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.
The Nature Of The Process. A gum-bichromate print is the result of the action of light in varying degrees on a thin surface of some sensitized colloid, such, for instance, as gum arabic. Other similar materials can be used, but gum arabic is the simplest and more generally employed. Such a substance when sensitized with bichromate of potassium or bichromate of ammonium becomes insoluble when subjected to light, the degree of insolubility varying according to the strength of light; consequently, with such a substance spread on a sheet of paper and placed under a negative, varying degrees of insolubility are produced, those parts of the coated paper under the dense parts of the negative being entirely soluble, owing to their having received no action of the light, while those parts under the clear glass of the negative will be entirely insoluble from having received the full strength of the light.
424. If, now, a pigment or a carbon in a finely powdered state be mixed with the gum solution, it follows that this pigment will be soluble or insoluble in the same proportion as the gum, for the pigment, present in the gum in a finely powdered state, will dissolve away in the same proportion as the gum solution. When such a pigmented piece of paper has been printed and developed in water, and the soluble and semi-soluble parts washed out, a picture will be left, which will consist entirely of pigment, and as carbon and most other pigments are permanent, a permanent picture is produced on the paper, similar in quality to the material left on a sheet of paper in a water color painting.
425. The Process in Detail. While almost any kind of paper can be used, such a paper should be chosen as is thoroughly well sized; otherwise the coloring material will sink into the pores of the paper and it will be difficult to obtain clear high-lights. The coloring material or pigment is mixed in proper proportions with a solution of gum arabic and bichromate of potassium. This solution is spread evenly upon a sheet of paper, which is then dried in the dark. In its wet state the coated paper is not sensitive to the light, but becomes sensitive when dry. When the paper is bone dry it is in condition for printing, and can then be placed back of a negative and placed into a strong light, according to the density of the negative-direct sunlight for a strong negative, diffused light for a weak negative.
426. With certain colors a faint image is visible on the paper. With other colors no image at all is visible; consequently it is advisable to print with an actinometer. However, this is not entirely necessary, as the process has tremendous latitude. An under-exposed print may develop out in cold water in ten or fifteen minutes, or less, leaving a coarse grained image. An over-exposed print may require to be left in the developing water for several hours, or the process may be hastened by the application of hot water; or, the hot water may be poured onto the print from a distance, or a soft brush may be used to bring out the image, if over-exposed. Thus there is little chance of losing a print unless it is very badly over-exposed or under-exposed. After development in water the print can be cleared in alum or sulphite of soda, and then hung up to dry, when it is finished.
427. Materials Required. The materials required are few and inexpensive. A few sheets of paper, some clean blotters, an ounce of gum arabic, an ounce of bichromate of potassium crystals, and a few tubes of water color or a small quantity of dry powdered colors. These are all the materials required for the preparation of a considerable number of prints.
428. Apparatus Required. Either a sheet of glass and a palette-knife, obtainable at any art store, should be provided, or a small pestle and mortar of glass or porcelain; one or two gutta-percha-bound bristle brushes, say of two-inch width, and besides these brushes a long Badger hair brush, of three or four inch width, which is styled a blender, the use of which will be described later. The Badger hair blender can be obtained at art stores or brush stores, and will generally cost about thirty-five cents per inch length. A camel's-hair brush is too soft for the purpose, and a bristle brush will not serve for blending the coating on the paper.
429. Large trays for developing should be at hand, and these can readily be made by the worker himself, out of shallow wooden boxes, lined with black or white oilcloth. A flat drawing-board, or pastry board, is very convenient on which to lay the print during development when special work is being done on the print. A long rubber hose, to attach to the faucet of the sink, is serviceable for manipulative work on the print during development. A package of push pins, or thumb tacks, to keep the paper taut during coating, is also advisable.
430. The Most Suitable Colors. For first work in the gum-bichromate process it is best to keep to a few colors, such, for instance, as lamp black, ivory black, Vandyke brown, Indian red, Hooker's green, burnt sienna, and some good blue color. Some workers use moist water colors in tubes, such as Winsor & Newton's, but a cheaper grade of tube can be employed without any disadvantages. Other workers prefer the dry powdered colors, obtainable at a paint shop. These can be bought, according to the color, from a few cents per ounce up, and a half ounce each of the colors necessary will last a very long while. Dry powdered colors should be put in little glass jars and properly labeled. A black coating is generally made up of a mixture of ivory black and lamp black, which gives more body than either one of these colors taken alone. The addition of a little Vandyke brown gives a warm black, and from there on the color can be changed to suit by the addition of Vandyke brown and a little red or other color. The color is largely a matter of individual choice.
431. The Best Papers for Use. Almost any kind of paper can be used for the gum-bichromate process, provided it is well sized, so that the coating stays on the surface and is readily removable in the developer. If the paper is porous the coating will sink in, and it will be impossible to obtain clear high-lights, as the color will stay in the paper. Any good water-color paper is recommended, such as Michallet, Lalanne, etc., and these can be obtained at art stores, for a few cents a sheet. Papers with a slight grain on the surface should be used at first, as it is easier to coat such papers evenly than a perfectly smooth paper.
When the worker has gained some skill in coating, and desires to obtain a finely detailed print, then smooth bond papers may be used. For rough effects Whatman's papers, which should be first sized, give excellent results. Tinted papers are often used for special results.
432. Preparing Solutions. The simplest formula for gum bichromate printing calls for a solution of gum arabic and a solution of bichromate of potassium, with the addition to these two of a little coloring matter and good results can be obtained with just these three ingredients. The gum arabic, which can be bought at any drug store for five cents an ounce, should first be finely powdered with the pestle in a mortar; or if that is not at hand the gum can be put between heavy sheets of paper, or in a small linen bag, and pounded fine with a hammer. The object of reducing the gum to fine powder is to enable it to dissolve quicker. Place five ounces of water in a wide mouthed bottle, and then add the powdered gum gradually to the water, stirring vigorously. Or, the gum may be poured into the water and left to dissolve for a couple of days, the bottle being shaken now and then. When thoroughly dissolved it should be strained through muslin, to remove any impurities.
433. Such a solution of gum arabic will not keep in very good condition after a couple of weeks. A mold will begin to form on the surface and it will sour and grow thin, but this can be prevented by adding a little salisylic acid to the solution, which will tend to preserve it. However, as it is inexpensive it is best to make it up fresh every now and then, or each time gum-bichromate prints are to be made. The ounce of bichromate of potassium crystals is dissolved in ten ounces of water, which makes a saturated solution. Bichromate of potassium is poisonous, and the worker should keep his hands out of the solution. The bichromate solution should also be strained or filtered through a wad of cotton.
Gum Solution .............................
Potassium Bichromate (Saturated Sol.).......
Or Pigment Red........................
Note. - Bichromate of ammonium may be used in place of potassium.