PhotoHistory

THE BARNET BOOK OF PHOTOGRAPHY

The Gum-Bichromate Process

By: James Packham, F.R.P.S., 1898

Pictorial photography is answerable for the revival of this, one of the almost forgotten methods of printing. Results unacceptable to bygone requirements have been reintroduced with advantage, where suggestive individuality and artistic effect have been desired.

The gum process has an unlimited range of possibilities, it would be impossible to describe them all. The minutest details, or the broadest diffusion together with the power of working from the highest to the lowest keys of chiaroscuro are values that can only be realized when the infatuation consequent on successfully working the process is experienced.

This method of printing, as with the so-called “carbon process,” is dependent upon the characteristic behavior of the chromic salts when in combination with organic substances, such as gelatine, gums of various kinds, starch, etc.

When any of these mixtures are submitted to the action of actinic light, they become more or less insoluble.

This property was partially discovered as far back as 1798, by Vauquelin. Professor Sucrow, Mungo Ponton, Beauregard and others advanced its application to photography up to about 1840, but it was not until some ten years later that its great value as a photographic agent was definitely established.

Hunt, Fox Talbot and Poitevin, each worked indefatigably to bring the application of the chromic process to a successful issue; but to Poitevin must be accredited the honor of being the original inventor of the chromated pigment or carbon process. This brings us up to about 1855.

None of these investigators appear to have been remarkably successful, beyond having established definite, but valuable facts of the changes produced. This want of success may possibly be accounted for by the general employment of gelatine and direct printing. It was not until Pouncey and others, about 1859, employed gum as the colloid medium, that any great advance was made.

About this time an important commission of inquiry decided that to Pouncey, Garnier and Salmon, and Beauregard the honor of producing permanent prints must be equally credited, and accordingly divided the Due de Luyue’s prize between them, giving to Poitevin the credit of the priority of invention.

Pouncey appears to have followed up the process with some considerable success, as some of his existing examples are excellent; it is much to be regretted that we have not more detailed particulars of his methods of working; but he evidently was before his time and met with but little encouragement.

To Alfred Maskell and M. Demachy must be accredited the revival of this long neglected process, and during the last three years much advancement has been made towards perfecting it. Serious workers, both at home and abroad, are industriously exhausting the possibilities of the process, and crude as some of the earlier examples of this revival have been, improvements and simplicity of working are giving us productions of every description, of such excellent quality that it may soon be expected to satisfy even the caustic criticism that has so persistently opposed its re-introduction.

Dexterity in the various stages of practical manipulation is necessary before skillful efficiency can be secured, and in order to arrive at this, due consideration must be given to the selection of the paper the color most suitable to the subject and the effect desired.

Almost any kind of paper will be found workable, if it be of fairly good quality. Those that are thickly coated with soluble sizing media are unsuitable, for although they may give clear whites they sometimes produce harsh prints, the half-tones are also liable to be lost in development unless very deeply printed. Several of the continental kinds are well adapted to the process and work in an excellent manner, giving soft and even results; of course, it will be understood that for definition and fine detail the finer grained description’s are the best, but where diffusion is desired those of a coarser texture may be advantageously used, they give a granulation that tends materially to secure the peculiarities of gradation characteristic of this process.

A few of the continental papers that will be found to work with ease to the beginner, are as follows :

Michallet paper is rather coarse, but takes the gum coating easily, it has a series of lines running in both directions, which are rather objectionable for some subjects; but it is an excellent paper for first experiments.

Ingres, is also a paper of similar character, and can be worked with equal facility. Lallane is another paper of the same class, but much finer.

Allonge paper is entirely free from the markings peculiar to those previously mentioned. This paper is best worked on the reverse side, which can be distinguished by examining the name marked in one corner.

Among the English papers the ordinary cartridge, Whatman’s drawing papers and many others are adaptable, but it must be borne in mind that those with a toothed or grained surface are preferable.

There are two methods of working, and results of equal excellence have been produced by either. Some of the most proficient workers of the process adopt the easier one of coating the paper, without previous preparation, with a mixture of gum, bichromate of potass and pigment. Others adopt the precaution of first saturating the paper with a strong solution of bichromate, and when dry coating it with a mixture containing only gum and pigment.

Experience is in favor of the previous saturation of the paper, this is recommended especially for beginners, as there are several kinds of paper that will not work efficiently by the first method; but when skill and practical knowledge of the special behavior of the materials employed is acquired, either method can be adopted.

We may presume that the advantage of the previous saturation of the paper with the chromic salt is, that should there be any inequality in its structural character, or should it be unequally sized, the bichromate appears to act as a kind of resist to the penetration of the pigment, thereby securing an increased range of tone and a corresponding purity of the whites.

