The Chemistry of Pigment
Printing with Bichromated Colloids
It must be remembered that in all the processes based upon the action of light on a colloid containing a bichromate, the pigment which forms the image is unaltered and plays no part in the reaction. It is just as easy to produce an image in pure bichromatized gelatine as with one containing a pigment, only in the first case the image would hardly be visible unless stained up by the use of dyes. The reaction involved is therefore simply one between light and a bichromated colloid. The salts of chromium are not in themselves sensitive to light; it is only in the presence of light that they are reduced and the action that takes place may, according to the researches of Eder (Phot. Korr., 1878, 32, 48, 75, 98, 117, 144.), be represented as follows:
2K2Cr2O7 + H2O = 2K2CrO4 + Cr2(OH)6 + O Cr2(OH)6 + K2Cr2O7 = Cr2O3·CrO3 + 3H2O + K2CrO4
The first equation may be regarded as the primary light action; the chromium hydroxide — Cr2(OH)6 — is decomposed by an excess of bichromate, as in the second equation, to chromate of chromium — Cr2O3·CrO3 — which is the real agent that acts on organic substances such as gelatine, gum arabic and albumen to render them insoluble in water.
Howard Farmer in 1889 (B.P. 17,773 of 1889) found that in the presence of gelatine, the bichromates are reduced by finely divided silver, so that when a bromide print was immersed in a solution of bichromate, the gelatine in contact with the metallic silver constituting the image is rendered insoluble without exposure to light. This observation forms the basis of both the carbro and bromoil processes. From a chemical stand-point the two processes are essentially the same; the mechanism of the two processes, however, is somewhat different, as in bromoil the insolubilization of the gelatine takes place in the film containing the image, while in the carbro process the insolubilization has to be produced in another layer of gelatine by diffusion.
The probable reaction here is first the bleaching of the silver image, resulting in the formation of silver bromide and potassium ferrocyanide. This is reoxidized by the bichromate back to ferricyanide, the reduction of the bichromate resulting in the insolubilization of the gelatine. With copper salts the reaction is essentially the same, the cupric salt and the bromide reacting with the silver image to form cuprous bromide, which is reoxidized by the bichromate into a cupric salt, the reduction of the bichromate resulting in the insolubilization of the gelatine. Hypothetical reactions can, and have been (Venn, Brit. J. Phot., 1924, 71, 427; Tritton, Phot. J., 1926, 66, 126 and 1927, 67, 140; Schömmer, Phot. Rund., 1926, 63, 210 and Schiel, Phot. Rund., 1926, 63, 55, 97.), written to represent these reactions but the objection to all such equations is that while they show what can take place, there is little evidence to show that it does take place. Consequently, it does not seem worth while, at the present time, to attempt to represent the reaction in the form of a chemical equation.
The Carbon Process
Ever since its introduction as a practical process, carbon has been recognized as one of the finest of printing mediums. While it does not allow the same degree of control as some of the later processes, as gum-bichromate and the oil pigment methods, there is a good deal of latitude in carbon printing and if multiple printing is employed alterations of values may be effected to a considerable extent. In common with other processes, which depend upon the action of light upon chromic salts, the carbon process affords a wide variety of colors and surfaces. The Autotype Company, who are the principal makers of carbon materials, supply tissues for over thirty different colors. But even more important is the fidelity with which carbon reproduces the delicate tones of a negative. In this respect it is approached by no other medium and a carbon print will reproduce the fine tones of a good negative more truthfully than any other process extant.
There are two variations in carbon printing known as single and double transfer. In the first case the image is reversed from right to left, while in the latter instance the image is non-reversed. Carbon tissue consists of paper coated with gelatine and pigment. Before use, it has to be sensitized in a solution of potassium bichromate and is then dried in the dark. With the spirit sensitizer, manufactured by the Autotype Company, the drying is very rapid and sensitizing is an operation which requires very little time. When dry, the tissue is exposed to daylight under the negative, a photometer being used to regulate the exposure as the image is not visible. When exposure is complete the tissue is removed from the frame and allowed to soak until limp in cold water. In the meantime, a sheet of single transfer paper, or the temporary support if double transfer is to be made, is allowed to become pliable in the water. As soon as limp, the two are brought together and pressed into contact. After remaining under pressure a short while, the two are immersed in warm water and the tissue stripped off, leaving the gelatine and pigment adhering to the transfer paper, or to the temporary support in case of double transfer. Gentle washing in the warm water follows and the unacted upon bichromated gelatine with its pigment soon washes away leaving the image in pure insoluble pigment. As soon as development is complete, the print is removed, placed in a bath of alum to remove the bichromate stain and harden the gelatine and is finally dried.
