A heliotype print is made in an Albion or other type-printing press, from a film or skin of bichromated gelatine, which, having been dried upon a plate glass (finely ground and waxed), is stripped off, then exposed to light in an ordinary printing-frame, under a reversed negative, then removed from the frame, laid face down upon a piece of black velvet, and the back of the skin exposed to light for a short time. The skin is next mounted upon a pewter plate, coated with a thin film of India-rubber, and placed upon the bed of the press, where the skin is covered with water from a sponge, and is allowed to soak for twenty or thirty minutes. It is then inked up, first with one roller, and then with another of a slightly different composition; the margin of the skin is then covered with a mask, with a suitable opening cut in it, then the paper that is to receive the image is put into position upon the mask. The picture is then printed.
The above is a rough outline of the process; now for the working details.
First of all, it will be necessary to understand that the gelatine skin from which the picture is printed must be of such a size as to allow a margin outside the actual picture of at least two inches all around, so that the rollers used in inking up will not overlap the margin, and strip the skin away from the pewter plate upon which it is mounted. For pictures 10 by 12, and under, a convenient size for the heliotype skin will be 21 inches by 15 inches, so the present chapter will treat entirely of skins 22 by 16, and the sizes of rollers, etc., given, as well as formulas for quantities of gelatine, etc., will be those most suitable for that size of skin; so if larger skins are adopted, the quantities here given must be modified accordingly.
The first consideration will be the apparatus and materials necessary. The first thing that commands attention is the press—an Albion or other typographic hand press—(the pressure necessary to print from a heliotype skin must be vertical, not scraping, therefore a lithographic press will not do). The press must have the bed fitted with several thicknesses of Kamptulican sufficient to nearly bring a sheet of thick zinc type high—“nearly” is said, because {153} due allowance must be made for the pewter plate (one-eighth of an inch in thickness) upon which the heliotype skin is mounted. Inside the tympan place a piece of good blanket, and also provide a loose blanket to lay over the paper when placed in position for receiving the impression.
The press must be fastened down to the floor, as plenty of pressure will be required in printing; a bench will be required close to the press, upon which either one large inking slab or two small ones can be placed, as well as the stock of printing papers, and a dish of water. A couple of good sponges are also required, as well as a supply of thick plate paper, to blot off the superfluous water.
THE ROLLERS.
The quality of the print from a heliotype skin depends greatly upon having good rollers at hand for inking the image; therefore, the preparation of the rollers (two) must be made with every care. Get the usual printing office pattern, with handles in the centre. The stocks of these rollers are to be coated with India-rubber sponge, upon which is cast a mixture of gelatine, etc.
First of all, procure two 12-inch rollers with handles complete, and get the stocks coated with India-rubber sponge a quarter of an inch thick; then, with strong twine tie down about a quarter of an inch from each end, trimming off the edges outside the string, so as to make them nice and round; now place one of the stocks in position upon the base socket of a 15-inch seamless roller mould, then carefully smear the inside of the mould with a rag saturated with whale oil (taking care that the whole of the surface of the mould is smeared with the oil); now place the mould in position over the stock, put the loose socket in position at the top, when it is ready for receiving the composition for the inking roller, made as follows—
| Scotch Glue | 1 | pound. |
| Gelatine | 1 | pound. |
Soak all night in sufficient cold water to cover it. In the morning squeeze out the superfluous water, and in a suitable size glue kettle, “Baine Marie,” or a jar placed in a pan of water, melt over a fire or Bunsen burner, then add 1 pound of green treacle, mix, and add 2 ounces of castor oil, 1 ounce of glycerine; stir well until all are thoroughly mixed, then pour into a warm jug, and from the jug pour into the mould as gently as possible; when the mould is full, take a piece of wood, and tap the outside of the mould all round, from bottom to top. That will cause any air bubbles that may be imprisoned in the mixture to rise {154} to the surface; then, if the mixture in setting should shrink, pour in more until the mould is quite full.