GELATINE (Fr., Gelatine ; Ger., Gallerte, Gelatin) A colloid of extremely complex nature, con taining carbon, hydrogen, nitrogen, and oxygen, with a small proportion of sulphur. It is known commercially in fine shreds or thin, flat sheets, marked with the diamond pattern of the strings on which it is dried. Photographic gelatine is usually prepared from selected hides, whereas inferior sorts are prepared from bones, tendons, and cartilages. It is insoluble in cold water, which it absorbs and then swells up to a slimy mass ; it is soluble in all proportions in hot water, but insoluble in alcohol and ether. The principal constituents are two substances known as glutine and chondrine ; the former is not precipitated by the alums, whilst the latter is, and gelatines rich in chondrine are the best for photographic purposes. A simple test for this is to add to a warm so per cent. solution of gelatine an equal volume of a saturated solution of chrome alum, when the solution should in stantly set to a jelly. It is only possible to indicate the general characteristics of a gelatine suitable for photographic purposes, as the true test of its suitability is to make a practical trial with a small batch of emulsion. A good gela tine should absorb not less than six times its weight of water, and not much more than twelve or fifteen times.
There are three kinds of photographic gela tine : hard, medium or middle hard, and soft. Hard gelatines should not melt below 82° F.
(nearly 28° C.) ; if they do not melt at 88° F. (31° C.), there is a risk of their having been hardened with alum. Soft gelatines should melt at from 62° to 75° F. (say 17° to 24° C.). A i per cent. solution should set to a jelly when cooled down to 56° F. (i3.3° C.), and remain without any sign of putrefaction for twenty four hours.
The determination of the melting point is somewhat difficult, anal Child Bayley has sug gested an excellent practical device here shown in the testing position. This may be made of copper or zinc, and the sloping portion is to prevent heat from the Bunsen burner passing direct to the front of the tank. Across the front of the tank should be scratched a line about i in. from the top, and on this line should be placed some discs of gelatine. Gummed labels are cut into strips about f in. wide and about
if in. long, and their ends are then gummed together, with the gummed surface outside, so as to form rings. The tank should be placed with the front and marked surface up, and the paper rings placed on the line and then carefully filled by means of a pipette with a warm so per cent. solution of gelatine. When the gelatine is thoroughly set, the rings should be cut down with a sharp knife and stripped off, and the tank set upright and filled with cold water, and this heated by means of the Bunsen burner. The discs must be carefully watched, and when they begin to melt and run down over the line the temperature should be noted. A mean of six trials may be taken as correct.
Another method is to use a thermometer in a very narrow test tube, just s mm. ( in.) wider all round than the thermometer bulb, which should be of elongated shape. Then fill the tube with the gelatine solution and, while hot, immerse the thermometer well into the tube, and set. Afterwards place the tube in warm water, and gradually raise the temperature ; then when the gelatine melts, the tube will drop off, and the temperature can be noted.
Gelatine is used for preparing baryta paper, for emulsions, both negative and positive, in collotype, photogravure, and other photo mechanical processes, and it is the chief in gredient of an excellent mountant.
Solutions of sulphocyanides and barium chloride dissolve gelatine in the cold, as do also acetic, oxalic, hydrochloric, and sulphuric acids. Zinc chloride and chloral hydrate destroy its setting power. The setting is increased by the alums, magnesium sulphate, and numerous other salts. It forms 4 compound, gelatinate of silver, which is sensitive to light, with silver nitrate.
In process work, gelatine has numerous uses —namely, for coating collotype plates, for pre paring photo-lithographic paper, for the carbon tissue in the photogravure process, for the making of gelatine reliefs, for use as a substratum on glass plates, for the making of films for shading mediums, for making litho-transfer papers and films for tracing, for stripping negative films, for making colour filters, for glazing prints, etc. Particulars of these applications and uses are given under the respective subject headings.