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

by W. T. Wilkinson

One of the best vessels to use as a pourer is an invalid’s cup with a spout at the side.

The above quantity of mixture is sufficient for sixteen plates 16 × 13 inches; {166} therefore, if only four are to be coated, which is the quantity that the drying oven to be described will take, the above quantities must be divided by four.

Now if the first formula be used, take sufficient of the jelly, melt it and strain it through muslin; then take one of the plates from the drying oven—which plate should be just warm enough to be comfortably held on the palm of the hand—carefully brush it over with a dusting-brush, so as to remove any particles of dust that may have adhered and balancing the plate on the palm of the hand, pour over it sufficient of the warm gelatine to just cover it. A 16 × 13 plate will take a little less than two ounces of the gelatine mixture. Take care not to spill any. Next place the plate upon the hot slab, and coat the rest in the same way. When all are done close the door of the oven and do not open it again until the plates are dry.

With the second formula the modus operandi is the same, except, of course, that instead of melting the already made jelly, the mixture made as above directed is used directly after mixing. The plates being dry they are ready for exposure, which need not necessarily take place at once, as if stored away in the dark and away from damp they will keep two or three weeks.

The exposure to light is made in a box-pattern frame, the negative having a mask in front of it to protect the margin of the plate from the light.

The negative used must be reversed—i. e., the image must be in its right position on the negative when viewed through the film, not as ordinary negatives are—through the glass; therefore, a mirror must be used in making the negative, or a skin negative may be used; or, if neither is practicable, the negative must be reproduced by one of the methods already given.

The exposure to light in the printing frame will vary according to the density of the negative, and the strength of the light, and should be continued until the image can be seen well defined upon the gelatine film when examined in the dark-room, by opening one-half of the back of the frame.

The exposure being complete, lay the printing frame face down upon a table in the light, remove the back and packing, and expose the back of the film to the light for a short time, without in any way disturbing the position of the collotype plate or the negative.

This exposure of the back of the image to the light has a twofold advantage, one being that it helps to weld the sensitized gelatine film to the surface of the plate, and so enables it to stand the wear and tear of printing; the other, that it reduces the relief of the picture, and so gives the inking roller a better chance of feeding the deep shadows. {167}

The duration of the exposure of the back of the film to the light will vary very much with the subject in hand; if it be one with slight contrasts, a much shorter time will be required than where the picture contains a lot of very heavy shadows.