Photo-engraving, Photo-etching and Photo-lithography in Line and Half-tone / Also Collotype and Heliotype

by W. T. Wilkinson

The wet plate is covered with this two or three times, and then placed on a rack to dry. When all the plates in the batch are coated the rack should be removed to the drying oven and the plates dried in sitû, not lying down.

Be as careful as possible to prevent any of the beer and silicate mixture from getting on the back of the plate, as it interferes with the running of the image afterward.

Plates may be prepared with the preliminary coating in quantity, as the coating hardens and improves by being kept.

When the coating on the plates is quite dry rinse them under the tap and again dry them, this time quite spontaneously. Lastly, they are placed upon the slab of the drying oven to get sufficiently warm before being coated with the sensitive mixture.

The sensitive mixture must be made exactly as follows, as everything depends upon the time and temperatures named: First of all, take thirty ounces of pure water and add ten drops of a saturated solution of chrome alum; shake up well and allow to stand ten minutes; if the water is very slightly opalescent, it is {165} quite satisfactory, but if there is any tinge of green then too much chrome alum has been used, and it must be thrown away and another lot made.

Then add two and one-half ounces of fine gelatine and allow it to soak until soft, then place it in the “Baine Maire” and melt the gelatine, stirring all the time. Raise the temperature to 125° F. and keep it thus for fifteen minutes; next add 150 grains of bichromate of potassium in fine powder, stirring until dissolved, and raise the temperature to 150° F. Strain the mixture through muslin into a porcelain jar and allow it to cool, keeping it in jelly at least twelve hours before using it.

This sensitive mixture does not work well if used to coat plates with directly it is made; therefore, it must be made up some time in advance. It keeps well, in fact improves, with keeping, up to about seven days. In remelting for use, only melt just sufficient for the plates to be coated, as it does not improve it to melt it too often.

Plates coated with the above mixture are dried at a temperature of from 100° to 120° F. in from two hours to three hours, and have a splendid grain.

Here is another formula the mixture from which does not keep, but must be made and used directly. It is the formula most frequently used by the writer. Plates prepared with it can be dried, at lower temperatures, in from twenty to fifty minutes. It is also the best formula when copper plates are used instead of glass, of which mention will be made at the end of the chapter.

Soak two and one-half ounces of gelatine in fifteen ounces of water until quite soft, then place all in the “Baine Maire” and melt at as low a temperature as possible; when melted add 100 grains of bichromate of potash and 50 grains of bichromate of ammonia both in fine powder, stirring until dissolved; now put in the thermometer, and raise the temperature to 140° F. and keep it there for ten minutes; then allow to cool to 125° F. and add the following mixture, stirring vigorously the while: fifteen ounces of alcohol and five ounces of a saturated solution of borax in alcohol. Mix the two before pouring them into the hot gelatine. With some gelatines the addition of this mixture will cause a slight coagulation of the gelatine, and it will stick to the stirring rod; therefore, the solution must be stirred until the coagulated gelatine is redissolved; then strain it through muslin into the pourer and at once use it by pouring it over the warm plates.