The process may be divided into the following operations :

• Saturation or sensitizing of the paper
• Preparation of the gum mucilage
• Mixing and preparing the pigments
• Coating the paper
• Printing and exposure
• Development

For working by the previously chromated paper method, the sensitizing solution is made up of one part of bichromate of potassium dissolved in ten parts of water. This strength will not keep at all temperatures. Should the salt crystallize out, it is necessary to warm a portion of the solution and re-dissolve the crystals. The solution may be used repeatedly, but it will be necessary to filter it occasionally.

Before saturation it is convenient to cut the paper into the most useful sizes — quarter sheets are handy. Having decided which is to be the working side, mark the back distinctly. Into a dish of sufficient depth pour in the one in ten bichromate solution to a depth of about one inch, and immerse your paper sheet by sheet, until you have in it all you intend to sensitize. As each sheet is placed in the solution, remove air bells and turn it over and repeat this precaution. The time necessary for immersion is of no importance so long as the saturation is absolute, about five minutes being generally sufficient for the thickest of papers. By removing the bottom sheet to the top and passing through the whole in this manner, turning over each sheet and removing all air bells, even saturation is secured. Each sheet is carefully and slowly removed from the solution and dried in the dark. The paper is now very sensitive to actinic light, which must during all future operations be carefully guarded against.

After the paper is dry it will, if kept so, be in good condition for a long time.

To prepare the gum mucilage, take two ounces of Soudan or Turkey gum and dissolve it in five fluid ounces of cold water, strain out the floating impurities through fine muslin, and allow others, and finer to subside. This mucilage will keep in good condition in a well-corked bottle, for a considerable time. M. Demachy employs gum mucilage of twice this density.

Pigments in powder are more suitable than in any other form. If in cakes or paste the medium in which they are prepared does not work kindly with the gum, and it is also difficult to accurately measure quantities. No advantage is gained by using expensive colors, they can all be purchased at a good colorman’s, and at a small cost.

Lamp or any carbon black, red ochre, yellow ochre, burnt sienna, and raw sienna, all work well; there is some uncertainty with the umbers and sepias. It will be found that much time will be saved if a combination of these dry colors is made up in bulk, as for instance one hundred and seventy-five grains of vegetable black and one hundred of burnt sienna, give a rich soft brown color. These must be finely and intimately mixed with each other, which is conveniently accomplished by grinding with a small pestle and mortar; after which the mixture may be kept in a wide-mouthed bottle. Another advantage in thus keeping combinations of dry colors in bulk, is the absolute certainty of repeating the actual tint when required.

Various combinations of similar mixtures can be made. Of course it will be understood that any or all of the above-named colors may be used singly.

The grinding of the pigment with the mucilage is easily done on a stone slab with a palette knife. Take half a fluid ounce of the two in five gum mucilage, to which add the same quantity of water and thoroughly mix. Weigh out fifteen grains of the mixed pigment and place in a heap on the slab, add a few drops of the diluted mucilage, grind and regrind the mixture until it is completely smooth, then remove it to a cup, and clean the stone with another portion of the reduced mucilage, finally adding the whole of the ounce, intimately mix, and it is then ready for coating the paper.

For extra fine work on smooth paper, and in fact for all classes of work, the fine grinding of the colors adds materially to extend the range of gradation, and although the trituration may be carried out fairly well with a palette knife, when the finest possible grades are desired, recourse must be had to the muller and stone. Mullers are obtainable of any artist’s colorman, they are made in glass, and a convenient size is about one inch in diameter.

The most convenient brush for applying the mixture of combined gum and pigment to the paper, is of the description known as bear’s hair, these are usually set in tin; a flat one about two inches wide is a useful size.

In order to coat the paper evenly, pin it down to a drawing board by each corner with a double layer of blotting paper an inch or two larger than the paper to be coated. The blotting paper will absorb the excess of color at the margins and enable you to secure an even coating up to the extreme edge.

Take a fairly full brush of the mixture after thoroughly incorporating the color and spread it evenly over the paper, crossing and recrossing it with the brush. Allow the mixture to lie upon the paper for a second or two so that the paper may expand; now release each of the corners and pin the paper down again. Upon the next operation depends the evenness of the coating.

Take a four inch wide artist’s badger’s hair softener, hold it vertically and lightly by two fingers and the thumb about an inch and a half from the top of the handle, and pass it rapidly over the whole surface of the paper as quickly and evenly as possible. The motion producing the best effect is not the usual sweeping action, but a series of sudden short jerks, difficult to describe but easily acquired. Continue this softening down until the paper has an even semi-transparent surface without uneven cloudy spots. Allow it to dry spontaneously, but before it is stored for future use dry it carefully by the fire, but avoid overheating.

