The Barnet Book of Photography Part 23

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 pota.s.sium 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 that the saturation is absolute, about five minutes being generally sufficient for the thickest of papers. By removing the bottom sheet to the top and pa.s.sing 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 quant.i.ties. No advantage is gained by using expensive colours, they can all be purchased at a good colourman'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 colours 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 colour. 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 colours 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 colours 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 quant.i.ty 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 cla.s.ses of work, the fine grinding of the colours 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 colourman, they are made in gla.s.s, 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 colour 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 colour 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 pa.s.s 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 quant.i.ties 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 semi-transparent surface slightly gla.s.sy.

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 half-tones.

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 quant.i.ty of saturated solution of bichromate of pota.s.s; 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 circ.u.mstances 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 matt varnish, and by other methods of locally r.e.t.a.r.ding printing.

The greatest a.s.sistance 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 a.s.sistance, 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 colour 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 hyposulphite of soda, strength being immaterial with a careful after-washing. If the print is only just sufficient or only slightly over-printed, 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 colour is very tender and delicate.

_Jas. Packham, F.R.P.S._

_An Introduction to Carbon Printing for Beginners._

[Ill.u.s.tration]

In the article that follows next will be found a complete exposition of the carbon process, with its various adaptations from the preparation of the paper and material forwards.

Whilst at the present time carbon printing is more largely used by professional photographers, yet its simplicity, the absence of chemical formulae and complications combined with the beauty of the results, makes it eminently suitable for amateur workers, and hence it has been thought desirable that as an introduction to the subsequent article, a brief and simple outline of the process should be given for the benefit of those who have not hitherto made its acquaintance.

In the first place then let it be understood that in carbon printing instead of depending on light to make a visible alteration of the sensitive salts as in silver printing, we expose the prepared paper or "tissue," as it is called, under a negative and secure a positive in insoluble gelatine, the gelatine having combined with it a pigment, and hence we get an image in pigment, not in platinum, or silver, or gold, but in a simple pigment which may be of any colour.

If bichromate of potash is mixed with an organic substance such as gelatine, that gelatine becomes insoluble after exposure to light, and if that gelatine carries with it a pigment, then on becoming insoluble it holds the pigment with it. If now, paper or other material be coated with bichromate, gelatine, and pigment, and exposed to light under a negative in the usual way, the thin portions of the negative will admit of the light acting on this coating and making it insoluble, whilst the parts which are protected from light, as for instance the sky or white objects, will remain unchanged and soluble, and on being washed in water will dissolve away, leaving white paper, whilst the light-affected portions which have become insoluble remain in proportion as the light has penetrated the various densities of the negative. This then is how we obtain our print.

For fuller explanation of the paper or "tissue" and its manufacture the reader is referred to the next article.

The beginner will certainly first obtain his tissue ready made, and he can purchase it ready sensitized or otherwise. The former will be best at the outset, but it must be borne in mind that it should not be kept longer than can be helped before use, and never more than ten to fourteen days at the utmost. Various shades of blacks, browns, and reds are the usual colours, also grey, green, and blue.

The tissue is rather more sensitive to light than silver paper, and should therefore be opened and handled in subdued light. It must be kept as dry as possible. A rather vigorous negative is best for carbon printing, one not too strong in contrasts. Before placing the negative in the frame, we must give it what is termed a "safe edge."

This is done by making a narrow border, say of about a quarter of an inch or less, round the negative, either on the gla.s.s or film side, with opaque black varnish, or it may be done by gumming on narrow strips of paper, such as lantern-slide binders. If binders and not black varnish are used, they must be applied to the gla.s.s side.

The Carbon printing paper which will hereinafter be called the "tissue" will be found to present an unpromising appearance, and as the coating is the full colour of the pigment in which the print is eventually to appear, it follows that the progress of printing will not be visible, and a mechanical means of gauging the exposure must be resorted to. An actinometer, similar to that described in the article on Platinotype, will do, and another and simpler form is described in the next article.

Printing will occupy about one-third of the time occupied by gelatino-chloride of silver paper.

Development is conducted in daylight, but not too close to a window.

The absence of chemical solutions has been suggested as an advantage, in this process the developer being merely hot water.

It is not necessary to have this laid on, a can of hot water close at hand and a kettle on the fire or gas stove not far off are all that are required.