Sinee the Paris Exhibition of 1868, where Parkes obtained a prize medal for his show of articles manufactured from celluloid, and where the substance was first named Parkesine, the Americans have made considerable advance in the manufacture. A modification worth mentioning consists in employing camphor as tlae solvent of the pyroxyline. The lattcr is first reduced to a fine pulp, by grinding it in water in a maehine such as is used for grinding paper pulp, and to the pulp thus prepared, pulverized camphor gum is addetit in the proportion of one part by weight of camphor to two parts pyroxyline wheu dry. At the same time, is added any desired material for colouring the oolluloid, or modifying its specific gravity. Tho camphor is comminuted by grinding in water, trituration, or solution and precipitation. The camphorated mass is placed in 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 149° (300° F.), or the pulp in contact with the mould will become charred ; sometimes 66° (150° F.) suffices. The mixtmn should remain in the mould under heat and pressure till the conversion of the pyroxyline is completed ; 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.
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, dic., are used, and which would impair the strength, durability, purity, and firmness of texture essential in dental plates. Fifty parts at les.st by weight of camphor are added to one hundred parts of soluble pyroxyline; more camphor makes the compound more plastic. The plates formed are placed in a drying room heated to 65°-82° (150°480° F.), the latter being the maximum, to drive off the camphor. A temperature above 93° (200° F.) 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 unchange able ; it has no unpleasant taste ; it is absolutely non-injurious, and never shrinks or warps after setting.
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 conve nient sized cakes, about in. to 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 com pression 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 ernpty, a gradual discharge of the celluloid is effected, in the form of a continuous has or sheet as desired. The cylinder is unequally heated, in such a manuer 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 e,ompletely 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 hy a steam or hot water jacket. The former is supplied hy the escape pipe of the hydraulic engine. In the discharge ond 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 tbe central core. The discharge pipe is passed through an equalizing warm-water vessel, which keeps it sufficiently warm to prevent the material in contact with the inner surface cooling faster than the central portion, as the unequal cooling, and consequent unequal consistency, of the different portions of the material would cause the central and softer portion to move faster than the outer and harder portion, thus destroying the homogeneity of the mass, and rendering the surface rough and broken.
The soluble pyroxyline is first comminuted in a wet condition, and the excess of water is pressed out. The camphor and colours, as required, are then thoroughly incorporated with it by tbe mixing rollers. The compound, thus prepared, is tbrmed into cakes by means of a mould and follower, the bottom of the mould being made separate, and serving to transfer the formed cake to the pile. These cakes sre preferably made about 12 in. square, and to in. thick; it would be difficult to properly absorb the moisture from thicker cakes. These arelaid up in a pile with layers of blotting-paper between them, and are then placed in a hydraulic press to remove the water as far as necessary. During this process, the compound is protected from the air, to prevent evaporation of the camphor, and to avoid the chance of ignition. The rapidity with which this drying is effected ensures great saving of time and space. The dried material is ready for conversion into celluloid, for which purpose, it is transferred, with the solvent, to the converting cylinder. The heat from the steam-jacket surrounding the lower portion of this cylinder brings about the conversion of the pyroxyline to a homogeneous mass of celluloid, which is then forced through a discharge nozzle, constructed according to the desired form of the product, e.g. in bars or sheets, or directly into a mould of the article to he manufactured.