solution of potash silicate containing free potash. To prepare this solution, first make a 10 per cent, solution of caustic potash in water, heat to boiling in a suitable vessel, and then add pure silicic acid (free from, iron) as long as it continues to dissolve. On standing, the cold solution usually throws down some highly - silicated potash and alumina. It is left in well-stoppered glass vessels to settle. Just before using, it is well to throw in a few bits of pure potash, or to add 1 or 2 per cent, of the potash solution. If the plaster articles are very bulky, this solution can be diluted to 1/2 with pure water.
The casts are silicated by dipping them (cold) for a few minutes into the solution, or applying the solution by means of a well-cleaned sponge, or throwing it upon them as a fine spray. When the chemical reaction, which takes place almost instantly, is finished, the excess of the solution is best removed with some warm soap-water or a warm solution of stearine soap, and this finally removed with still warmer pure water.
The casts, which can be immersed or easily moved around, may be treated as above when warm; a very short time is required, but some experience is necessary. In every case it is easy to tell when the change is effected, from the smooth dense appearance, and by their feeling when scratched with the finger-nail. It is not advisable to leave them too long in the potash solution, as it may injure them. A little practice renders it easy to hit the right point. The fresher and purer the gypsum and the more porous the cast, the more necessary it is to work fast. Castings made with old and poor plaster of Paris are useless for silicating. These silicated casts are treated with soap as above.
In washing plaster casts prepared by either method, use a clean soft sponge, carefully freed from all adherent sand and limestone, wet with lukewarm water, and well soaped. They are afterwards washed with clean water. They cannot, of course, be washed until thoroughly dry and saturated with carbonic acid. The addition of some oil of turpentine to the soap is useful, as it bleaches the casts on standing. The use of hot soapsuds must be avoided.
Following is a new process of hardening plaster so as to adapt it to the construction of flooring in place of wood, and to other purposes for which it cannot be used in its ordinary state on' account of its want of hardness and resistance to crushing. Julte recommends the intimate mixture of 6 parts plaster of good quality with 1 part finely sifted, recently slaked white lime. This mixture is employed like ordinary plaster. After it has become thoroughly dry, the object manufactured from it is saturated with a solution of any sulphate whose base is precipitated in an insoluble form by lime. The sulphates best adapted for the purpose, from every point of view, are those of iron and zinc.
With zinc sulphate, the object remains white, as might be supposed. With iron sulphate, the object, at first greenish, finally assumes, through desiccation, the characteristic tint of iron sesquioxide. The hardest surfaces are obtained with iron, and the resistance to breakage is 20 times greater than that of ordinary plaster. In order to obtain a maximum of hardness and tenacity, it is necessary to temper the limed plaster well in as brief a space of time as possible, and with no more water than is strictly necessary. The object to be hardened should be very dry, so that the solution employed may penetrate it easily. The solution should be near the point of saturation, and the first immersion should not exceed 2 hours. If immersed too long, the plaster would become friable.