(4) A practical difficulty attending the use of the 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 acetic acid, and the celluloid may then be worked in the open air. The ordinary volatile solvents are improved by the addition of camphor.
(5) When using nitro-benzol, the commercial article should be distilled off hydrochloric acid or chloride of lime, say 6 lb. of either to 1 gal. 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 chloride of lime - and the excess of solvent is removed by hydraulic pressure. The other solvent is then added in the proportion of 10 to 50 parts of prepared nitro-benzol or aniline, together with 10 to 50 parts of camphor, and 150 to 200 parts of oil, preferably cotton-seed or castor. This mixture is forced 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 to 200 oil to 100 pyroxyline. In making celluloid with glacial acetic 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 semi-fluid consistency.
(6) Usually the pyroxyline requires to be dried. before dissolving it. The conduct 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, are to be derived from dissolving it in a moist state. For this purpose, the pyroxyline is prepared in the usual way, and when rendered soluble 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 continued, to throw out the surplus water. Or the pyroxyline, after conversion, may be placed in the centrifugal machine, and there deprived of the acids, and, without removal, be thoroughly washed by admitting a copious supply of water, the operation occupying from a few minutes to an hour.
When the pyroxyline does not contain more than 5 to 10 per cent, of water, it is dry enough for solution in naphtha, etc.
(7) Instead of evaporating the solvent used in making the celluloid, it may be removed by precipitating the pyroxyline by means of water, mineral naphtha, etc. There is thus obtained a semi-solid mass, containing a small quantity of the solvent, which is passed through grinding rolls or other disintegrating machinery, and then worked up as usual. The celluloid is placed in a vessel containing a revolving agitator or beater, together with water or mineral naphtha in the proportion of 1 lb. of celluloid to 1 qt. of liquid, and the agitator is set in motion. After a short time, the celluloid is let out in a curd-like form, and submitted to pressure (not excessive), to separate the liquid. It is convenient to place it in a vessel of cylindrical form, and about 12 in. in diameter, provided with a movable and perforated bottom, covered with several layers of wire gauze. This is filled with the curdlike celluloid, upon which a plunger is forced down, and a cheese-like block is produced. This is rolled down between rollers heated by steam, and any pigment, etc, is worked in by them at the same time, the mixture being passed through and through till perfected. The solvent used is preferably mineral naphtha, as free from smell as possible.
The solvent taken up by the liquid is recovered by distillation, if water has been used; but in the case of naphtha, the greater part will separate on standing, and may then be decanted off.
(8) In order to make celluloid in imitation of pearl, fish-scales are mixed with the dissolved pyroxyline, and a pearly-lustrous material is thus produced. To form a thin veneer of artificial pearl, 1 part of this material is mixed with 100 parts of pyroxyline. The latter is first ground with a solvent and oil to a doughy consistency, the pearly compound is then added, the solvent is separated, and the celluloid is worked up in the ordinary way. But when the celluloid is required in a semifluid condition* the solvent must be increased instead of removed, and a much larger proportion of the pearly material will be needed. The best lustre produced is that made in France from the scales of the whiting. In producing a coloured celluloid, preference should always be given to dyes - especially aniline - rather than pigments. The brightest and most delicate colours may be imparted.
(9) To manufacture celluloid so as to resemble ivory, the following plan is adopted: - The celluloid is made without any colouring matter, and is kept as clean and white as possible; when in a doughlike state, it is rolled into sheets 1/16 in. thick. Meantime another celluloid is prepared containing carbonate of strontia in the proportion of 1 part to about 200 parts of pyroxyline, and this also is rolled into sheets. These sheets are placed alternately one over another, to produce any desired grain. A good plan is to lay a transparent and an opaque sheet one over the other, and roll them up together, then take the roll and twist it, pass it through heated rollers, and roll it down into a slab, for cutting knife handles or whatever may be required.
(10) In working white or light-coloured celluloids, or those in imitation of pearl or ivory, it is necessary that porcelain or glass vessels should be used in its manufacture as far as possible, and the rollers through which it is passed must be covered with platinum, as other metals are acted upon by the celluloid. A coating of platinum 1/16 in. thick will be very durable.