The American Carbon Manual/Details of Manipulation
DETAILS OF MANIPULATION.
We shall endeavor to make plain the manipulations in the various stages of producing a carbon print.
PREPARING THE NON-SENSITIVE TISSUE.
The tissue is prepared by machinery, by which a perfect and uniform coating is secured. Each piece of paper is made into an endless band revolving round rollers, which keep it stretched, and repeatedly pass it over a surface of melted gelatine, sugar, and pigment, until a perfectly even coating of the right thickness is applied to the whole length. The trough of gelatine is kept at proper temperature by means of steam. By repeated contact with the gelatine, a thin coating being applied each time it passes over it, a much more perfect surface and even thickness of the gelatine is secured than could be obtained by any plan which applied the full thickness at once. By the arrangement adopted, waves of irregular draining are entirely avoided. These lengths of gelatine are then cut up to specific sizes, and will keep ad infinitum, ready for sensitizing when required.
It is important that the paper employed should possess a fine surface, and be quite free from inequalities and imperfections, in order that it may receive an even layer of the pigmented gelatine, as any imperfection in this layer may result in a blemish in the picture. It is also desirable that it (the paper) shall be sufficiently permeable by the water to facilitate its removal from the gelatine prior to development. To the quality and proportions of the color and gelatine we refer in another chapter. As the tissue is prepared and sent out ready for use by the patentee, it is not important to enter into more minute details of its preparation here.
The tissue is prepared in three distinct varieties of color; and in each scale there are three gradations of intensity, to suit negatives of various kinds. The colors are described as Indian-ink, sepia, and photographic purple.
The Indian-ink tissue is a pure black, nearly neutral in tone, but inclining to warmth rather than coldness.
The sepia tissue is of a rich deep brown, of a warm sepia tint.
The photographic purple tissue is of a tint resembling that common in gold-toned silver prints, generally of a purple-brown character, or in its extreme depths a purple-black.
Each of these tints is made in three qualities, to suit the different degrees of intensity in negatives, on a principle first pointed out by Mr. Swan, and which it may be well here to explain in detail.
In this method of pigment printing, although the best picture will result from the best negative, it is possible with a very intense hard negative, possessing very abrupt contrasts, to produce extremely soft and harmonious prints; whilst, on the other hand, brilliant prints may be obtained from a feeble negative possessing very little contrast or intensity. The principle upon which these effects are secured is this: The reader has seen that, as the gradations in the picture are obtained by different thicknesses of a translucent colored film resting on a white ground, the deepest shade being secured by the greatest thickness of this material, most completely covering up the white ground, it follows that the greater the proportion of color present in the film of a given thickness, the deeper will be the tint secured; and the less the amount of color present, the thicker must be the layer of the material in order to get depth. If, then, we take a sample of tissue prepared for a good negative, and print with a hard, dense negative, sufficient thickness of the colored translucent film is rendered insoluble to produce deep shadows and well-marked half-tones in the deepest gradations, long before the more delicate half-tones have been formed at all. If the printing be continued until these are secured, the lower half-tones forming the details in the shadows are obscured, sufficient thickness being rendered insoluble in the lighter of these shades to completely mask the underlying white ground. If with such a negative, however, we employ a tissue containing a much smaller proportion of color, it permits a considerable thickness to be rendered insoluble before the deeper half-tones are obscured; and in the time required for this, sufficient light has penetrated through the dense parts of the negative to render the details in the lights properly. On the other hand, by increasing the proportion of color, great contrast may be secured in the print, although little contrast may exist in the negative, as a slight thickness of the translucent material will, if it possess a large proportion of color, give great depth; and by the time the light has passed sufficiently through the thin deposit of a feeble negative to produce details in the lights, sufficient color will have been secured in the shadows to give vigor, without continuing the printing further, and so degrading the picture by rendering insoluble a further layer of color in the lights.
It will be seen, then, that by forming the picture in a thin film of insoluble matter of intense color, vigorous contrasts and perfect gradations from light to dark may be secured with a thin negative; and that by using a thicker film of insoluble matter, less intense in color, the excessive contrasts of a hard negative may be softened, thus materially ameliorating the faults of bad negatives in either direction.