Double transfer is a little more complicated. After development on the temporary support, the image is hardened and allowed to dry. It is then again placed in water and brought in contact with a sheet of double transfer paper. The two are allowed to dry in contact and the paper may then be stripped from the temporary support carrying with it the image. The introduction of spirit sensitizers has rendered carbon a comparatively simple and straightforward process and instructions unfortunately make it appear more involved than it really is.
Carbon Tissues
The Autotype Company of England are the principal manufacturers in the world of materials for the carbon process and supply everything necessary for working the process. Over fifty different tissues in thirty colors are made by this company and there are a large number of different transfer papers to select from, giving practically all useful tones and surfaces. The following is a list of the more important tissues of the Company:
- Terra cotta
- Ivory black
- Warm sepia
- Standard brown
- Standard purple
- Gray green
- Dark blue
- Blue black
- Cold bistre
- Warm black
- Engraving black
- Sepia
- Red chalk
- Ruby brown
- Platine black
- Italian green
- Cool sepia
- Portrait purple
- Portrait brown
- White cameo
- Talbot sepia
- Sea green
- Brownish black
- Vandyke brown
Much may be done to enhance the effectiveness of the print by judicious choice of a color appropriate to the subject. Thus, Dark blue or Sea green is suitable for pictures at sea or on large bodies of water. Suitable colors for landscapes are found in Engraving black, Ivory black, Italian green, Vandyke brown, Warm black and Gray green. Portraits appear to advantage on Red chalk, Sepia, Standard brown, Warm black and Brown black. A decided advantage of the colors obtained by the carbon or any other pigment process, over those obtained by toning, lies in the fact that the tones are purer and may be duplicated with ease and certainty which is rarely, if ever, the case with toning processes.
Double and Single Transfer
Carbon prints from glass negatives are reversed from right to left unless double transfer is employed. In the majority of pictorial subjects this is not objectionable and single transfer is quite suitable. Non-reversed carbon prints may be made from films by printing from the back side and with little or no loss in detail or definition. If carbon printing is proposed at the beginning the negative may be reversed in any one of several ways. The plate may be placed in the plate holder with the glass side facing the lens; a reversing mirror may be used, or the film may be stripped from the negative and reversed. This latter is a rather risky method to employ but some appear to have good success with it. At any rate it is best to use the special stripping plates for the negative as there is then less danger of trouble in the operation of stripping and reversing.
On the whole, double transfer is to be preferred to any of these methods where it is necessary that the picture appear in the same manner as seen by the eye; i.e., non-reversed.
Sensitizing the Tissue
The tissue is supplied in packages of a dozen sheets in nearly all standard sizes and also in bands 2½ by 12 feet. In commercial establishments, the tissue is usually bought by the band but it is more convenient for the beginner to buy the ready cut tissue. Since the tissue tends to curl, it should be kept under pressure until used, and as the solubility becomes less with age until complete insolubility is reached, no more tissue should be purchased at one time than may be used up in a few months at the most.
The sensitizing bath consists of pure potassium bichromate. Only the purest form of this chemical should be used. That sold for storage batteries etc. is unsuited. The strength generally advised for average negatives is four per cent solution, although with weak negatives better results will be secured with tissue which has been sensitized in a bath of lower concentration, as 2 per cent or 3 per cent. The sensitiveness depends upon the strength of the sensitizing solution and also upon the tissue. Thus tissue sensitized in a 1 per cent bath of potassium bichromate requires about three to four times more exposure than that sensitized in a bath of 4 per cent, while a color such as Red chalk requires more time for exposure than Turquoise blue, owing to its greater opacity to actinic light. However, generally speaking, the tissues all require about the same exposure.
On the whole, the use of the Autotype Company's Spirit Sensitizer is to be advised for the amateur or occasional worker. For commercial work, where the proper facilities are available for drying the tissue after sensitizing, the plain bichromate bath is satisfactory but as there are generally not at the disposal of the amateur, it is recommended that he make use of the spirit sensitizer, which dries very quickly, requiring no elaborate drying cabinet, and permits the tissue to be used within an hour.
