The American Carbon Manual/Historical Notes on Carbon Printing, 1814 to 1868
HISTORICAL NOTES ON CARBON PRINTING.
Photographic Carbon Printing may be said to have commenced with the labors of M. Nicephore de Niepce, in 1814. His pictures were produced by the action of the solar rays upon certain hydro-carbons, which were rendered insoluble in the usual menstrua, wherever they had been submitted to the influence of light.
In 1839, Mr. Mungo Ponton announced, in the Edinburgh New Philosophical Journal, a process of producing images by the action of light on paper which had been impregnated with a solution of bichromate of potash.
M. Becquerel shortly afterwards investigated the action of chromic salts on organic substances under the influence of light, and arrived at the conclusion that the coloration and insolubility were due to the reaction which took place between the chromic acid and the sizing matter in the paper, as, on using unsized paper, the effect could only be produced in course of time. Upon this, he used another modification of the process: employing a paper sized with starch, he subsequently treated the image, obtained by the aid of the bichromate, in a weak alcoholic solution of iodine, and so obtained a blue tint. In 1852, Mr. Fox Talbot patented a method of photo-engraving, in which he availed himself of the reaction between organic matter and chromic acid under the influence of light.
In 1855, M. Poitevin announced the first carbon process. He proposed to render the reaction between a chromic salt and organic matter available in producing photographs in permanent pigments, the image being formed either of coloring matter miscible in water, or of a fatty ink.
In the same year, M. Lafon de Camarsac announced his discovery of methods of producing photographs in enamel colors, in which, although very vaguely stated, are involved the elements of a carbon process.
In 1857, Dr. Phipson conceived that certain volatile oils, such as those contained in the oil known as huile de Dippel, or those which accompany naphthaline, and which blacken when exposed to the light and to the air, might be used to obtain photographs.
In September, 1857, Mr. Thomas Sutton proposed adding the finest pulverized charcoal to an albuminous solution of bichromate, so as to form a black mixture of about the consistency of common shoe-blacking, which was then applied to paper. This was to be exposed under a negative, and the unaltered material subsequently removed by repeated washings, and a carbon print produced.
Later in 1857, M. Testud de Beauregard, a gentleman who had devoted much attention to the production of photographs without salts of silver, patented a process, not differing from that of M. Poitevin in principle, but having some modifications in detail. In his specification he re-states the fact as already recognized, that if a mixture of gum or gelatine, bichromate of potash or ammonia, and an insoluble coloring matter, such as carbon, black-lead, vermilion, indigo, etc, be exposed to light, it is rendered insoluble, the coloring matter or pigment being imprisoned or retained by the mixture, and that if such exposure be effected under a negative, the portions not acted upon by light may be washed away, leaving an image in pigments.
Mr. Pouncy appears to have been the first in England to produce carbon prints by means of photography. The earliest examples exhibited were shown at the April meeting of the London Photographic Society, in 1858: but the author declined to describe the process. An allusion to it and its results, without details, had appeared in one of the journals a few weeks earlier, viz:
“He prepared the paper, or other surface, for having the picture produced on it, by applying over its whole surface the coloring matter which formed the picture, and together with this coloring matter applied a substance which is acted on by the light. The following is the manner in which he proceeded when printing positive pictures on paper from negative pictures: He coated the paper, or surface which received the picture, with a composition of vegetable carbon, gum arabic, and bichromate of potash, and on to this prepared surface placed the negative picture, and exposed it to the light in the usual way. Afterwards, the surface was washed with water, which dissolved the composition at the parts on which the light had not acted, but failed to affect those parts of the surface on which the light had acted. Consequently, on those parts of the surface the coloring matter remained in the state in which it was applied, having experienced no chemical change. Sometimes, for the vegetable carbon, he substituted bitumen.”
MM. Henri Gamier and Alphonse Salmon, in August, 1858, laid before the French Photographic Society, a “Method of Carbon Printing,” in which paper was treated with a strong solution of citrate of iron, and after drying in the dark, was exposed under a transparent positive. The parts acted upon by light were rendered insoluble, whilst the parts protected by the dense portions of the cliché remained soluble and hygroscopic; and when tested with powdered lampblack, plumbago, or other pigment in fine powder, and then breathed upon, the color adhered to the hygroscopic portions, forming the shadows, but not to the lights, which had been rendered insoluble by the action of light. After washing in clean water, the print was perfected.