With a good negative, neither weak on the one hand, nor too intense on the other, there is no difficulty in producing perfect results, rendering between pure white and deep black every minute gradation, from the most delicate demi-tint in the lights to the least illuminated detail in the shadows.
The tissue is prepared, therefore, in each tint to suit negatives of three qualities. These are numbered, Nos. 1, 2, and 3. No. 1 possesses the smallest proportion of color, and is suited to the production of harmonious prints from negatives in which, from the nature of the subject, from under-exposure or over-intensifying, the contrasts are abrupt. No. 2 is suited to good negatives of normal character, in which the densest parts are not absolutely opaque. No. 3 possesses a large proportion of color, and is suited to thin, soft negatives, a little lacking in force and intensity. By a classification of the negatives, and the use of a suitable quality of tissue for each, it will be found possible to secure more complete control over the character of the prints, and a more perfect uniformity of result than is possible in ordinary silver printing.
The tissue should be kept in a cool, dry place, packed flat, and kept under a weight. If suffered to be exposed to the atmosphere, it will be apt, in hot weather, to curl up and become unmanageably horny; whilst in damp weather it would, being a hygroscopic substance, absorb moisture.
SENSITIZING THE TISSUE.
This and other subsequent operations will of course be conducted in the "dark-room." A nearly saturated solution of bichromate of potash is employed. As the strength of a saturated solution varies with temperature, Mr. Swan prefers to make a solution of definite strength, by dissolving such a quantity of bichromate of potash as will not, during cold weather, crystallize. Such a solution is formed by dissolving one pound of bichromate of potash in twelve pounds of water.
The tissue is immersed by drawing it (face up) under the solution (contained in a dish two or three inches deep), care being taken to avoid the formation of air-bubbles. After immersion the sheet is turned, and a flat camel's-hair pencil is employed to remove the bubbles that form on the back, which, being apt to repel the aqueous solution in small points, should be brushed over until all parts absorb properly. After the displacement of the bubbles from the back of the tissue, it is again turned, and is drawn repeatedly through the solution. It then has clothes-clips attached along one of the edges, and is slowly withdrawn, so that the solution drains off without being repelled from the face of the tissue, and running off in streams. If the sheet is large, a thin lath of wood may be laid along the edge of the tissue that is first withdrawn from the trough, the tissue and lath being clipped together with clothes-clips. The time of immersion may vary from one to three minutes, depending somewhat on temperature and on the facility with which the tissue absorbs the solution. As a rule, as soon as it is quite limp from the thorough permeation of the solution, it should be removed. The longer the immersion, within certain limits, the more sensitive will be the tissue; but if too much prolonged, there is danger of two serious evils. In the first place, the paper becomes rotten, the gelatine also loses toughness, and the large quantity of water absorbed renders it liable to tear with its own weight. In the next place, long immersion in a saturated solution is apt to produce a crystallized surface in drying, which, of course, renders the tissue quite useless. As a rule, perhaps, two minutes will be about the average time of immersion; but a knowledge of the degree of pliancy required will be gained from two or three experiments.
The tissue should be placed to dry in a dark room, through which a current of dry air is constantly passing. In the first stage of drying, the temperature, of the air must not be above 60 or 70 degrees Fahr., for otherwise the gelatine, already softened with water, would melt. During damp weather, the air of the drying-room may be raised 10 degrees after the tissue has become half dry. If the drying be slow, the development of the image afterwards will be extremely slow or altogether impossible. As the result of much experience, Mr. Swan has arrived at the conclusion that keeping the sensitive tissue for a long time in a moist condition, or drying it slowly, results in a decomposition analogous to that effected by light, producing uniform and complete insolubility. We are able to confirm his conclusions from our own experiments. After complete desiccation, the sensitive tissue may be kept for several days. We have kept it for a fortnight without change. However, Mr. Swan strongly recommends that the tissue be used on the first or second day after sensitizing. By keeping too long, a discoloration of the print results, precisely analogous to that produced by keeping sensitive chloride of silver paper too long. The print develops tardily, and the lights are not clear. Excessively prolonged immersion in the bichromate solution of course retards drying, and should therefore be avoided. As a rule, by sensitizing in the evening, a supply of paper may be prepared for printing next day; twelve hours' suspension in a dry atmosphere being amply sufficient for the necessary drying.
Perfect desiccation, so as to make the tissue horny and unmanageable, is not desirable. It is in such a case difficult to get perfect contact in all parts in the pressure frame, and difficult to mount the tissue before development. Should the tissue by accident be rendered too dry, it is desirable to hang it for a few minutes in a damp place, when it will quickly become sufficiently pliant to permit easy manipulation. On the other hand, it is obvious that the tissue must not be too damp, or retain the slightest capacity for adhesion, or ruin to the negative would be the necessary consequence.
EXPOSURE UNDER THE NEGATIVE.
As the prepared side of the tissue is placed in contact with the negative, it is manifest, as we have just seen, that if it retained the slightest adhesiveness of surface, it would be dangerous to bring them together. Care must always be taken, therefore, not to use damp tissue.
For the exposure it is not necessary to use pressure frames with hinged backs, as the print is not, of course, examined in progress, the sole guide as to time being afforded by the photometer. The pressure of the back should be comparatively light, and the backing should be smooth and level. Fine cloth forms an excellent backing. Where the padding of the back is coarse, a piece of smooth cardboard may be placed at the back of the tissue. Too heavy pressure causes a kind of mottle of dark patches at points which have been pressed into absolute contact with the negative. If the tissue be quite dry, there can be no objection to sun printing; but if the slightest moisture were left in the gelatinous film, prolonged exposure to a hot sun with a dense negative would soften the film, and cause it to adhere. As, however, this tissue is much more sensitive than albumenized paper, printing in diffused light will generally
be more convenient, as well as safer. As a rule, the exposure is from one-third to one-half of that usually required for albumenized paper. In direct sunlight we have found the exposures with different negatives vary from one to ten minutes; in diffused light, from ten minutes to an hour, or upwards. In using the actinometer, it is scarcely necessary to observe that it must be exposed to the same light as the prints, the progress of which it is intended to indicate.
MOUNTING AND PREPARATION FOR DEVELOPMENT OF THE IMAGE.
The term development is used for convenience, although it is essentially different from the operation usually known as development, in which the reduction of a metallic salt on which light has acted produces an image, the developer completing or developing an operation which light has commenced. Here light has completed the chemical action; and the operation which follows is a mechanical one, which, by the removal of the sensitive compound where light has not acted, at once makes visible the image, and prevents the further action of light.
As we have seen that the washing away of the superfluous compound must be effected at the side opposite to that which was in contact with the negative, before we can commence development, the tissue must be mounted on another piece of paper with a material which is not affected by water, in order that the paper on which the compound has rested up to the present time may be removed, so as to expose the hitherto protected surface to the water.
As the paper upon which the tissue has to be supported, during the future operations, is placed in contact with the surface which will eventually be the surface of the finished print, it is desirable that it should be smooth and free from blemish; and it should be sufficiently tough to bear the treatment necessary in hot water. Fine Saxe paper answers well.
A solution of India-rubber is used for mounting the tissue. Pure India-rubber should be cut up into fine shreds, and dissolved in pure benzole at the rate of about ten grains to one ounce of the solvent. When properly prepared, it forms a thin varnish, but it leaves a palpable film of India-rubber on the paper to which it is applied. We have at times met with samples which dissolve very tardily in benzole. Where it is found desirable to hasten the complete solution, covering the shreds of India-rubber with a little chloroform will quickly reduce them to a pasty mass, which will be readily dissolved by the addition of benzole.
The India-rubber solution is poured into a flat dish, and the paper floated (till saturated) upon it, or rather drawn over it, so as to secure an even coating on the whole surface. The paper is then hung up by the aid of clothes-clips to dry. The tissue is now floated over the surface of the India-rubber solution in the same manner, care being taken not to allow it to sink below the surface; otherwise the back of the tissue would be coated with the India-rubber, and so retard subsequent operations. The tissue is then hung up to dry for about an hour. When the India-rubber on the paper and on the tissue is dry, the extreme edge of the tissue is cut off with scissors, and the two coated surfaces are carefully brought into contact and pressed together, when they cohere very tenaciously. In order to secure perfect contact and cohesion, they are next submitted to heavy rolling pressure. It is necessary to remember that any want of cohesion will issue in blisters, which will mar in greater or less degree the effect of the finished picture. It is important, therefore, that this operation be performed with care. The coated surfaces should be preserved alike from dust and from contact with fingers, or anything which could impair the cohesion of the India-rubber surfaces. In bringing the tissue into contact with the India rubber coated paper, the tissue should be bent back, so that contact is first made with the middle of the print; the ends of the tissue being then allowed to fall after first contact. Shifting the position after the tissue has touched the paper is inadmissible; it must, therefore, be laid on straight. After being placed, the back of the tissue may be lightly rubbed with the hand or a pad, the rubbing being from the centre outwards. It is an advantage to prepare a stock of paper in advance, and to use it about an inch larger than the tissue, and to fold this upon itself, so as to form a double thickness half an inch wide. Several prints may be attached to one piece of paper.
The press used for this and subsequent operations, in Messrs. Mawson & Swan's establishment, is a powerful copper-plate press (by which a pressure of several tons can be applied), having a plate of polished steel on the bed of the press, whilst a piece of thick elastic felt is placed between the print and the roller, compensating for possible inequalities in the tissue, paper, etc, and securing perfect contact in every part of the paper and print. It is probable that the more inexpensive rolling machines might be applied to this purpose without disadvantage; but very heavy pressure is indispensable. We find any ordinary bed-plate press, with the plate moving between the rollers, to be applicable to this stage of the process. The plate should be entirely level. In rolling, the India-rubber coated paper is laid on the steel plate, and a blanket of thick felt is laid over the tissue, which is uppermost, whilst it passes through the press.
We may remark here, that whilst the prepared surface of the sensitive tissue must be always carefully shielded from light, when once that has been covered up by mounting, it may be submitted to a dull, diffused light with impunity, care being taken that the back of the original tissue, which has now been rendered very non-actinic by the yellow color of the bichromate with which it is saturated, be uppermost. This permits the rolling of the mounted tissue to be effected in a moderately light room. The back of each print should be examined, and any India-rubber solution removed by rubbing with a piece of India-rubber. The object of this precaution is to secure uniform development. If some portions were rendered waterproof by patches of India-rubber at the back, such spots would be protected from the action of the water for a time, and would be incompletely developed when the other portions of the print were finished; the result of which would be a patch of a deeper tint than the remainder of the picture.
DEVELOPMENT AND WASHING.
The print is now ready for development. To effect this, a plentiful supply of warm water is necessary. In the Newcastle establishment, a series of three large wood-troughs are used. These are provided with hot and cold water-taps and waste-pipe. Into these troughs the prints are passed in succession. This ready supply of water and facility of securing any temperature is very desirable, as a matter of convenience, wherever the operations are conducted on a large scale. But the same result could be easily obtained on a more limited scale in photographic dishes, and having at hand a large vessel of hot water, as well as the ordinary cold water supply.
Where hot and cold water are not convenient, the following arrangement will be found very useful:
A wooden frame-work is made of the desired height, similar to a table with an open top, and a cross-piece in the centre. A and B are metal pans four to six inches deep, placed in the open top, suspended there by their rims, and heated by a gas-burner or the stove C. One of these is used for cold or tepid, and the other for hot water. It is convenient to have a thermometer in each pan.
The prints are first immersed in cold water, all air-bubbles being carefully removed. Here they are left for half an hour or more, as may be convenient, to permit the water to penetrate and soften the gelatine; after this, they are placed one by one in water of from 80 to 100 degrees Fahrenheit. This immediately loosens the backing paper upon which the tissue-compound was originally coated, and which, having now completed its office of supporting the tissue until it is no longer needed, is stripped off. It is separated from the tissue at one edge, and lifted gently away. If it still adhere tenaciously, a little longer soaking in the warm water will be necessary to effect the removal of the paper; but this is always a bad sign. The back surface of the tissue, opposite to that which was exposed, is now uncovered; and the next operation is to remove all gelatine, pigment, and chromic salt which have not been rendered insoluble.
The operation of developing, up to this period, has been conducted in a subdued or yellow light. As the sensitive surface is now exposed, it is obvious that strong white light should be avoided until the bichromate has been washed out of the film. This is rapidly done. A large portion has been removed whilst the print was soaking; and now that the gelatinous compound is exposed to the warm water, the salt is rapidly diffused in the water. The process of clearing may be accelerated by allowing a gentle stream of the warm water to fall on the surface of the print, or by laving the water on to it with the hands, so as to produce slight attrition between the surface and the water. This, however, is not necessary, as, if the print be left face down in the warm water, it will be found, in the course of from five minutes to a quarter of an hour, to have parted with nearly all the superfluous gelatine and color, presenting the image in all its proper gradations, and only requiring a little further washing to complete the operation.
The usual temperature for development is from 80 to 100 degrees, Fahrenheit; but there are circumstances which modify this. If, from over-exposure, the picture appear too dark, or from some tendency to insolubility in the compound, the image appear slowly, the temperature may be raised, when necessary, even to 150 degrees Fahrenheit; but high temperature must not be used until all the development has been effected that can be effected by water of a lower temperature.
The development is best commenced at as low a temperature as possible; and then, as soon as the image is fully made out, the print should be removed to cold water, in which the residue of bichromate will be washed away without risk of injury to the delicate half-tones, which would, with an under-exposed print, disappear in hot water. After two or three hours' immersion in cold water, the prints are one by one re-immersed in water at 80 or 90 degrees. Those which show signs of under-exposure are very carefully rinsed in merely tepid water, say 80 degrees, to clear away the soluble gelatine and adherent color; after which they are suspended to dry. The more fully-exposed prints remain longer in the warm water, in fact, until they become light enough. Any that are overexposed are put into hotter water, and are allowed to remain until the depth is sufficiently reduced. By judicious management of the development, using merely tepid water (not over 80 degrees), at the commencement of the operation, any under-exposed prints are discovered and saved. Then, by the use of hotter water to the more fully-exposed prints, these are speedily lightened to the required degree, and thus very few prints are lost either from under- or over-exposure.
When sufficient gelatine and coloring matter have been removed, and the prints are fully developed, they are hung up to dry. In the developing operation, several prints may be placed in one vessel; but as the image, although no longer soluble in water, is still slightly gelatinous, it is liable to abrasion; and care should be taken to avoid the prints dragging over each other, or over the bottom of the dish.
There are a few precautions to be carefully observed in development. It is most important to preserve uniformity of action. If, for instance, an air-bubble form, at any period before development is complete, the film of air protects the spot from the solvent action of the water, and the picture will be darker in that place. If the picture be suffered to float with the face partially out of the water, the same thing will happen. It is desirable, therefore, to keep the face downwards until the operation is completed, and to remove air-bubbles whenever they form. It should further be remembered, in observing the depth of the picture, that it is now seen on a ground considerably degraded by the coating of India-rubber, which gives the paper a brown tint, and that when transferred to pure white paper, it will possess much greater brilliancy.
TRANSFERRING THE PRINTS.
The picture, up to the present time, presents an image in which right and left are reversed. It is now necessary, therefore, to transfer it from the paper which has supported it temporarily for the purposes of manipulation, to its final resting-place, in which operation the right and left will resume their proper relations. The image may be either transferred to a sheet of cardboard, so as to require no further mounting, or to paper; in the latter case, it is simply in the position of an ordinary print, and will require subsequent mounting. Each method has certain specific advantages, but generally the transfer to paper is to be preferred.
