Pure celluloid is nitro-cellulose with a composition that is nearly allied to gun-cotton, and is prepared by submitting cellulose to the action of a mixture of nitric and sulphuric acids. Both cotton and paper are varieties of cellulose, and are used for making nitro-cellulose. Cheap solvents of cellulose are a mixture of methylated spirit and camphor acetone, and a mixture of amyl acetate and petroleum spirit. The two former are usually employed for rendering the celluloid plastic so that it can be moulded into shape, while the latter is used for making a transparent solution.
In making celluloid the first operation is the preparation of a nitrated cotton, which is similar to gun-cotton. Two parts of strong sulphuric acid are mixed with 1 part of concentrated nitric acid in a shallow stoneware vat. The cotton or tissue paper is weighed out and put into small perforated jars, each provided with a lid, and when the temperature of the acids is 60° C., the jars are placed in the acid, which penetrates through the perforations to the cotton. After about forty-five minutes the jars are removed and placed in a vat of water, and fresh water is run through the vat to wash out the acid, the last traces being removed by a little ammonia or carbonate of soda. The nitrated cotton is next dried at a low temperature, and is mixed with camphor and spirit of wine or with acetone which will soften it. The mass is then kneaded and pressed into square blocks, Moulded articles are made from the plastic celluloid, but some articles are cut or turned from the solid. The celluloid soon hardens in contact with air owing to the loss of the solvent. As a rule, a little castor oil is mixed with the celluloid to make it more flexible.
In one method of making celluloid the pyroxyline is obtained from cigarette paper of very good quality. This paper in rolls 13 in. in width and 33 to 35 lb. in weight, is unrolled mechanically and immersed in a mixture of 5 parts of sulphuric acid of 66°B., with 2 parts of nitric acid of 42° B., kept at a temperature of about 85° F. (35° C.). The cellulose of the paper after 12 or 15 minutes' immersion, becomes changed into nitrocellulose, which is soluble in a mixture of alcohol and ether. The solubility is tested by a hasty trial. The product is then removed from the acid bath, the liquid is expressed from it, and it is thrown into water. After a preliminary washing it is placed along with water in a pulp vat, and triturated for 2½ to 3 hours in order to obtain a homogeneous paste. The pyroxylinethen has to undergo bleaching, the operation being effected by the use of a solution of potash permanganate. When contact with this reagent has been sufficiently prolonged, the excess of permanganate is eliminated by washing. Then the mass is treated with a solution of sulphurous acid in order to dissolve the oxide of manganese, and the operation is finished by a series of washings in water. The whitened pyroxyline is put into boxes lined with filtering cloths, and then submitted to mechanical drying. On being taken from the hydro-extractor, the material only retains about 43 per cent, of water and is found to be in a state fit for the preparation of celluloid. It is then passed through a mill having metallic runners, first alone, and afterwards mixed with the proper quantity of camphor (which has been first rolled), and with colouring matter if it be proposed to make opaque celluloid. After a dozen successive grindings, the mixture is moulded in a metal frame, by hydraulic pressure, so as to give slabs, that are arranged and pressed between 10 to 12 sheets of thick bibulous paper. The water in the mixture is then gradually absorbed by the paper, the latter being renewed 12 to 15 times. The slabs, thus dried and reduced to a thickness of about 1/10 in. are broken up between bronze cylinders armed with teeth. The pieces are allowed to macerate for about 12 hours with 25 to 30 percent, of alcohol of 96°, and then the colouring matters soluble in alcohol are added, if it be proposed to have transparent coloured celluloid. The mixture is then passed through the rolling mill, the cylinders of which are heated to about 122° P. (50° C.). The operations are performed upon 12 to 28 lb. at once. The rolling takes 25 to 35 minutes, and terminates when the material has become homogeneous. There is then obtained a sheet of about ½ in. in thickness, which is cut into pieces of 23½ by 31½ in. The latter are superposed on the table of a hydraulic press in a metallic box having double sides and being tightly closed, and allowing the heating to be done by a circulation of hot water. The box is heated to 140° F. (60° C.) during the whole duration of compression, which lasts about 4 hours. At the end of the operation, a current of cold water is passed into the box, the pressure is removed, and there is obtained a very homogeneous block of celluloid about 5 in. thick. The blocks are taken to the planing machine, and shaved into sheets varying from 0 â€¢ 008 to 0'12 in. in thickness according to the purpose for which the product is designed. These sheets are next placed in a ventilated stove, heated to 131° F. (55° C.), where they remain from 8 days to 3 months, according to their nature and thickness.
