| Distilled water | 8 | parts. |
| White of egg—whisked | 7 | ” |
| Silicate of soda (of the consistency of syrup) | 3 | ” |
This mixture should be well beaten to a froth or put in a bottle with pieces of broken glass, well shaken, and placed aside to settle for half-a-day. Then filter through paper, which should be replaced so soon as the pores become choked. To facilitate the operation the filter paper should before use be moistened with distilled water. The surface of the plate should be flooded with water, and coated as evenly as possible with the above solution, a small glass rod being made use of to assist the flow. Air bubbles must be removed, and by quickly tilting the plate the superfluous solution run off; this should not be used to coat other plates without being again carefully filtered. If the substratum be thin, it is not necessary to wash the plate after drying, but after standing aside two days to harden, with dusting and warming, they may be coated with the chromated solution of gelatine. If small crystal formations appear on the surface the plate should be well washed and again dried before proceeding further. The following formula for substratum is due to M. Thiel, senior; it contains less silicate, and the washing before applying the sensitive coating is never necessary:—
| Distilled water | 36 | parts. |
| White of egg | 20 | ” |
| Silicate of soda | 6·2 | ” |
Stale Beer.—Albumen is sometimes replaced by beer, from which by frequent outpourings the carbonic acid has been removed, and to which caustic potash has been added until it fails to show an acid reaction when tested with blue litmus paper (about three to five grammes of caustic potash to the litre of beer will be necessary). To a litre of beer so prepared is added 1/3 kilo. of silicate of soda. The main points in this and also the following operations are careful filtration, even coating and drying in a clean place entirely free from dust. Plates previously used or somewhat scratched must before further use be thoroughly cleaned and the face reground with the finest emery powder.
The Sensitising Solution.—This is manifestly the most important matter in the Collotype process, for on it depends not only the durability of the printing plate, but also the method of procedure in the subsequent operations. It may be here intimated that plates prepared by the very best formula cannot be expected to yield good impressions if the whole of the manipulations are not carried out with the greatest care, and it should be noted
1. That neither too much nor too little of the chromated gelatine should be allowed to remain upon the plate. It is impossible to lay down a hard and fast rule, and the exact amount must be determined by the experience of the operator. Of equal importance, and naturally connected with this operation, is
2. The even distribution of the solution, the flow of which may be assisted or directed by the aid of a small glass rod, and by allowing it to run off more or less at each of the four corners of the plate. Considerable experience has demonstrated that the coating is applied most easily and in the evenest manner by resting the plate—previously warmed to about 45° C.—as horizontally as possible on the cork of a heavy bottle, the latter standing in the centre of an earthenware dish. A sufficient quantity of the solution, carefully filtered, and heated to 45° to 50° C., is then poured on to the centre of the plate and rapidly spread over it with a glass rod, previously well warmed. So soon as the surface of the plate has been entirely covered a further supply of the hot gelatine solution is quickly poured on and partially run off at each corner. A scarcely perceptible inclination should be given to the plate, otherwise the layer will be too thin; it ought to be about the thickness of four or five sheets of Rive paper of 8 kilo. per ream. It should be borne in mind that the addition of isinglass will necessitate a comparatively thinner layer than when gelatine is used alone.
To obviate the gelatine coating becoming uneven great care must be exercised in removing the plates—when coated—to the drying-box, the levelling screws of which have been adjusted with great accuracy. The box should be heated to a temperature not exceeding 50° C.
To satisfy the adherents of isinglass, two eminently satisfactory formulæ are here given—
| With Gelatine only. | ||
| Collotype gelatine | 6 | grams. |
| Water | 700 | ccm. |