Transferring to Cardboard.—The face of the dried print is very evenly coated by floating, or by means of a flat camel's-hair brush, with the following preparation:
The gelatine should be melted and carefully cleared of air by long heating, and skimming the froth; after which the glycerine is added. It will at all times, of course, require melting by heat and straining through wet flannel or muslin before use; it is then applied evenly to the surface, either by floating (which is best), or with a broad camel's-hair brush, and afterwards hung up to dry. Sometimes in coating the print with the gelatine solution, there is a disposition manifested by it not to adhere on all parts of the surface of the print, but to “creep” off in certain spots. When this happens, there can be no adhesion at those points between the gelatinous coating and the pictures; consequently, the film, composing the picture, will be torn off from such places, when the rubbered Saxe paper is removed. To avoid this non-adhesion and “creeping,” it is best to coat the prints in a warm room, where the hot gelatine will not chill, nor “creep,” upon being brought in contact with the surface of the print, as it would do if the print were in a cold apartment. When dry, the print is trimmed to the required shape. A piece of stout cardboard of the required size, pure in color and fine in surface, is passed through clean water, and then drained. Upon the moistened surface the print is laid, face downwards, exactly in the position it is designed to occupy, and the card is removed to the rolling-press and placed on the polished steel plate, print-side downwards, the side on which the print is placed being in contact with the plate, and a felt blanket on the back of the card; it is now submitted to a heavy rolling pressure, and then put aside to dry.
The quality of the cardboard and the exact condition of dampness are of considerable importance. It must be perfectly moistened all over, as, if any point or patch were omitted, the adhesion of the print in that place would not be secured. There should be an absolute film of water on the surface, so that as each part is submitted to the rolling pressure, a wave, infinitely small, however, is driven before the pressure, effectually displacing air, and securing perfect contact. It is, however, undesirable to have excess of water. There should be no delay in applying the pressure after the print has been placed in contact with the moistened surface, inasmuch as the gelatinous, although insoluble image, by absorbing moisture and becoming soft, might, under the heavy pressure, lose something in sharpness.
As each print is passed through the rolling-press, it is placed upon the last, and when the pile is completed, a weight is placed upon the whole heap. By adopting this course, the prints dry without warping or cockling; and at the expiration of about twenty-four hours the print is ready for the final operation.
This consists in removing the paper which has supported the image during the operations of developing and washing. The picture must be quite dry before the operation is attempted. A piece of clean cotton-wool is saturated with pure benzole, and the caoutchouc-coated paper which covers the print is rubbed pretty hard with it. An edge of the caoutchouc-coated paper is then gently raised with the point of a blunt knife, care being taken to commence at a black part of the picture where the film of the compound forming the image is thickest. The raised edge is then taken hold of, and pulled so as to tear it gently and steadily off the print. Instead of removing the paper with an upward or lifting motion, it is better to turn it backwards, so that the strain is in a horizontal direction, as there is, in this method, less danger to the surface of the print at any point in which the adhesion in mounting is imperfect. As a general rule, especially when the benzole is used sparingly, the paper brings away with it all the India-rubber coating; but any traces remaining may be rubbed away with the finger or with a piece of India-rubber. It is best always to rub a sponge, dampened with benzole, over the surface of the picture as soon as the Saxe paper is removed, even when there are no perceptible adhering spots of varnish on the print. Under ordinary circumstances, the picture is now finished. If required for coloring, the print may be coated with plain collodion, or a suitable sizing preparation.
It is important to remember that defective manipulation in the mounting operation seriously mars the beauty of the finished picture. It is necessary that the pressure should be perfectly uniform in order to secure evenness of texture in the surface of the picture. If the transferring coating of gelatine were laid on in uneven patches or streaks, the effect of this will be apparent in patches or streaks of greater brightness or dulness of surface, the thickest parts receiving the highest pressure, and consequently having the brightest surface. Any unevenness of pressure in rolling will produce a similar result.
