When dry, the tissue is generally wrinkled, brittle, breaks easily in handling and cannot be laid flat on the cliché; but by [pg 91] holding it over a basin of boiling water, the steam in a few moments rendering it sufficiently pliable to lay it flat between glass plates, where it should be kept under pressure until wanted for use.
The writer always dries the tissue in the following manner, which he devised about sixteen years ago.[27] And not only the least trace of reticulation is avoided, but the tissue, drying quite flat, lies in perfect contact with the negative, which is quite important to obtain proofs exactly sharp all over.
A clean glass plate is rubbed with talc, or, which the writer prefers, flowed with a solution of[28]
| Yellow wax, pure | 1 part |
| Benzine, pure | 100 parts |
then strongly heated, allowed to cool and rubbed clean (apparently) with a piece of flannel. After once more repeating this operation the plate is coated with the following plain collodion:[29]
| Ether, conc. | 250 parts, in volume |
| Alcohol, 95 deg | 250 parts, in volume |
| Pyroxyline | 3 parts |
When the film is set, the plate is immersed in filtered water until greasiness has disappeared, when on its removal from the bichromate bath the tissue is laid, without draining, upon it and pressed into contact with the squeegee to remove the excess of liquid and, with it, the air bubbles interposited. The tissue is then allowed to dry in the air on the collodionized plate in the cold season, or, when the weather is warm and damp, in a box in the bottom of which is placed a quantity of quicklime in earthen dishes. When dry, the plates are placed one upon another, wrapped in paper and kept in a dry place. When wanted for use the tissue is stripped off and will be found quite flat with a beautiful surface to print upon.
One should avoid to keep the sensitized tissue in a moist and warm atmosphere, for in less than ten hours it becomes insoluble even in complete darkness. It should neither be kept in the air contaminated with gaseous reductive matters, such as the products of the combustion of coal gas and petroleum, sulphydric or sulphurous emanations from any source, the fumes of turpentine oil, etc., which, by reducing the chromic salt, cause the insolubilization of gelatine, prevent the print to adhere on the support or the clearing of the image, which may even refuse to develop.
The sensitive tissue keeps well for three or four weeks in cool and dry weather, and no more than eight or ten days in summer unless well desiccated and kept in a preservative box. If kept too long the image cannot be developed.
The Photometer.—The time of exposure is regulated by means of a photometer. Of all the photometers which have been devised for that purpose we do not know any one more practical than that suggested in 1876 by Mr. J. Loeffler, of Staten Island. It is made as follows: On a strip of a thin glass plate, 6×2 inches, make four or five negatives, 1½×1¼ inch, exposing each one exactly for the same period and developing in the usual manner, but without any intensification whatever. It is even advisable to reduce the intensity if they were opaque. Fix, etc., and apply a good hard varnish. Now cover the back of these negatives with strips of vegetable paper or transparent celluloid, or, better, of thin sheets of mica, in such a manner as there be one thickness on the second negative, two on the third, three on the fourth, etc., leaving the first one uncovered. Then place on the whole a glass plate of the same size as the first and border like a passe-partout.