By exposing a plate in this manner there is a far better chance of getting absolute contact between the two plates, as the rays of light are parallel.
For making transparencies for printing upon copper-plates, first of all make a grained negative the proper size, then when this is varnished and touched up, make from it as above a transparency upon a gelatine chloride dry plate, by which process it is more certain to get a suitable result for the process than on a gelatine bromide dry plate.
No formula will be given here for developing these gelatine chloride plates as each packet has full instructions for development which must be followed implicitly.
For developing gelatine bromide dry plates for negatives, or for transparencies, the following, a modification of the Ilford formula, will be found better than using formula where the ammonia and bromide are mixed together.
| Solution No. 1. | ||
|---|---|---|
| Pyrogallic Acid | 1 | ounce. |
| Meta Bisulphate of Potash | 1 | ounce. |
| Bromide of Potassium | 1 | ounce. |
| Water | 80 | ounces. |
| Solution No. 2. | ||
| Liquor Ammonia | 1⁄2 | ounce. |
| Water | 10 | ounces. |
For a normal exposure equal parts of these are mixed just before using, and for an uncertain exposure begin with two parts of No. 1, one part of water, and one part of No. 2.
THE REPRODUCTION OF NEGATIVES.
For the production of suitable collotype prints, the first consideration is the negative, from which the printing surface is to be made. The requirements are, a negative in which all the half-tones, both in the lights and in the shadows, are reproduced in their proper gradations; the lights must not be too dense, or the shadows too clear; the blackest portion of the negative must be that part representing the highest light, and the only clear glass, that which represents the deepest shadow; these negatives may be either wet collodion, or on ordinary gelatine-bromide plates and reversed—i. e., the mirror must be used behind the lens. If it is desired to produce a collographic block from a negative already in existence, and which may not be retaken, then it will be necessary to make another negative. This may be done in many different ways; but whichever way is adopted, the first and most important step is the production of a transparent positive.
This transparent positive may be made on an ordinary gelatine-bromide plate, or on a gelatine-chloride plate, and then from the transparent positive a negative is made by transmitted light (in the enlarging camera), either the same size, reduced or enlarged in size; or instead of using this class of transparency, by far the finest results can be got by making the transparency in carbon tissue, which can be used for both transmitted light, for enlarging or reduction in size, and it can also be used for contact printing, when the reproduced and reversed negative is required to be the same size as the original.
On page [29] directions are given for the manipulations when gelatine dry plates are used, so here we will confine ourselves to the production of negatives from the carbon transparency. First of all, procure some carbon transparency tissue, ready sensitized, cut this into sizes at least half an inch larger all round than the negative, then cut a brown paper mask to fit all round the negative, and to protect the margin of tissue from the action of light; this acts as a safe edge to the tissue, enabling it to adhere to the glass support during the operation of development. If it is only intended to reproduce a portion of the {32} original negative, the safe edge may be made by putting the mask just round that portion of the image, but if the whole of the negative is required, put it in a printing frame with a plate glass front, fix the brown paper mask outside the negative, then place the sensitive carbon tissue the black side next the negative film, and put in the backing, and back, and fasten it up. Next expose it to the light for four or five times the time requisite to get an ordinary silver print.