In place of the spirit sensitizer of the Autotype Company the following may be used:
| Ingredient | Quantity | |
|---|---|---|
| Ammonium bichromate | 60 | grams |
| Water to make | 1 | liter |
A more sensitive tissue requiring from one half to one third the exposure of that sensitized with bichromate alone can be prepared by adding to each 100 parts of a 2½ per cent solution of potassium bichromate 2 parts of a 10 per cent solution of cerous chloride (Tritton, Brit. J. Phot., 1929, 76, 381). Immersion of the exposed tissue in a 2 per cent solution of cerous chloride before transfer, however, is equally effective as regards decrease of exposure and is more satisfactory in practice.
The operation of sensitizing with a spirit sensitizer is as follows: pour an ounce or two of the sensitizer into a saucer or cup and dip the Blanchard brush, supplied with each bottle of sensitizer, in the same and then apply to the tissue which should be pinned down on a board with pushpins. The solution must be evenly distributed and rapidly as it dries quickly. First cover the tissue lengthways and then dip the brush in the solution again and go over the tissue a second time in the opposite direction, namely the short dimension.
For a large print, a special brush should be made in order that the surface may be more rapidly covered with the sensitizing solution. When the first sheet of tissue is surface dry (several sheets may be coated in the meantime), it should receive a second application in order to insure a uniform coating. When finished throw away the remaining sensitizer and wash out the brush and keep for future use. The sensitizing bath should be kept in dark when not in use. The tissue should be hung on a line in a dark room to dry, which will take from ten to twenty minutes. The use of an electric fan will hasten drying as will also moderate heat. Of the two, the first is safer, as heat may render the tissue insoluble. The tissue should be thoroughly dry before it is placed in the printing frame. If it is intended to keep the tissue for any time, it should be placed in the storage tube supplied by the Autotype Company. Sensitized carbon tissue is at its best, however, as soon as dry and can only be kept in good condition with safety for a week or so, even in the special storage box.
The tissue may also be sensitized by immersion but then requires much longer to dry. It is, however, more sensitive than that sensitized by brushing and increases in sensitiveness with age (for methods of sensitizing using dyes, see Meisling, Brit. J. Phot., 1916, 63, Feb. 23; Dansk. fotografisk Tidskrift №s. 9 and 10, 1916 and Warburg, Phot. J., 1917, 57, 169).
Exposure
No special type of printing frame is required, but since there is no necessity for examining the print during exposure, the back need not be made in two pieces as usual.
The tissue must be kept dry while exposing and for this purpose sheets of waxed paper or waterproof sheets of vulcanized rubber, as used for the same purpose with platinotype, may be employed. The springs of the frame need to be strong and for this reason many of the cheaper frames known as “Amateur” will be found unsatisfactory since the springs are weak and unable to hold the rather stiff pigmented tissue in perfect contact with the negative.
Printing is usually done in the shade as direct rays of the sun may crack or cause the tissue to become insoluble. For commercial work, the Cooper-Hewitt mercury vapor lamp is a satisfactory light.
Before printing, the negative should be provided with a “safe edge.” This is a narrow opaque border on all four sides of the negative which insures a soluble margin to the picture by protecting the tissue from the action of the light. This “safe edge” may consist of a strip of opaque paint on the negative or the black paper masks sold for the purpose of producing prints with white borders.
As the image is invisible, an actinometer is used to gauge the time of exposure. Several forms are obtainable. Three popular types, Burton's, Sawyer's, and Johnson's, are illustrated in Figure 188.
In Johnson's actinometer a small roll of sensitive paper is contained within the cubical box. This is pulled forward and exposed to light beside the frame until the tint of the paper and the standard tint register and a new piece pulled into position. A thin negative may be sufficiently exposed in one tine, a medium one in two or three, while denser ones range higher. Such actinometers are known as intermittent and are not so convenient as the continuous type of which the Sawyer is an example. In this instrument there is a graduated scale of increasing opacities ranging from 1 to 9. The paper is exposed beneath the graduated scale and each number darkens in succession to the standard tint so that there is no necessity for moving the paper during an exposure. The Burton instrument is similar but has several portrait negatives of increasing density. Sensitive paper is placed under the negative in the actinometer which appears to resemble in density the negative to be printed and the two exposed until the test paper appears sufficiently dark.