Uncertainty of result is a defect often brought into argument against this process; but absolute uniformity is not difficult if strictly accurate quantities only are employed. With constant strength of bichromate and gum, uniform weights and combination of pigment, similarity of repeats are obtained: but these can only be secured when each sheet of paper is coated identically with its fellow. To get this evenness the badger hair softener must be washed out and dried after coating each sheet. This is very quickly accomplished by an energetic shaking and drying upon a smooth towel. If the paper has been coated properly, it has an even semitransparent surface slightly glassy.

Failures often occur from using an excess of pigment and allowing the gum to become too thick in consequence of evaporation. Excess of pigment gives dense heavy shadows and increases the difficulty of printing; excess of gum gives clear high-lights, tending to hardness and easy solubility endangering the halftones.

The paper, if it has been correctly coated will work satisfactorily, if on steeping a small piece of it downwards upon cold water, the pigmented gum dissolves and drops from the surface leaving the paper nearly clean. From ten to fifteen minutes should complete this test.

The method of working without previously chromatizing the paper is as follows: Take half a fluid ounce of four-in-ten gum mucilage and add to it an equal quantity of saturated solution of bichromate of potass; to this, with all care as to grinding and mixing, add the pigment; coat the paper as before directed. This method will be considerably slower in printing than that in which the paper had been previously saturated with the bichromate; neither are the whites as a rule quite so clear; but it will possess a peculiar grain and softness not otherwise obtainable, which is much approved by some workers of the process.

Exposure is so much dependent on circumstances that it is difficult to give precise directions, being governed by the density of the negative, the thickness of the coating and the intensity of the light. Even and not too dense negatives are the more suitable, for if the intermediate and high-lights are over dense the shadows are considerably over printed before the lighter parts can be brought out. Skill in development can do much to overcome these defects, but they may be considerably modified by the judicious employment of mat varnish, and by other methods of locally retarding printing.

The greatest assistance in obtaining uniformity in printing is the employment of a reliable actinometer, Wynne’s print meter is probably the most useful for this purpose, with ordinary gelatino-chloride paper as a register; from twelve to sixteen numbers will be mostly sufficient for an ordinary negative, on not too thickly coated paper. Another method of judging exposure is by the appearance of the shadows; they may frequently be seen by transmitted light, and when well out printing may be judged to be correct, but this is a slovenly method and only approximately correct at the best.

If the bichromate is used only in the pigmented gum, without previous saturation of the paper, exposure must be much more prolonged.

By no other process is it possible to obtain such diversity of effect as by this; much will, however, depend on the skill which is exercised in development. Should the printing exposure have been fairly correct it is a simple procedure. The print is floated face downward upon cold water contained in a deep dish; see that all parts are equally acted upon by the water, and that no air bells exist; if any, they may be easily removed by gently raising the print and immersing it again once or twice. After it has been soaking some five or ten minutes it may be examined; if all is going well, and the exposure has been approximately correct, the pigmented gum on the unexposed margins will have left the paper, and possibly some of the high-lights and half-tones may be making an appearance, if so, the treatment must be of a gentle character, and the print may be safely left for some time longer in the same position face downward; never allow it to lie either in or out of the water face upwards for any long time, or unremovable stains will be developed. Many prints will develop almost entirely without assistance, or with only an occasional laving of water if allowed to lie in this position for a long time. On the other hand some may, even when only slightly over-printed, give no indication of development. When this is the case remove the print from the water and place it face upwards upon a thin, smooth board, fix it in position with one drawing pin on the extreme margin, then gently lave cold water over it; should some of the darker parts still resist this action, longer soaking will be found advantageous. If there are still parts on which the color will not move, recourse must be had to the brush, and for this purpose nothing is better than a large camel’s hair mop. Keeping the brush always full of water, touch where necessary very softly; do not sweep it up or down, but just dab here and there as may be required, constantly flowing over the surface a copious supply of water.

If there are still parts in the shadows, or even in the high-lights that will not move, a jet of water from an enema syringe or from the household service pipe is very useful.

As a last resource a prolonged steeping in water of varying degrees of temperature, even up to the boiling point, may be resorted to, but the application of increased temperature requires judicious management.

When the print is sufficiently developed, if the creamy yellowness of the chromate stain is not desired, the print must be cleared or bleached, either in a solution of alum, sulphite of soda, or hypo-sulphite of soda, strength being immaterial with a careful after-washing. If the print is only just sufficient or only slightly overprinted, care must be taken that the clearing bath is not acid, neither must the washing be too prolonged, but if the print is first dried and submitted to light, this precaution is unnecessary.

Always allow the prints to dry spontaneously. It will not do to use any kind of pressure or blotting paper, for the surface of the color is very tender and delicate.