At the same meeting, M. Gabriel de Rumine described a method of obtaining prints analogous in some respects to that of MM. Gamier and Salmon, but rather resembling the patented method of M. Testud de Beauregard. He treated paper with a solution of gelatine and bichromate of potash, and, when dry, covered the surface with black-lead. After exposure under a negative, the print was removed to a dish of boiling water, which removed the soluble portions of the gelatine and the adhering color. M. Brebisson, a few months later, proposed a similar method, applying the color after instead of before exposure.
Mr. Charles Seeley, M.A., editor of the American Journal of Photography, proposed a method with gum, carbon, and a bichromate, mixed, and applied to paper, which he found successful. On learning that M. Poitevin had anticipated this method by several years, he ceased to prosecute it.
In the following year, the Due de Luynes' prize was awarded, an event interesting in the history of carbon printing. In 1856, this nobleman had offered certain prizes for improvements in connection with photography, including one of 2000 francs ($400), for a method of producing permanent prints. A number of processes were forwarded to the Commission, consisting of modifications of silver printing processes; but these were put aside as unsatisfactory, and attention chiefly given to the methods of carbon printing, the competition being practically confined to the processes of M. Testud de Beauregard, MM. Gamier and Salmon, and Mr. Pouncy.
The decision of the Commission was, that the process of MM. Gamier and Salmon and that of Mr. Pouncy were about equal in result, the latter requiring, however, an exposure of nearly four times as long as the former; but being at the same time simpler in manipulation. M. Poitevin had not entered the competition or contributed specimens; but recognizing in him the originator of all the processes of carbon printing, the Commission felt bound to acknowledge his merit. Instead, therefore, of awarding the whole amount as one prize to any individual, it was resolved to divide it, awarding a gold medal, value 600 francs, to M. Poitevin as the originator of carbon printing; a similar medal to MM. Davanne and Girard for contributions to the improvement and stability of silver prints; a silver medal, value 400 francs, to MM. Gamier and Salmon, for their carbon printing process; and a similar medal to Mr. Pouncy, for his process.
A further prize of 2000 francs, offered by the Due de Luynes for the same purpose, was awarded to M. Poitevin in 1862. In 1867 he received the prize of 8000 francs, offered for the best mechanical printing process with fatty ink, based upon photography.
Early in 1864, a suggestion for the production of carbon prints was made by Mr. Obernetter, of Munich. In his process, paper is treated with a solution of sesquichloride of iron, chloride of copper, hydrochloric acid, and water. After drying, the paper is exposed under a negative, and then developed in a solution of sulphocyanide of potassium, sulphuric acid, a little of the sensitizing mixture and water, and then washed. The image is formed of sulphocyanide of copper: if the print be exposed to an atmosphere of chlorine, the image is converted into a chloride of copper. The prints so produced were of a chestnut brown.
We now come to another definitely-marked feature in the history of carbon printing. In all the efforts hitherto made, there was one signal defect,—the absence of perfect gradation of tone from light to dark. A certain granular grayness in patches represented half-tones in some of the processes; but the tendency was to abrupt steps from white to black, any definite approach to delicately-marked gradation being rarely obtained. This was believed to be owing to the nature of the materials employed: the notion prevailed that the finest mechanical subdivison of a pigment could not equal in delicacy the fineness of the deposit obtained by the reduction or precipitation of a metallic salt. We know now that this was not the cause of the difficulty, which was due to the mode rather than to the material, as will be seen from the explanation which follows. When the surface of a film of bichromated gelatine is exposed to light, all portions upon which light acts in the slightest degree, whether through half-tones or shadows, are rendered insoluble at that surface, the only difference being that the light penetrates deeper in the shadows, and therefore produces a thicker layer of insoluble matter. When the exposed print is placed in a solvent, the whites, having been protected from the action of light, are laid bare at once; and the water then penetrating laterally, dissolves the soluble layer underneath the thin insoluble film, which forms the half-tones; these being thus deprived of their contact with the paper, float away, leaving only the deep shadows, in which the light has penetrated quite through the film. The picture thus consists of masses of black and white, without true gradation. The only wonder is that any approach to half-tone at all was ever secured by such a mode of operating. Such gradation as was obtained appears to have been dependent upon the fact that a thin solution and an absorbent paper were employed, the bulk of the sensitive material being in absolute contact with the fibre of the paper; so that any portion, on which light had acted at all, was not readily removed from the paper. Certain it is, that the principle of washing away the unaltered material from the side opposite to that which was exposed to light is vital to the perfection of gradation in carbon printing. As the discovery of this principle has been the subject of a little misapprehension, it is interesting to trace it here to its first enunciation, and mark how its gradual recognition, and the discovery of practical means of applying it, have led to the perfection of carbon printing.
