Exposing the Tissue.—The tissue can be exposed behind the negative in an ordinary printing frame, or in special frames having no joint in the back, as no image is visible. The negative must be furnished with a safe edge, made by painting an edge about one-eighth of an inch round the negative with black varnish, or by pasting on strips of red or black paper. Exposure must be judged by an actinometer. A very suitable instrument for timing the exposure of carbon tissue is Sawyer's actinometer. It consists of a rectangular tin box with a glass lid, bearing twelve tints graduated from slight discoloration to a degree of opacity, representing the extreme amount of deposit upon the lights of the densest negatives, each division of this screen of tints bearing a number in opaque pigments; and a roll of sensitive paper is placed in the box, and the end pulled forward so as to pass under the tints. When this arrangement is placed in the light, the silver paper commences to discolor underneath the graduated screen, beginning of course at the lightest, but the number on the tint being in an opaque pigment is preserved white, and serves to register the progress of printing; for if, when the lid is opened, the number one, for instance, shows clearly on a tinted ground, the instrument is said to have registered one tint; by that time the number two will have begun to make its appearance, and, if sufficient exposure be given, the light will print through the whole scale by successive steps, and show up the numbers, one to twelve. With an instrument of this kind it is evident that, by exposing alongside the carbon tissue and determining the number of tints required for the proper exposure of that negative, the same number of tints with the same negative will always prove right. A little practice will enable one to judge the number of tints required for every class of negative.
It will be well to remark here that freshly sensitized tissue will produce inferior pictures to that used a day or two after; the pictures are not so hard, and there is less danger of the high-lights being washed away.
Continuing Action of Light.—If the carbon tissue after exposure to the light, be kept in the dark for a little time the effect on the print will be precisely the same as if the exposure to light had been prolonged. This continuing action of light may often be utilized to advantage. Pictures known to be under-exposed will, if kept till morning, by that time have acquired the same force as if they had received the proper exposure.
Development consists simply in dissolving the gelatine unaffected by light, with hot water as the solvent.
Immerse the exposed tissue in a bath containing cold water. It will first of all curl up, but afterward lay flat and limp. It is then placed in another bath containing cold water together with a sheet of glass which has previously been coated with a 5 per cent. solution of gelatine. Bring them together face to face, draw them out, and force into close contact with a large squeegee; then place between blotting paper for five or ten minutes. In squeegeeing, the tissue should be uppermost, and a sheet of American cloth laid over it to prevent the squeegee from damaging it.
Development should not be attempted for at least twenty minutes, during which time the glass, with the tissue on it, should be placed between sheets of blotting paper, and kept under pressure to insure its adherence to the glass support. After that time it is placed in a dish, and water heated to a temperature of 100 deg. F. added. The colored pigment will at once commence to ooze out of the edges, and after a little time the paper originally holding the carbon film may be removed with the hand. Then, by gently leveling the picture with the hand, the superfluous gelatine will be washed away, and if the exposure has been correct a perfect image should remain. A certain amount of control can be kept over an autotype picture. An over-exposed print will show itself by insolubility of the gelatine, and the high light refusing to be washed clear. The temperature should be raised considerably, and hot water poured over with a jug. If this fails to reduce the intensity, add a little ammonia to the water as a last resource, though the better plan is to make another print, giving less exposure. Under-exposure results in over-solubility of the gelatine. The half-tones will be washed clean away. It is rarely an under-exposed print can be saved. All that can be done is to reduce the temperature of the water. Development should never be hurried; the slower it is the better the gradation of tone in the results.
After development is complete the bichromate salt is discharged, and the image rendered perfectly insoluble by well washing in cold water and placing in a dish containing a 5 per cent. solution of potash alum, after which it is again washed and dried.
Another method of making a transparency and one that involves less trouble is by means of the transparency plates which are now in the market. Of these we have tried Carbutt's with the greatest success. For these the following instructions are given:
The requisites are, a deep printing-frame a size larger than the negative to be used, with a flat glass bottom clear and free from scratches (crystal plate is best), a dark-room Lantern, or other artificial light, and Keystone Gelatino-Albumen Plates. Transparencies can be made same size of negative by contact and exposure to artificial light, or enlarged or reduced in the camera by daylight, with equal perfection in result. To make transparencies by contact place one of the Keystone thin crystal glass transparency plates over the negative in printing-frame, lay piece of dark soft material over it, close down the back, and expose to the light of the lantern or to a gas flame or other artificial light, for 10 to 30 seconds, according to density of negative, at a distance of 20 inches from the flame. Use the following developer: