Any corrections or alterations can be made on the image before it is transferred to the jelly. Lines which are to be erased can be covered with a solution of gum arabic ; parts of the design which require modification can be covered with a print from a rectified tracing, using suitable register marks. If desired, the prints can be made on tracing paper, in which case the ink is reinforced with powdered bronze, which only adheres to the inked lines. Transfer inks can also be used for printing in conjunction with special papers which allow of the inked image being transferred to stone or zinc by the ordinary lithographic processes ; this method is used if a large number of copies have to be made.
Printing having been done, the jelly is washed with a wet sponge to remove the adhering ink, scraped with a putty knife, and re-melted, together with a suitable proportion of fresh jelly, for use again.
624. Printing on Paper from Ordinary Negatives. The ferro-prussiate papers, as prepared for copying tracings, are, with the exception of the slowest varieties, not usually very suitable for making full-tone prints, and are only sold in quantities which are too large for ordinary photographic use, there being a great risk of such papers deteriorating before a roll is finished. Nevertheless, use may be made of these papers for record purposes, the prints forming a kind of " register " of the negatives.
The sensitizing of papers and fabrics (linen and cotton) by the ferro-prussiate process, in small quantities, can be carried out by the amateur very simply. The formulae for sensi tizing given below are suitable for obtaining vigorous, full-tone prints of good gradation, but such papers are very much slower than those obtained by the use of the formulae used in the manufacture of industrial papers. It should be borne in mind that the papers and fabrics sensi tized in this way are only suitable for printing from vigorous but not excessively hard nega tives.
Any drawing paper or writing paper can be used for sensitizing, also coated papers (of the quality used for printing) and gelatine coated papers (double transfer papers for the carbon process). These last two kinds of paper give prints with more detail in the half-tones, but uncoated papers are usually preferred.
Papers for personal use are sensitized with a mixture of potassium ferricyanide 1 and an iron ammonium citrate, using one of the two com mercial varieties (brown or green). 2 Very much
faster papers may be obtained with the green citrate (about 5 minutes instead of 15 minutes for printing under a negative in the sun). The keeping qualities are only very slightly reduced, but the prints obtained are of a very much brighter blue (E. Valenta, 1897).
After making a series of experiments, we consider the following proportions and concen trations to be the best for the purpose of com pounding sensitizing solutions with green and brown citrate respectively— Of the two papers prepared in this way, that made with the green citrate is more suitable for use with vigorous negatives than that prepared with the brown The sensitizing solution is most easily applied to the paper with a brush consisting of a wad of Cotton wool fixed on the end of a glass tube by a loop of thread passing up the tube and fixed at the other end, e.g. with a cork. The solu tion is distributed over the sheet of paper as in applying water-colours, the paper being fixed to a board or a piece of stout cardboard which is inclined at an angle of 30°. (The paper may be fixed with clips, but pieces of cork or card board should be interposed so as to avoid any contact of the sensitizing solution with the metal.) The sensitizing should be done in very weak or artificial light, using a clear solution prepared immediately before use. The sheets are put to dry forthwith, hanging them on tightly stretched lines in a room from which daylight is excluded. Care should be taken to prevent the drippings staining clothing or floor coverings.
The considerable falling-off in the depth of the image which occurs during washing should be taken into account when printing.
if required, a ferro-prussiate print may be drawn over with pencil or indelible ink, the image being then removed by immersing it in an approximately 5 per cent solution of oxalic acid, followed by washing in several changes of water. The same solutions can be used for sensitizing fabrics (linen or cotton) by immer sion. The working methods arc identical : the fabric should be stretched during drying, and, after printing and washing, should be ironed while slightly damp. 2