The first recognition which we find of this principle is in a paper on the use of linseed oil as a sensitive agent in photographic engraving, by M. L'Abbé Laborde, communicated to the French Photographic Society in July, 1858. In this paper, the Abbé announced his discovery that linseed oil which had been treated with litharge was sensitive to the action of light, becoming insoluble under its influence, in like manner to asphaltum. In the course of his experiments with this substance, he had discovered that the insolubility caused by the action of light commenced at the surface exposed, and gradually penetrated through the film in direct proportion to the intensity of the light. He acknowledged the loss of half-tone, although he did not suggest a mode of meeting the difficulty.
In November, 1858, Mr. J. C. Burnett, in a communication on carbon printing, pointed out the same fact still more clearly, and indicated the direction in which effort must be made in order to overcome the difficulty. After speaking of the application of a mixture of gelatine, bichromate, and pigment to paper, he says:
“It must be observed that the possibility of producing half-tones by this plan rests on the power of the insolubility—causing actinism to penetrate, with a certain degree of facility, the mixture of pigment with bichromate and gelatine, or gum, the gelatine or gum being in consequence rendered insoluble, to a greater or less depth on different parts of the picture, according to the varying depth to which the actinism has been allowed, or had time, to penetrate; this, again, being dependent on the varying translucency of the different parts of the negative.”
It was at this juncture of the history of carbon printing, we learn, that Mr. Swan commenced his experiments. Without being aware of Mr. Burnett's suggestions, and prior to the publication of Mr. Blair's letter (presently to be mentioned), he attempted to obtain half-tone by coating a plate of glass with a mixture of lamp-black, solution of gum arabic, and solution of bichromate of potash. After drying the coated plate, he exposed it in the camera, with the uncoated surface of the glass turned towards the light passing through the negative and lens. The plate was then washed in water, with the view of removing, from the back of the sensitive coating, those portions which the light had not rendered insoluble. The experiment was not successful (probably in consequence of the too feeble action of the light); it shows, however, that Mr. Swan not only, ab initio, recognized the true principle upon which the production of half-tone in carbon-printing depends, but, furthermore, applied it in a very elegant manner.
Early in the following year, Mr. Blair, of Perth, made the same discovery; for it is worthy of note that in each case the recognition of the fact upon which the loss of half-tone was based, seems to have been the result of independent observation. Having applied a thick coating of the sensitive gum and carbon to his paper, he noticed, after exposure, that “the outer crust was more sunned and hardened than the inner,” and that, by the time the paper was sufficiently steeped, the inner surface, which had been least sunned, was too soft, and was washed away, carrying with it the “outer crust.” It occurred to him, therefore, that if he could get the inner surface rendered insoluble first, he could overcome the difficulty. He accordingly attempted to print through the coated paper, so that the inner surface was acted upon first, leaving the soluble portions on the outer surface to be removed by washing. This gave a certain amount of success; but the exposure was long, and the print looked granular from the texture of the paper through which the light passed. Subsequently, he tried the use of waxed paper as a support for sensitive material. This lessened the exposure, and in some degree lessened the granulation; but the lights, consisting of waxed paper, were not good in color, and certain practical difficulties remaining, the method did not come into general use. The principle of securing half-tone in carbon prints was now known, but an efficient and practical mode of applying it remained unknown. To the subsequent modes of utilizing this principle, we shall refer presently.
Towards the close of 1859, M. Joubert called attention to a process of carbon printing which he styled phototype. An example, published in the journal of the Photographic Society, in June, 1860, showed that the process possessed much promise of excellence, the results far surpassing anything till then seen. That they could be printed with facility in large numbers was manifest from the circumstances under which the specimen was issued. The details were not published, and remain to this day the secret of the inventor.
