The American Carbon Manual/The Sensitive Collodio-Gelatine Tissue
THE SENSITIVE COLLODIO-GELATINE TISSUE.
The method of carbon printing with this tissue is better suited to the amateur than to the professional photographer. It involves more trouble than the use of the paper tissue, but the results are very beautiful; and as the photographer, in employing it, mixes his own preparations, he has certain points in color and intensity more completely under his own control than he could have in purchasing a ready-prepared tissue.
To prepare the Sensitive Collodio-Gelatine Tissue.—Take a sheet of plate-glass, free from blemishes or scratches, and clean it perfectly, finally rubbing the surface with a saturated solution of beeswax in ether. This is then wiped off with a clean cloth, leaving a scarcely perceptible coating of the wax. This coating may be omitted; but it tends to facilitate the future removal of the tissue from the glass.
Now coat the glass with a plain collodion, giving a thick, tough, transparent film. The pyroxyline should be of the kind which yields a film free from opacity or opalescence. About ten grains in an ounce of solvent, consisting of equal parts of ether and alcohol, will answer the purpose. This film is, of course, suffered to dry before applying the tissue compound.
Next make a solution of gelatine and sugar, as follows:
|Pure gelatine,||2 ounces,|
|White sugar,||½ ounce,|
The kind of pigment to be employed, and the proportion in which it is to be added, will depend much on circumstances, into the details of which we enter in another chapter; but it is especially important in the preparation of this kind of tissue, that the pigment employed be so finely divided that no subsidence will take place during the period the tissue compound remains in the fluid state upon the glass. The preparation in this state may be kept ready for use. It should be kept in a well-corked, wide-mouthed bottle; in hot weather it is apt to decompose if kept long. It may, if desired, be poured into a flat dish to the depth of about half an inch, and, when nearly dry, cut into shreds, and thoroughly dried; in which state it may be kept without risk of injury. When required for use after drying, it must be soaked again in eight parts of water.
The proportion of gelatine and of sugar given is that which is found to answer best under ordinary circumstances. But these proportions will be influenced by the quality of the gelatine, the temperature, and varying conditions, in regard to which experience must be the guide. In very dry weather, for instance, the proportion of sugar may be increased, its chief office being to give pliancy and elasticity to the tissue, and prevent the horniness of the gelatine when perfectly desiccated.
To prepare the tissue compound for use, heat must be applied until it is quite fluid, when one part of a saturated solution of bichromate of ammonia must be added to every ten parts of the gelatinous compound, after which the whole should be strained through flannel. It is desirable, after the chromic salt has been added to the gelatine, to avoid applying a greater heat than is necessary to preserve fluidity, as excess of heat tends to produce spontaneous insolubility. About 100° Fahrenheit will generally answer the purpose. It should be further remembered that frequent and continued application of heat to gelatine destroys its setting powers, which would render the preparation useless.
The thickness of the tissue, and the proportion of the mixture necessary in forming it, depend very much on circumstances. If the tissue be too thin, the finished picture will not possess its proper depth of shade in its darkest parts, unless it has had an unusually large proportion of coloring matter. If too thick, drying is retarded, and it is intractable in mounting and other manipulations; it will also require a longer time in development. As different qualities of gelatine will produce different results, something must be left to experience in determining the amount of the tissue-compound necessary to form a given amount of tissue; as a general rule, however, it may be stated that about two ounces will be required for each superficial foot.
Immediately previous to the preparation of a sheet of tissue, the piece of “patent plate” glass should be placed in a levelling stand, in a perfectly horizontal position, a spirit-level being used in the adjustment. The tissue-compound, warmed to 100°, should be strained through a piece of moist flannel or muslin, and when the compound is ready the plate should be warmed until it is of the same temperature as the compound. The proper amount is then poured on the collodionized plate, and caused to flow over its surface, a glass rod being used to spread the solution. A little care is necessary to prevent the formation of bubbles of air, which, when once formed, are not easily disengaged from the viscous solution, and, unless eliminated, result in defects in the tissue, producing white spots in the picture. The coated plate is then left on the levelling stand until it is quite set. As will easily be seen, a very little inclination will cause the coating to run into uneven waves, or to accumulate and form a greater thickness in parts, the disadvantage of which is manifest.
When once thoroughly set, the plates may be placed away in an upright position to dry. The more quickly the drying is effected, provided heat be not applied, the better. The temperature should not exceed 60° or 70° Fahrenheit, as a higher temperature may cause the gelatine to run, and form uneven waves. A very low temperature, which would retard drying, is, of course, undesirable. A damp place is especially to be avoided, as the protracted drying, caused by a damp atmosphere, materially tends to the production of a spontaneous decomposition and general insolubility of the tissue. In a dry, well-ventilated dark-room, kept at a temperature of about 60° Fahrenheit, drying will generally take place within twelve hours, and without any danger to the solubility of the tissue. It may be found desirable in damp weather to use a drying box, containing chloride of calcium, sulphuric acid, or other substance having great affinity for water.
When the tissue is dry, it is ready for printing; it is, therefore, removed from the glass and placed in the pressure-frame, with the collodion surface in contact with the negative. The proper exposure is ascertained by the aid of the photometer described on another page. Before development, the tissue is coated with India-rubber solution in the same manner as the paper tissue referred to in another chapter, and is, in like manner, mounted on paper coated with India-rubber. It is then developed, washed, dried, and transferred as already described; the film of collodion, in this instance, forming the surface of the finished print.
Instead of coating the glass-plate with collodion, it may be rubbed with ox-gall, or with the solution of wax before mentioned, and coated with the sensitive tissue compound. When this is dry, it may be coated with collodion, removed from the glass, and treated in the manner already described. Or it may, instead of being coated with collodion, have a sheet of wet paper applied to it, and pressed in contact so as to adhere. It is then suffered to dry, and treated as the paper tissue in all respects, its only difference consisting in the fine surface communicated by the plate-glass, which becomes, finally, the surface of the transferred picture, and possesses a little more delicacy of effect than that produced by the ordinary paper tissue.