(11) For producing a white celluloid, without unduly increasing its specific gravity, the dissolved pyroxyline and other ingredients are mixed with white starch, either from wheat, rice, potatoes, etc, or with arrowroot, tapioca, or other amylaceous substance, or with wheat flour, or with cotton ground and bleached.
(12) To remove the solvent remaining in the celluloid, which imparts a slight odour to articles made of it, and renders them liable to shrink in course of time, such articles are seasoned, while in a partially manufactured state, by soaking them in a liquid which will dissolve out the solvent without affecting the pyroxyline, such as bisulphide of carbon, chloride of lime, or benzol. The articles so soaked are afterwards placed in a vessel from which the air is exhausted, and the curing liquid is thus drawn out, condensed, and recovered.
(13) The process employed for making billiard balls is as follows: - To 100 parts of pyroxyline, dissolved, ground, and strained as usual, are added 300 to 500 parts of the usual solvent - alcohol 100 parts, naphtha 50 parts; 100 to 150 parts of arrowroot or starch; 50 to 100 parts of the best zinc-white. The solid matters are added to the plastic solution of the pyroxyline, and the whole is placed in a closed rolling or grinding apparatus, the rollers being heated by steam, and the compound is ground up till most of the solvent is driven off. The latter is recovered by conveying it through pipes to a Liebig's condenser. The mass is now about as stiff as clay, and may be moulded or rolled, and placed in a warm place for seasoning. When well seasoned, the ball may be turned. When less specific gravity is required, it is best to employ as much amylaceous substances as possible, they being lighter than the zinc. Ground and bleached cotton fibre may be rubbed up with the plastic pyroxyline, in the proportion of 100 parts disintegrated cotton to 300 parts pyroxyline paste.
(14) A modification worth mentioning consists in employing camphor as the solvent of the pyroxyline. The latter is first reduced to a fine pulp by grinding it in water in a machine such as is used for grinding paper pulp, and to the pulp thus prepared pulverized camphor is added in the proportion of 1 part by weight of camphor to 2 parts pyroxyline when dry. At the same time is added any desired material for colouring the celluloid of modifying its specific gravity. The camphor is comminuted by grinding in water, trituration, or solution and precipitation. The camphorated mass is placed in a mould, and heated to a sufficient temperature to liquefy or vaporize the solvent, and is then subjected to heavy pressure. The temperature should never exceed 300° F. (149° C), or the pulp in contact with the mould will become charred; sometimes 150° F. (66° C.) suffices. The mixture should remain in the mould under heat and pressure till the conversion of the pyroxyline is completed; it is then left to cool under pressure in the mould.
When first taken out, it has the consistency of sole leather; but is easily softened by heat till the camphor has evaporated, when it grows as hard as horn.
(15) For dental purposes, the transformation of the pyroxyline is effected by camphor, and without the use of fixed oils or fusible non-solvent gums, which are required to be combined with the material when ether, alcohol, etc, are used, and which would impair the strength, durability, purity, and firmness of texture essential in dental plates. 50 parts at least by weight of camphor are added to 100 parts of soluble pyroxyline; more camphor makes the compound more plastic. The plates formed are placed in a drying-room heated to 150° to 180° F. (65° to 82° C), the latter being the maximum, to drive off the camphor. A temperature above 200° F. (93° C.) will expand the material, and make it porous and brittle. It is said that this compound is lighter and stronger than dental vulcanite or indiarubber; its colour is the same as the natural gum, and is unchangeable; it has no unpleasant taste; it is absolutely non-injurious, and never shrinks or warps after setting.
(16) The following process is adopted in practice to dissolve the pyroxyline in camphor, eliminate the solvent, and form a solid mass of celluloid at one operation. The prepared mixture of soluble pyroxyline and camphor is first dried, by compressing the moist pulpy compound into convenient sized cakes, about 1/4 in. to 1/2 in. thick, and arranging them in a pile with intermediate layers of paper, or other absorbent material, and. subjecting the pile to pressure in a hydraulic press. By this means, the material is uniformly and sufficiently deprived of its moisture, while the compression of the material and exclusion of the air prevent all danger of ignition when exposed to the sun or the heated air of a drying-room. The mixture of pyroxyline and camphor is subjected to pressure by means of a plunger in a heated cylinder provided with a discharge nozzle or pipe, the cylinder being of sufficient length to cause the conversion of the pyroxyline to take place while the material is being gradually forced through it, so that by replenishing it as it becomes partially empty, a gradual discharge of the celluloid is effected, in the form of a continuous bar or sheet.
The cylinder is unequally heated, in such a manner that the mixed material will first be compacted in the colder portion, before the solvent is melted and the process of transformation commences. The air is thus allowed to escape more freely, and is more completely expelled, while the conversion of the pyroxyline is effected in another and hotter portion of the cylinder, as the mass is forced through it. The upper or receiving end of the cylinder is cooled by being surrounded by a cold-water jacket; and the lower or discharging end is heated by a steam or hot-water jacket. The former is supplied by the escape pipe of the hydraulic engine. In the discharge end of the converting cylinder is a central heating and distributing case, constructed with radial pins or projections, by which the material, before it escapes from the cylinder, is caused to pass through the annular space around the central core, and in contact with the heated surface of the cylinder, while the spurs or pins divide and mix the material, and at the same time serve to conduct the heat from the cylinder to the central core.