The next step in the history of carbon printing is based upon the discovery of another effect produced on certain bodies by the action of light, and is again due to M. Poitevin. It will be noticed that the processes already described depend on the action of light in rendering a soluble body insoluble. This is the effect on asphaltum, upon a mixture of some organic bodies and a chromic salt, and upon some other substances. In the new process discovered by M. Poitevin, a body previously insoluble is rendered soluble and hygroscopic by the action of light. The first results of this process were shown to the French Photographic Society, in July, 1860, the process having been patented the month previously. The details were published in the following November. In this process a mixture of perchloride of iron and tartaric acid, ten parts of the former to four of the latter, dissolved in one hundred parts of water, is the sensitive preparation. It is poured on a plate of glass which has been previously coated with collodion or other suitable material; it is then left to dry in the dark, and becomes spontaneously insoluble. Submitted to the action of light, however, it again becomes hygroscopic. After exposure under a negative, if breathed upon, the parts upon which light has acted become moist and tacky, in degree and depth proportioned to the action of light. Finely-powdered carbon, applied with a brush, adheres to the image in greater or less proportion, just in the degree in which moisture is absorbed, thus giving an image with a just gradation of half-tone. For producing enamels, a vitreous powder has to be applied; and for the production of images in printing ink, a fatty acid or a resin has to be applied. The print, if in carbon, was subsequently washed with water containing a little hydrochloric acid, and a piece of paper coated with gelatine applied to its surface, attached to which the collodion film, with the image upon it, was removed from the glass. In some cases the sensitive fluid was applied direct to the glass, and when the picture had been produced by the application of carbon to the exposed film, a coating of collodion was applied, and the whole eventually transferred to gelatinized paper. This was the first process in which a collodion film was used for the purpose of transferring the carbon picture from a glass plate to paper.
In the following November, M. Fargier brought before the French Society a process for which, in the September previous, a patent was obtained. It consisted of an ingenious combination of previously published discoveries. A plate of glass was coated with a mixture of gelatine, bichromate, and carbon, and, when dry, exposed under a negative. The exposed film was then coated with tough, plain collodion; and, after allowing the film to set, the whole was plunged into warm water. This dissolved the portion of gelatine which remained soluble, and detaching the film from the glass, removed the unaltered pigment and gelatine, leaving an image attached to the collodion film with perfect gradation of half-tone from white to black. The film was next attached to a sheet of gelatinized paper, collodion side uppermost. The results were exceedingly beautiful, far surpassing in delicacy and gradation anything which had previously been produced in carbon printing.
In the spring of 1863, M. Poitevin made another important advance in carbon printing, based upon his last discovery,—that soluble organic substances might be rendered insoluble by the action of metallic salts, and recover their solubility under the action of light. It will be seen that in this operation the reactions are just the reverse of those in the chromo-gelatine processes. Paper, coated with the sensitive salt and pigment combined, simply needs exposing to the direct action of light under a transparent positive, and washing in water. The dark coating forming the shadows retains its insolubility; and in the half-tones, the film being rendered soluble through a part of its thickness, is washed away in due proportion, whilst the light having penetrated quite through the film in the whites of the picture, the color is washed away entirely, leaving the bare paper.
The mode of proceeding is as follows: Five or six parts of gelatine are dissolved in one hundred parts of water with gentle heat, and to this the necessary proportion of carbon (or some inert pigment), is added, and the paper is coated with the mixture. When required for use, these are impregnated with a solution containing ten parts of perchloride of iron, and three parts of tartaric acid in one hundred parts of water. This paper is left to dry in the dark, when it becomes insoluble, even in boiling water. It is then exposed under a positive cliché, and, under the influence of light, becomes soluble, commencing at the surface of the film. A short exposure is sufficient, and the print is then immersed in warm water, which removes the soluble matter, leaving the print with its true gradations of light and shade. It is now necessary to remove the tint given by the iron salt to the paper, and this is done by washing in a dilute solution of hydrochloric acid. The print is then rinsed and dried. To prevent injury to the gelatine film, which would become further soluble by the action of light, it is rendered wholly insoluble by any of the known methods, such as immersion in a solution of alum, bichloride of mercury, &c. This process has not hitherto been successfully worked.
In the same communication, M. Poitevin, mentioned another mode of carbon printing. Paper treated with perchloride of iron and tartaric acid, without pigment or gelatine, is exposed under a positive cliché. The parts treated with these salts possess the power of precipitating casein, which, after insolation, they lose. The coloring matter is therefore mixed with milk, and the exposed print immersed in it; the casein, and pigment with it, are precipitated on the protected parts, which form the blacks of the picture.
