Even more convenient than this is a paper actinometer of the so-called terrace pattern. To make this, procure a strip of glass; a bit of an old negative answers admirably. This should be one inch wide and about six inches long. Procure also some typewriting paper; the thin "onion skin" variety is excellent. Cut this into strips slightly, say one-sixteenth of an inch, narrower than the glass and of the same length. Then arrange these strips in step or terrace fashion, as shown in the diagram (Fig. 14) in which G represents the glass, and P P a few of the paper strips. The width of the steps is not of much importance; about three-eighths of an inch will be plenty. This entails cutting a piece off each strip and if these are laid one on top of the other, the result will be about sixteen steps, each one increasing the density by one thickness of paper.
The steps should preferably be numbered or lettered, and the numbers can be written on a piece of fixed-out negative film with waterproof drawing ink, or may be similarly written on the paper itself. Small figures or letters of ready-gummed black paper may be obtained commercially. This figuring or lettering is not absolutely essential, but it facilitates the reading of the tint in practice. Finally a strip of clear celluloid should be cut the same length and width as the glass, placed over the papers, and the whole bound up with narrow lantern slide binding strips, the papers being pressed into as close contact with the glass as possible.
Printing-out paper may be used with this as with the other form; or plain white paper may be sensitized with the bichromate bath and used instead, and then the numbers or letters will appear yellow on a brown tinted ground. Such a meter can be used in a printing frame, but another strip of glass may be hinged to it with cloth or lantern slide binding strip along its length on one side, thus forming a little frame. A couple of metal clips serve to keep the paper in close contact with the celluloid while printing. Again, trial and error must be resorted to, to ascertain the necessary exposure; but, as a rule, from five to ten steps will be sufficient under the above conditions. Of course, if the paper selected be thicker, then it will naturally stop more light and not so many steps will be required.
Practically the carbon tissue is about the same speed as a print-out gelatino-chloride paper; or one can take the yellow print as a guide, as the image can be seen on this, and when all the details are visible in the highlights of this print, the printing may be interrupted. It is as well to expose all three tissues at once, and actual sunlight must not be allowed to fall on the negatives. The negatives must be provided with a safe-edge, and the easiest way is to use black lantern-slide binding strips.
Stick them to about half their width on the gelatine of each negative all round, and, when the strip is quite dry, shave off the projecting part with a knife. The safe-edge prevents the tissue from washing up at the edges during development.
As the carbon prints are to be developed on a temporary support and subsequently superimposed in register, it is necessary for this support to be transparent, or sufficiently translucent to enable the outlines of the images to be seen. Such temporary supports are obtainable commercially, or celluloid sheets ten one-thousandths of an inch thick may be used. The temporary support should be polished with a solution of beeswax in turpentine, such as:
Beeswax 20 g.
Resin 20 g.
Turpentine 1000 ccm.
Melt the wax in a pot in a water bath, add the resin, preferably in powder, and finally add the turpentine slowly with constant stirring. Make a pad of absorbent cotton, wrapped in a piece of linen cloth, pour a little of the wax solution on the pad and rub over the surface of the celluloid, which can be pinned to a board by the corners; then polish off the wax with a cloth. It is as well to use two or three clean pads, as but the merest trace of wax is required. Then hang up the prepared sheets for twenty-four hours to dry. Sheet celluloid of the given thickness can be obtained from any dealer, either with polished surfaces, or one side polished and the the other matt. As the final print assumes the same surface as the temporary support, it is thus possible to obtain either matt or glossy pictures. The celluloid sheets should be obtained an inch larger each way than the tissue, as this facilitates handling.
The exposed tissue must not be left too long between exposure and development, as the insolubilizing action, initially set up by light, proceeds in the dark; so that if the exposed tissue be allowed to lie undeveloped for some hours, the final prints will be actually darker than they should be, becoming obviously overexposed. The exposed tissue should be immersed in a bath of cold water (150 C. or 6o° F.), and care taken that no air-bells adhere to the surface, which can easily be done by carefully passing the fingers over the surface both back and front. At first the tissue will have a tendency to roll itself up into a spill with the gelatine side inside; then it will gradually straighten itself out, and then the edges will turn back with the paper inside. This is the psychological moment to take advantage of. The waxed celluloid sheet should be slipped into the dish with the waxed side toward the gelatine of the tissue, the two brought into contact under water, and the print shifted until it is fairly central; then both should be lifted out, being held in contact by the thumbs. The two should then be placed on a pile of paper, preferably stout blotting paper; a sheet of blotting paper should be placed over the print, then a sheet of ordinary hard paper, and the two thoroughly squeegeed into contact; the flat squeegees are better than the roller type for this. The print should then be placed between two sheets of blotting paper and under a moderate weight, such as a pile of big and heavy books, for about fifteen minutes, and it is then ready for development.