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

by W. T. Wilkinson

Some collotype blocks require the ink far thinner than others, therefore it will be the aim of the printer to try various degrees of ink with each plate until the best result is attained. The best prints are got, as a rule, from a roller that seems to have very little ink upon it, but the ink thoroughly well distributed over both slab and roller.

To print from a collotype plate it is necessary that those portions of the film which have not been exposed to the light—and from them in proportion as the gradation of tone progresses to the shadows—must be charged with moisture to enable them to resist the greasy ink; if plain water be used for this it would {173} be troublesome to get sufficient in the film to resist the ink, and at the same time enable the roller to feed the image without slipping. Therefore glycerine is used, mixed with water, and with various hygroscopic substances, to which the term “etching fluids” has been given. This term is a misnomer, and quite out of place, “damping fluids” being the best.

In addition to the damping fluid previously given, a few others are here appended, each having their advantages under certain conditions:

	Glycerine alone or with water.
or,
	Glycerine	5	ounces.
	Water	5	ounces.
	Hyposulphite of Soda	1⁄2	drachm.
or,
	Glycerine	5	ounces.
	Water	5	ounces.
	Nitrate of Potash	1	drachm.
or,
	Glycerine	5	ounces.
	Water	5	ounces.
	Ammonia	10	drops.
or,
	Glycerine	5	ounces.
	Water	5	ounces.
	Common Salt	30	grains.
or,
	Glycerine	5	ounces.
	Water	5	ounces.
	Chloride of Lime	1	drachm.

The last one must only be used when it is desired to take away a tint, and if used too often, will be very likely to destroy the image altogether.

During the printing from a collotype block, the number of impressions that may be pulled without redamping, will vary very much with each block. Sometimes as many as eight or ten may be pulled, but the average will not exceed two. As soon as there is a tendency to flatness, take a sponge containing a little of the damping fluid, and go over the film with it, then wipe it off {174} with another sponge, or a piece of damp cloth (soft), then blot it off with a piece of good blotting paper, and ink up again. In fact, as a rule, it is best to damp for each print.

Good printing paper must be used for collotype prints, and for very fine work a fine neat enamel paper will be best. Coarse hand-made papers, however much admired, must soon break up the delicate image.

Fig. 22.

I will describe another drying oven, the heat­ing ar­range­ment of which is the same as the other, but the super­struc­ture is much smal­ler. It will re­quire more care during the time the coated plates are drying to keep the temp­er­a­ture down, but it has the ad­van­tage of al­lowing a ther­mom­e­ter being in­serted in the lid in such a man­ner as to allow the temp­er­a­ture of the in­ter­ior being seen at a glance. Still another ad­van­tage is, it takes up much less room. It measures inside 18 inches wide by 32 inches long, and 9 inches deep to the top of the slab. The thick­ness of the slate slab and of the copper tank should be the same as in the larger oven; the lid at the top should be made solid and have an orifice in the centre through which the ther­mom­e­ter is pushed. The sides of the oven should be covered with one or two thick­nes­ses of good red flan­nel.