Early in 1863, Mr. Pouncy called attention to a new mode of carbon printing, which he had patented in the previous January. In the course of the summer the details were published, and were found to embrace an important new principle in carbon printing, inasmuch as the picture was formed of a fatty ink, similar to that used in ordinary mechanical printing. Thin, transparent paper, like tracing paper, was coated with a mixture of carbon or other pigment, fatty matter, such as tallow or oil, bichromate of potash, or bitumen of Judæa, or both, and turpentine, or some equivalent body. When dry, this is exposed under a negative, with the back in contact, according to the now recognized principle of obtaining half-tone. After exposure, the unaltered matter was removed by means of turpentine or other similar solvent, leaving an image with perfect gradation, in a material analogous to printing ink. The picture thus obtained was then mounted on white or tinted paper. The chief drawbacks to this process were: first, the impure lights which resulted from the tracing paper on which it was necessary to produce the prints to secure sufficient transparency, in printing through the paper. This has, however, been overcome by adopting a transferring process in which the image is removed from tracing paper to any ground which may be chosen. The second difficulty was the long exposure, which was about three times as protracted as that necessary for silver printing on albumenized paper. Many of the results we have seen are, however, very excellent.
Later in the same year, Mr. Blair expressed a conviction as to the disadvantages of any method of producing prints on waxed, oiled, or varnished paper, and proposed a method on plain paper, in which he endeavored to compensate for the tendency to lose half-tone when the prepared surface was presented direct to the light, by the mode of preparing the paper. It was at first coated with gelatine; then, when dry, with albumen and syrup (containing a little transparent color, to give a delicate half-tint to the paper). Subsequently the surface was coated with carbon powder, which was made to adhere by moistening the back of the paper. Finally, when required for use, it was floated, back downwards, on a solution of bichromate of potash. When dry, it was exposed with the face in contact with the negative. The washing away of unaltered sensitive material and color was effected by cold or warm water, and a brush, to which sometimes a little ammonia, or acetic acid, was added. Mr. Blair describes this method as giving, with care, pretty good results.
The next step in carbon printing marks an important epoch in its history: we refer to the introduction by Mr. Swan of a prepared tissue for producing the pictures, which permits exposure on one side, and washing away on the other. This step, together, with the complete system of operations connected with its use, made carbon printing practicable as a useful art. The process was first announced in the Photographic News, early in 1864, and during many succeeding months continued to occupy a large share of public attention. The results were as perfect as the most fastidious could desire, and the process was so simple in itself, and so clearly stated by Mr. Swan, that for the first time in the history of carbon printing, many experimentalists gave attention to the subject, and produced excellent pictures. As the mode of working is given in another chapter, it is not necessary to state it here.
In the course of the discussions elicited in the photographic journals by the publication of Mr. Swan's process, we learn that Mr. Davies, of Edinburgh, had, in the course of experiments in photo-lithography, produced transferred carbon prints as early as 1862; allusion to which was made in a paper read at the Edinburgh Society in February, 1863. A series of circumstances, however, prevented the publication of his process until July, 1864. He then described a method analogous to that already patented by Mr. Swan, namely, coating paper with gelatine, bichromate, and pigment; exposure with the prepared surface next the negative, mounting with a solution of shellac and Venice turpentine in alcohol, or with albumen, and then coagulating; soaking until the original paper leaves the gelatine and pigment, and then developing with hot water.
It is not necessary to mention all the minor modifications of Swan's method which were proposed; nor the various suggestions which grew out of the discussion of his process. We may, however, mention one or two of the latter. Mr. Frank Eliot suggested taking advantage of M. Poitevin's last process with perchloride of iron and tartaric acid. He proposed working with black paper, coating it with gelatine and white pigment, sensitizing with the iron salts and tartaric acid; then exposing, and developing, to obtain an image in white pigment on a black ground. If any attempt were made to carry out such a scheme, it is obvious that a pigment must be found which would not decompose the iron salt or be affected by it.