Transferring to Paper.—The manipulations here are very similar to those which we have just described, but are a little more easy. It is not necessary to trim the print to its proper size or shape, as this will be done in the final mounting. The mounting papers are carefully immersed in water, air-bubbles being brushed away, and then laid one upon another while in the water; they are then drawn out in a pack, and are suspended to drain for some hours, or submitted to pressure to remove the superfluous water; a perfectly even film of moisture is thus secured. The transfer is effected by laying the print face up on the steel plate of the press, and over the print is laid the moistened paper, and on that a felt blanket. The press is then “pulled.” The print is next immersed for an hour in a bath, containing five per cent, of alum, and is afterwards well washed in water and dried, after which it is uncovered as when mounted on cardboard.
By transferring to paper, it will be observed that facility is afforded for performing the last-mentioned operation, by which an additional source of stability is secured. The only possible source of deterioration in the prints produced by the method we have described, exists in the thin coating of gelatine with which the print is attached to its final support. By means of moisture and friction the print could be removed; this, it is true, is destruction, not fading or instability in its usual sense. But it is happily possible to remove even this susceptibility to injury. Although the transference of the print direct to cardboard has the advantage of making an exceedingly neat finish to the mounting (the print being slightly recessed in the cardboard), and although it has the further advantage of reducing the number of operations required to complete the picture, yet Mr. Swan greatly prefers, and almost invariably adopts, the method of transfer to paper, chiefly because this method secures the most uniform adhesion, and because it allows the gelatine (used to cause the print to adhere to the paper), to be rendered water-proof—a property not possessed by the prints mounted direct to card. One of the means used by Mr. Swan to render the gelatine insoluble is quite novel, and constitutes one of the first applications of his discovery of the property possessed by salts of the sesquioxide of chromium of rendering gelatine insoluble. A solution of common alum has, to a certain extent, the power of waterproofing the prints, and generally fixture with alum is quite sufficient. Where, however, more thorough waterproofing is demanded, the prints, after transfer, should be treated with a one per cent, solution of chrome alum. Mr. Swan has shown us some prints very successfully transferred without a press. The transfer was effected with the gelatine solution ordinarily used, to which has been added one-twentieth of a ten per cent, solution of chrome alum. Prints intended for coloring in water colors should be chrome-fixed.
Mr. Swan generally adds a small proportion of a white pigment to the gelatine with which the transfer is effected, in order to give brilliancy to the whites of the picture, and to avoid the intervention of a transparent film between the under surface of the print and the paper to which it is attached.
- We say, that by the employment of a kind of tissue possessing a suitable proportion of pigments, the faults of weakness or of hardness in the negative may be ameliorated. We must not be understood to say altogether corrected; for after all that can be done in the way of compensation, defective negatives will produce defective carbon prints. And here it may be well to mention that the kind of negative which suits best for Mr. Swan's process is a negative of full average density, with full detail in the shades, such as is got by ample exposure and development. There should be some, although little, absolutely bare glass; but whatever deposit of silver there is on the deepest shades should be a pure photographic deposit, and not “fog.”
- First wipe the surface of the tissue with a tuft of cotton, piece of cotton flannel, or soft linen, taking care not to touch the printing surface with the hands.
- Never sensitize more than a dozen 4-4 sheets of tissue in thirty ounces of solution, and the same proportion for a larger or smaller quantity. Then throw the solution away. Immediately before sensitizing, said solution must be thoroughly shaken or stirred; otherwise, the lower portion will be stronger than the top, and no good results can be obtained. Never put more than two sheets of tissue in the sensitizing solution at the same time, unless you can keep them widely separated from each other. If by dipping them vertically you can keep the entire surfaces of the sheets half an inch distant from each other, any desired number may be immersed and sensitized at the same instant.
- We have always practised brushing on the India-rubber solution, i.e., “Hydro-carbon Varnish,” with a soft camel's-hair blender. To do this, the print is fastened to a glass by means of clips or sticking paper, and then an even coating brushed over it. It is very difficult to float the tissue.