It will be evident to anyone that the fancy forms of sensitizing have been carefully avoided—floating on the back, floating on the face, etc., etc. All the results desired can be obtained by immersion. If a hard negative has to be dealt with, a stronger solution, or longer soaking in the bichromate solution, is all that is needed; for weak negatives vice versa.
Note.—In the dry frosty air of winter, sensitized tissue will dry without heat, and continue soluble for a considerable length of time, often as long as a month, or even longer.
In hot weather it is recommended that the solution of recrystallized bichromate be made immediately before using, as in dissolving the crystals a considerable reduction of temperature is produced. Should the temperature then be over 60° F., ice must be used, not in the solution, but roughly broken up and mixed with salt in an outer vessel. If ice is placed in the bichromate bath allowance must be made by keeping out part of the water. The ice should be encased in several thicknesses of fine muslin to prevent the solid impurities it generally contains getting into the solution. When recrystallized bichromate is not procurable, a few drops of liquid ammonia added to solution of crude bichromate is recommended. As bichromate is cheap, a fresh solution should be made for each large batch of tissue.
PRINTING THE NEGATIVE.
Any negative that will yield a thoroughly good albumen print is suitable for carbon work. The thinner negatives now made for P.O.P. and similar processes are less satisfactory for direct prints in carbon, for enlargements and reproductions such negatives can be made to yield most satisfactory results by modifying the transparency and the enlarged or reproduced negative. The latitude in this direction is great. No matter how flat the original negative may be, if all the grades are present it can be manipulated in such a way that the most brilliant result will be produced.
PREPARATION OF THE NEGATIVE FOR PRINTING.
The negative is prepared for printing as in all other processes by removing all defects such as pinholes, streaks, etc. For the carbon process the negative requires to be further provided with what is termed a "safe edge;" this is a line of black varnish, from one-eighth to half an inch in width according to the sizes of the negatives, painted on its margin, either on the film or glass side. In the case of original negatives masks of opaque paper are used instead of the painted edge, the masks having openings cut in them slightly less than the size of the negative. The purpose of the safe edge is to secure a margin on which light has not acted, as such a margin gives greater freedom to the operator in the process of development by preventing the more deeply printed portions of the picture leaving the support when the backing paper is removed.
DOUBLE TRANSFER PRINTING.
It may be explained in a few words why an ordinary (non-reversed) negative must be printed by double transfer. In all other solar processes when the print is removed from or taken off the face of the negative, it is turned over to view, it therefore follows that the details on the left side of the negative are found on the right side of the positive print; with the carbon print no such turning occurs, it is mounted upon its support in the same position as it lay on the negative, developed in that position from the back and leaving the position of objects the same as in the negative. In some cases this reversal of the position of details is unimportant.
We will first consider the double transfer. In all double transfer processes a temporary support must be provided. Such supports are of two kinds, flexible and rigid. When a matt-surface print on paper is required, finely grained opal glass is used. For the enamelled surface patent plate, for intermediate or only slightly glazed surface, a flexible support is used. Flexible support yields a surface similar to an albumen print without special preparation. When the higher glaze of the double-albumenized print is desired, the printed tissue is coated with thin collodion before mounting on the flexible support. Rigid supports, zinc or ground-glass plates, have been used, but owing to the difficulty of seeing the details during development their use is practically discontinued. Flexible temporary support is always used in transferring to canvas, wood panel, opal, ivory, etc., etc. In the case of canvas, the double transfer process has two great advantages. First, staining is avoided, the bichromate has been thoroughly got rid of in the process of development. Secondly, the canvas is prepared to receive the print by a substratum that allows the carbon image to expand and contract with the expansion or contraction of the canvas and not in opposition to it. There are also two advantages in adopting the double transfer process for the production of pictures on ivory. The first, is freedom from bichromate stains. Secondly, the ivory is not distorted by washing in hot water, such distortion generally takes place when the single transfer process is adopted. Wood panels are prepared in a similar way to canvas. Stains are avoided, and as there is not the expansion and contraction of canvas to provide against, the substratum is modified in composition and greatly reduced. The stains above alluded to are caused by the chemical combination of bichromate with the lead of the paint, forming chromate of lead or chrome yellow. In the case of opal, opaque celluloid, and similar substances, no staining takes place, the double transfer is only required to restore the image to its proper position.