During the following year, Mr. M. Carey Lea published working details of two processes, both of which were analogous to some of the earlier processes. In the first, the paper was prepared with a mixture of gelatine, glycerine, bichromate of potash, and water; and, after drying, was exposed under a positive cliché. The parts intended to form the lights become hardened and insoluble; the shadows, being protected from the light, swell and soften, but do not dissolve on immersing the print in cold water. The prints are left soaking, to remove as much of the color of the reduced chromic salt as possible from the lights; and afterwards, finely-powdered lampblack is applied, which adheres to the softened gelatine, and forms the picture. Mr. Lea points out that, as in this process the lights are embodied in the shadows, and no part is washed away, there is no danger of losing the half-tones, as in some processes; and that he thinks it is possible to obtain some degree of gradation by this method.
The second process proposed by the same gentleman is avowedly for the reproduction of subjects without half-tone. In this process he employs a mixture of gum-arabic, albumen, glycerine, double chromate of potash and ammonia, powdered graphite, and water. The mixture is of the thickness of honey, and is applied to the paper with a broad brush. When dry, it is exposed under a negative of any subject in line or stipple. After exposure, the right amount of which may be ascertained by examining the back of the print, it is developed by soaking in cold water, which removes the unaltered material, leaving clean lights and good blacks.
A more recent suggestion for a method of carbon printing was made in the course of last year by Dr. Gotschalk. He observes that graphitic acid, a substance prepared by the action of nitro-sulphuric acid upon graphite, is sensitive to light, which deoxidizes it, reducing it again to the condition of graphite. Hence it was proposed to prepare paper with a solution of this body, and expose it under a negative to the action of light. The first difficulty in the way is its insolubility, or sparing solubility, in any available menstruum. The trace, dissolved by water, applied to paper, is sufficient to color the paper brown when exposed to light. A method of checking its action, or fixing the print, would also be required. No practicable application of this substance for this purpose has yet been found.
Although Mr. Swan has perfected a process for us, by which we may secure uniform and elegant carbon prints, yet he has not given us all we want. What is needed to make the carbon entirely supersede the silver process, is a direct carbon printing process—one that will enable us to print directly upon a permanent surface without the necessity of making transfers.
Mr. V. M. Griswold, Peekskill, N. Y., has been experimenting in this direction for the last three years with varied success, persistently overcoming one difficulty after another, until he is able to produce passable results always, and very fine and promising ones in some cases. He has shown us prints which possess elegant detail and half-tone, yet lacking purity in the whites and brilliancy. His principal trouble seems to be in the matter of exposure, and to discover the best method of sensitizing his paper or other medium. The prints we have seen are of cabinet size, larger and smaller, upon iron and paper. He secures them with equal facility upon wood, canvas, silk, etc.
The details of his process are not yet made public. We are assured that it is based upon an entirely new and beautiful principle, as well as quite an original one.
To avoid the transferring, Mr. M. Carey Lea recommends printing through or upon glass by reflected light. He places the frame, holding the negative and the pigmented glass, against the wall beside the window at which the sun enters. A mirror is then so placed that the rays fall upon it almost perpendicularly, and are reflected upon the frame. After printing, all that is necessary is to wash off the superfluous pigment, and the picture is finished.
Here we leave this wonderful process, for the experimenter to follow up and perfect.
- Comptes Rendus, June 11, 1855.
- “Photographic Notes,” vol. ii.
- It is right to place on record here that this has been denied. Mr. Portbury, in a letter to the Photographic News, Nov. 23, 1860, and personally at a meeting of the Photographic Society, Nov. 4, 1862, claimed the production of the first carbon prints, stating that he was at the time an apprentice with Mr. Pouncy.
- Bulletin de la Societé Française de Photographie, August, 1858.
- “Journal of the Photographic Society,” vol. v, p. 84.
- “Photographic Notes,” vol. iv, p. 45.
- “Photographic News,” vol. iv, p. 331.
- “Photographic News,” vol. iv, p. 390.
- “Photographic News,” vol. vii, p. 124.
- "Photographic News," vol. vii, p. 169.
- It is worthy of note here that a very similar mixture appears to have been used at a very early date in obtaining a sensitive surface for photo-lithographic purposes. In January, 1863, Mr. A. Mactear read a communication to the Glasgow Photographic Association, describing a process of photo-lithography employed by Mr. Gibbons in 1859. The sensitive compound applied to the stone consisted of copal varnish, raw linseed oil, bichromate of potash, Brunswick black, mastic varnish, and turpentine, ground up together.
- "Photographic Notes," vol. viii.
- “Philadelphia Photographer,” vol. ii.
- “Photographic News,” vol. ix, p. 459.
- “Philadelphia Photographer,” April, 1868, p. 104.