COAL TAR.—A tar produced in the destructive distillation of bitumenous coal, and from which many of the aniline colors—napthalene, phenol, etc.—are prepared.
COATING.—The term is largely used in photography to define the act of covering glass, paper, etc., with a film of any kind. In the dry-plate process the glass is coated with a gelatine emulsion (this is described under Dry Plates) and the paper is coated with a layer of albumen or gelatine. The simplest method of coating glass is to pour the necessary quantity of the solu tion on to the top right-hand corner of the glass, and by moving the glass in different slanting directions cause the solution to run to the top left-hand corner, and from there to the bottom left-hand, and finally to the lower right-hand corner, from whence the superfluous solution can, if necessary, be poured off into the bottle, or if the whole quantity of the solution is 'required to be kept on the glass the latter is, after coating, placed on a level stand. In coating glass with gelatine it should be warmed to about the same heat as the solution. Instructions for covering glass with collodion will be found in the description of the collodion process.
The coating of paper is a very simple matter if properly carried out. In coating with col lodion the paper should be laid upon a sheet of glass, and the edges turned up all around. It will
then be an easy matter to coat in the same manner as with glass. Another method is to place the paper between two suitable frames hinged together, which serve to hold the paper and stretch it tight. The simplest method of coating paper with gelatine is to place the solution in a dish, and draw two pieces of paper placed back to back through it. The pressure of the liquid keeps the two papers together, and there is no fear of the solution getting in between. A thin coating may be obtained by passing the paper slowly through the emulsion, and a thick film by drawing rapidly. Another way is to float the paper face downwards upon the solution placed in ft flat-bottomed dish. With this method the formation of bubbles is extensive, and they should be carefully taken out with a camel-hair brush dipped in the solution, and the paper replaced into position upon the emulsion. Another method of coating paper with gelatine is to first coat a glass, previously rubbed with chalk, with the emulsion, and lay the sheet of paper over it. The gelatine adheres to the paper when dry, and leaves the glass with a very high polish. Vari ous methods of coating paper will be found under Collodio and Gelatino Chloride Printing and Processes.