In this description it has been only a question of celluloid of a uniform colour, either transparent or opaque, imitating pale tortoise-shell, coral, ebony, turquoise, etc. When it is desired to obtain a product to imitate amber, jade, spotted tortoise-shell, etc., each of the ingredients, of uniform colour, which is to compose the material, is prepared separately and then mixed, to be afterwards united by pressure.
Celluloid is also formed of divided cotton waste, or similar substance, dissolved in one or more of the following solvents: Vegetable naphtha, nitrobenzol, camphor, alcohol, and glacial acetic acid. Sufcient of these solvents is used to make a soft, plastic mass, which is then or subsequently subjected to hydraulic pressure, and mixed with oils, gums, and colours. By this means, any degree of hardness or flexibility can be given to it, and it can be made white and transparent or of brilliant colour. It can be made as hard as ivory, or retained in so soft a condition as to be spread in layers over textile fabrics much in the same way that paint is laid on. The substance is water-proof, acid-proof, and air-proof. It can be worked in a soluble, plastic, or solid state. It can be pressed and stamped, planed as wood, turned in a lathe, cut with a saw, carved, inlaid, woven into fabrics, or applied as a varnish. It can be made either transparent or opaque, and is capable of bearing a high polish. When dyed, the dye runs through the whole substance, and cannot, consequently, be rubbed or washed off.
The manufacture may be divided into two distinct stages:
The production of the so-called "pyroxyline;"
The treatment of this compound with solvents, in order to make it plastic, and give it other desired qualities.
The first stage of the process suffers but little variation. A convenient quantity of cellulose or woody fibre, such as disintegrated cotton waste, paper, etc., is fed into an open vessel called a " converter," and treated with an acid mixture composed of 1 part of nitric acid, sq. gr. 1.420, and 4 to 5 parts of sulphuric acid, sq. gr. 1.845, mixed in a separate vessel, and kept as cool as possible. The acid mixture is pumped or forced up into the converter, while the fibrous substance, previously placed in a hopper over the converter, falls gradually into it by an opening in the top. The charging of the cotton into the converter occupies about 10 minutes, and at the end of 20 to 30 minutes at most, it is chemically converted into the so-called pyroxyline or nitro-cellulose. This, together with the excess of acids adhering, is then allowed to fall through an opening in the bottom of the converter, and is caught in a large box provided with a false bottom of perforated iron or wire gauze, at about 6 in. above the real bottom. On this the wet mass remains for an hour, to admit of the excess of acids draining away as far as possible; the still remaining impregnations of acid are then expressed by placing the pyroxyline in a cylinder with a perforated bottom, and subjecting it to hydraulic pressure. The result is a hard cylinder of pyroxyline, containing from 5 to 20 per cent, of the acid mixture, in which state it is stored for future use. When required, the cylinders of pyroxyline are torn into dust by special machinery, such as that employed for grinding paper pulp, and the disintegrated mass falls into a large tank, where it is well washed with water, to remove the last traces of acid. It is then again placed in the cylinders with perforated bottoms, and pressed to remove the water, leaving in from 5 to 20 per cent. The solid cylinders of soluble pyroxyline are again broken up in the disintegrating machine, preparatory for the treatment with solvents, which forms the second stage of the manufacture.