One of the first solvents employed on a large scale was wood naphtha, distilled with chloride of lime, in the proportion of 1 gallon of the naphtha to 2 to 6 lb. of fused chloride ; the more of the latter used within these limits, the stronger will the solvent be. The first 3 quarts of the distillate are eollected for use ; the remainder is caught in a separate vessel so long as any spirit comes over, and is distilled again at the next operation with more fresh materials. The deposit remaining behind in the still ie chloride of lime, dissolved in water, and contaminated with some tarry matter. It is run into an open iron vessel, heated by a fire beneath, to evaporate away the water, and fuse the chloride of limo ready for re-use.
Tho solvent thus prepared is applied to the pyroxyline, in such proportions as to make a pasty mass; but if used alone, the resulting celluloid would soon become hard and brittle. To avoid this, a certain quantity of oil is added to the mass, and kneaded up with it in the mixing machine. The proportiun of oil will vary with the desired degree of toughness. To produce a consistency suitable for coating telegraph wires, or for spreading on textile fabrics, the proportion of oil may equal half the weight of the pyrnxyline. If the oil used be first treated with chloride of sulphur, the compound is rnuch more elastic. It is thus treated by mixing with 2 to 10 per cent. of liquid chloride of sulphur, according to the degree of elasticity required ; but the chloride of sulphur ehould first be diluted with an equal bulk or more of mineral naphtha, or hisulphide of carbon, to prevent too violent action. The prepared oil is compounded with the dissolved pyroxyline, in various proportions, but seldom exceeds 20 per cent.
To increase the hardness and modify the colour of the product, sometimes a small portion of gum or resin, such as shellac or copal, is added, but seldom more than 5 per cent. The wood naphtha may be replaced by alcohol, and the chloride of lime by chloride of zinc, or manganese fused or dry. For economy sake, a small quantity of light spirits from coal may be mixed with the solvent, but it is not preferable. For the oil, may be substituted gum ballata, treated with chloride of sulphur—usually not more than 5 per cent. of the chloride. The combustibility of celluloid thus made may be corrected by tho addition of chloride of zinc, or tungstate of soda. Ten per cent. of either effectually prevents burning ; but usually much less will do, especially when pigments are used. The same end is attained by employing iodide of cadmium, oxalate of zinc or manganese, or gelatine dissolved in glacial acetic acid.
A practical difficulty attending the use of tho above process is that the solvents employed are so volatile. Large masses of celluloid may be prepared better, quicker, and with less consumption of solvent by adopting nitro-benzol, aniline, or glacial acetie acid, and the celluloid may then be worked in the open air. The ordinary volatile solvents are improved by the addition of camphor.
When using nitro-benzol, the commercial article should be distilled off hydrochloric acid or chloride of lime, say 6 lb. of either to 1 gall. of nitro-benzol, which is thus rendered purer and sweeter. One hundred parts of pyroxyline are then moistened with ordinary solvent—preferably naphtha distilled off ehloride of lime—and the excess of solvent is removed by hydraulic pressure. The other solvent is then added, in the proportion of 10-50 parts of prepared nitro-benzol or aniline, together with 10 to 50 parts of camphor, and 150 to 200 parts of oil, preferably cottou-seed or castor. This mixture is formed between rolls, heated by steam being admitted into them, till the whole forms a well-combined dough or paste, which will be more or less stiff, according to the quantity of solvent used. For a hard compound, the oil should be less than the pyroxyline ; for a soft one, it should exceed the latter—say, 150-200 oil to 100 pyroxyline. In making celluloid with glacial acetie acid, 100 parts of pyroxyline are dissolved in 50 parts of the acid, for a stiff paste ; or 100 to 300 or more parts, for a serni-fluid consistency.
Usually the pyroxyline requires to be dried before dissolving it. The conduet of this operation on large quantities requires much care and time, and a very large space of drying room, so that great advantages, on the score of cost, ease, and safety, aro to be derived from dissolving it in a moist state. For this purpose, the pyroxyline is prepared in the usual way, and when rendered eoluble by the addition of hydrocarbon solvents, it is taken out of the acids and placed in a hydraulic machine, by which as much as possible of the acid is expressed. The cake of pyroxyline is then taken out of the press, opened out, put into a centrifugal washing machine, and washed with water until clean ; then the rotation of the machine is con tinued, to throw out the surplus water. Or the pyroxyline, after con version, may be placed in the centrifugal machine, and there deprived of the acids, and, without removal, he thoroughly washed, by admitting a copious supply of water, the operation occupying from a few ininutes to an hour. When the pyroxylin° does not contain more than 5 to 10 per eent. of water, it is dry enough for solution in naphtha, &e.