This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Sensitizing The Paper. When ready for sensitizing, decant or filter sufficient of the solution into the tray for sensitizing the tissue that you expect to use within a few days. If only a few prints are sensitized this bath may be poured back into a bottle and used again. If many sheets have been prepared, then we advise that you throw away this bath each time after using, as the material is not expensive and better results are obtained from fresh baths. A bichromate bath containing glycerin, carbonate of ammonia, or other organic substances, will be reduced by the action of the light, therefore should not be used too often, as it is more economical to make up the bath frequently.
199. There are a few points that must be remembered when sensitizing tissue. Weak prints are generally caused from weak sensitizing baths, providing the negative is of the proper strength. Therefore, sort your negatives, using a strong bichromate bath for those that are somewhat hard and contrasty. Where you ordinarily would use two ounces of bichromate increase this quantity to four. For average negatives three ounces of the bichromate is generally sufficient. Again, the weaker the negative to be printed from, the weaker should be the sensitizing bath. The action of light will be much slower, resulting in more vigorous prints.
200. It is essential, when sensitizing the tissue, that no air-bells exist upon the paper. The tissue, or paper, is immersed under the solution, face side down, and with a camel's-hair brush air-bells are removed from the back. After the print is immersed for one minute face side down, turn it over with film side up, again expelling any air-bells that may appear. The print must at all times be kept under the solution.
201. When the tissue shows signs of curling backward, you are sure of its being sufficiently sensitized. There are times, however, when the tissue will lay flat and will not curl at all. Therefore, do not wait for this to take place, as the time for sensitizing should not exceed three minutes. Anywhere from two to three minutes is generally sufficient.
Mopping And Drying The Tissue. Great care must be exercised in drying the tissue. Lifting it from the bath and placing face downward upon the squeegee plate, cover with a rubber cloth, and with the squeegee swab the surface lightly at first, and then more vigorously, to remove all the superfluous solution as thoroughly as possible from the face and back of the tissue. Remove the rubber cloth and with a clean blotter mop up the remaining solution. Then, catching the print by the corner lift it from the plate.
203. While ordinarily a photo clip attached to one corner of the tissue will hold the film flat, yet, there are times when the paper will curl considerably, and to avoid this curling we advocate using small strips of wood. Lay the strips across the upper edge, the tissue between the two strips, and clamp them together in the center with one photo clip. Arrange the bottom in a like manner. These strips will hold the tissue perfectly flat, and it will dry nicely in this way.
204. If you have but a few sheets to sensitize, you may allow the tissue to remain squeegeed flat upon the squeegee plate and place in the drying-room to dry. This method protects the film from dust, injurious gases and vapors that might be present in the room, and when it leaves the plate its surface is as smooth as glass, usually resulting in a better and more perfect contact with the negative.
205. Drying-Room - The drying of the sensitive carbon is one of the most important parts of the process. The room used for this purpose must be clean and dry. Have plenty of fresh air, if possible. Any foul gases, vapors, or stench of any kind is ruinous to the sensitized tissue.
206. The room must be perfectly dark. A ruby light, or the light of a candle, is all that should be used in this room. Do not use kerosene lamps, as the odor from the oil. will have a bad effect upon the tissue. The proper drying of the tissue determines, to a certain extent, the sensitiveness and solubility of the tissue and brilliancy of the picture for which it is employed. Therefore, strict attention must be paid to this part of the process.
207. For the temperature of the drying-room, from 50° to 60° Fahr is about right. In the winter months it can be increased to 70°. Paper sensitized in a strong bath should be dried quicker than that sensitized in a weaker solution. The weaker the solution the slower the paper should dry. Ordinarily the proper length of time occupied in drying is from five to eight hours-it should never be less than five, or more than eight. Tissue that has been dried rapidly adheres readily to any support, and will develop more readily in water of a low temperature. Such a tissue, however, if printed from immediately when dry will lack in gradation and half-tone.
208. In case you have such paper already prepared and must use it, you can improve upon the general results by exposing the film side of the tissue to the light for a few moments, thus tinting it a trifle. Do this before placing upon the negative. In this way you can produce good half-tones, which otherwise would be lost. Better results can be produced from such paper when it is three or four days old.
209. The most sensitive carbon tissue is that which is dried quite slowly-say six to seven hours-and the prints are much softer providing you use good, snappy, contrasty negatives; otherwise the results will be mushy and sunken appearing, and the development will be slow and difficult. Too long drying causes the film to become insoluble, to lose its adhesive properties, sometimes to blister in the developing, and often becomes entirely worthless.
210. On the other hand, if the tissue is allowed to dry too quickly it becomes brittle and is liable to crack, and at the same time its sensitive properties will be greatly impaired. Should the paper become brittle by rapid drying, this can be alleviated by subjecting the tissue to the action of moist atmosphere. Scattering the tissue out on. a table in a damp room for a few minutes will make it pliable and suitable for working satisfactorily.
211. Sensitized tissue that is not to be used at once should be stored flat in a plate-box, and held down by a heavy sheet of glass. Better still, a tin box can be arranged with a double perforated bottom, in which is stored some asbestos fibre soaked in calcium chloride. This prevents the tissue from becoming too dry. Or, the sheets can be rolled up and placed in a tin platinum tube, in which the soft lumps of preservative have been left.
212. Another method is to lay a sheet of glass in a printing-frame; over the glass lay a piece of opaque paper, and on this, face down, lay the sheets of sensitized tissue. The back is then clamped on, and the tissue will be kept flat and in good condition for several days.
213. A few points would be well to remember at this time: The stronger the bichromate bath the more rapidly can the tissue be dried; the weaker the bath the slower it should be dried.
214. A most convenient way to work is to sensitize the tissue in a normal bath the evening before you expect to use it. Have the room at a temperature of not more than 60° Fahr., when the paper should dry in from seven to eight hours. In doing this the paper should be gathered up the first thing in the morning and placed in a light-tight box, weighted down with a glass weight.
Colors Of Tissue To Use. Standard brown, warm black and sepia are generally considered the best colors to use for portraits. For landscapes and seascapes, sea-green, marine-blue, and sometimes red chalk will give you a very pretty effect. It is essential that you select the proper color of tissue to suit the negative or view from which you expect to print. As each color has some little peculiarities of its own, we consider it advisable that you try to perfect yourself on as few standard colors as possible, and only use the odd colors for experimental purposes.
216. When employing red chalk, sepia, or any warm-colored tissue, as these colors are generally used for sketchy effects, they require a strong bichromate bath, for the reason that they are not printed so deep, and unless the bath is quite strong they will bleach out in the developing. For this reason use a stronger bath, doubling the amount of potash given for the normal bath.