Carbon tissue sensitized on a 4 per cent bath of potassium bichromate is about three times as rapid as P.O.P. The beginner will find it necessary to make two or three tests and after development the proper exposure can be judged. The number of “tints” required may then be marked upon the negative so that the proper exposure at any future time can be readily determined with the actinometer.
After exposure the print should be developed as soon as possible as if left to stand the action of light will continue even though the print be kept in complete darkness and over exposure will result. This is known as the continuing action of light and was first observed by Johnson, and Abney later showed that the principle might be used to advantage in increasing the speed of printing in dull light. It is possible to work out a system whereby one third or even less of the original exposure may be given and the print allowed to stand several hours before development. Unless absolutely uniform conditions can be maintained, this method is not to be advised however and the beginner will do well to develop immediately after exposure.
Development
The development of a carbon print is a comparatively simple operation. No chemicals are needed, hot water and a large tray being the principal requirements. No dark room is required and the operation may be carried out in subdued daylight. Up to this stage there is no difference in double or single transfer but before development it becomes necessary to transfer the pigment to either its transfer paper or to a temporary support from which it will later be again transferred to its final support. Transfer is necessary because the insoluble pigment which has been formed by the action of light is upon the surface of the tissue while the insoluble pigment which must be washed away in order to reveal the image lies beneath the image. It is, therefore, necessary to transfer the pigment so that the soluble pigment will be on top where it can be washed away without affecting the pigment forming the image.
We will first consider single transfer as it is the simplest and best for the beginner. After he is able to make single transfer prints with satisfaction, he may experiment with double transfer.
A sheet of single transfer paper is placed in cold water at about 16°C for several minutes until it is limp. Very rough or thick transfer papers should be allowed to soak for an hour before being squeezed into contact with exposed tissue. With thin and smooth papers ten to fifteen minutes will be sufficient. The exposed tissue is then placed in the same water. The tissue will at first curl inward and then outward until it becomes practically flat. At this point it should be removed from the water and placed face down upon the transfer paper, which should have been previously placed upon a flat surface with the side coated with gelatine upwards. When the two are in contact, a squeegee is passed over the same from center to the margin with moderate pressure in order to eliminate air and moisture. After being squeegeed into contact the tissue and its support may be placed under blotters and allowed to remain for fifteen to twenty minutes before development.
To develop, immerse the print and its support in water at about 35–38°C. In a few seconds the pigment will begin to ooze out around the edges. When this begins, separate the corner of the tissue and its support by lifting it with the finger nail and pull off the paper that originally held the pigment gelatine. This tissue may be discarded. Holding the print by one corner, gently splash warm water over the surface. The soluble pigment will gradually wash away leaving the image. Care should be taken not to touch the print with the hands or any hard substance as the gelatine is very soft and easily injured at this stage. If the print is under exposed, the pigment will wash away very easily while if exposure is excessive the pigment dissolves with difficulty and warmer water must be used. Highlights may be lightened or detail in dark shadows may be brought out at this stage by squirting water from a blow tube against the print, or hot water may be poured on the desired portion.
When development is complete, place the prints in clean cold water for a minute and then transfer to a five per cent solution of alum to remove the bichromate stain and harden gelatine. Porcelain steel enameled or hard rubber trays may be used for the alum solution but tin or zinc vessels, such as may be used for development, are to be avoided. The time required in this bath varies but sufficient time should be given to make sure that all of the bichromate stain has been removed as any trace which is left will be more noticeable when dry than while wet.
After clearing and hardening in the alum bath, the print is removed and well rinsed in water and then hung up to dry. Carbon prints should not be forced in drying by heat as there is a danger of the gelatine cracking. An electric fan, however, may be used to hasten the process.
Double Transfer
So far we have considered only single transfer which is quite suitable in all cases in which reversal of the image is not objectionable. The operations prior to development are the same in both single and double transfer. Before development, instead of being attached to the transfer paper, the exposed carbon tissue is fixed to a temporary support. This temporary support may be opal glass or the special coated paper supplied by the Autotype Company. This latter product is made in two grades: Thick №112 for general use, giving either medium gloss or matt; and Thin №112 which is advised for thick and rough surfaced transfer papers.