SELF-TONING PAPERS 545. Types of Self-toning Papers. The addi tion to the emulsion of the gold salts necessary for toning prints on print-out paper was sug gested before 1860, but it was not until 1896 that P. E. Schocnfelder and E. Kehle worked out a commercial process. For some years past a certain number of self-toning or auto-toning papers have been prepared which, instead of gold, have salts of selenium or tellurium in the emulsion.
Emulsions for self-toning papers are made with gelatine or collodion, and correspond to gelatine and collodion print-out papers respec tively. Gelatine papers arc generally preferred for glossy prints and collodion papers for matt prints.
The extreme simplicity in the handling of these papers and the permanence of the prints when the treatment has been correct have made these papers very popular with amateurs who do their own printing. So simple, indeed, is the manipulation that there is a tendency to take liberties, and so to depreciate the quality and permanence of the prints.
546. Working Methods. Self-toning papers should be used only with vigorous 1 negatives. The final tone of the image largely depends on the qualities of the negative ; within certain limits, the more vigorous the negative, the richer the colour of the print.
Self-toning papers may be placed in the fixing bath without any washing or other preliminary treatment. Most of them can be treated with a solution of sodium chloride (table salt) before fixing to give purplish tones. In both cases, the colour of the image depends essentially on the concentration of the bath, and on the volume of bath used for a given amount of paper. To ensure uniformity of colour when many prints of the same subject are being printed, they should not be treated one by one in the same solution; it is better to measure into a dish the amount of solution required for all the prints, and then to introduce them rapidly one by one into the bath for simultaneous treatment. The prints must be kept in move
ment, a good method of doing this consists in continually transferring the print at the bottom of the heap to the top.
The usual strengths of baths arc 5 per cent of sodium chloride for the salt solution and from io to 20 per cent of sodium hyposulphite for the fixing bath. Acid or alum fixing solutions should on no account be used. The fixing bath should preferably be rendered alkaline by the addition of a pinch of bicarbonate of soda in accordance with the recommendation of most manufacturers.
Very cold baths should be avoided ; the best temperature is in the neighbourhood of 65° F. If the toning is done in the fixing bath, and if the temperature is too high, the strength should be reduced by addition of water to avoid the toning being too rapid for easy control.
The volume of bath generally recommended is from i to It oz. for a print 7 X 5 in. (from 30 to so c.c. for a print 13 x x8 cm.) (4 to 7 oz. per sq. ft., or 12 to 20 c.c. per sq. dm.). Used baths, which have practically no value, should not be used many times in succession, nor should fixing baths which have been used for other sensitive material ever be used for self-toning paper.
Where the above instructions are found to differ in any respect from the makers' instruc tions, the latter should, as a rule, be followed.
Some self-toning papers become discoloured between the times of their manufacture and use more readily than do other print-out papers, but the colouration disappears during fixing ; paper should not therefore be thrown away, owing to its appearance, until it has been tested.
After fixing, the prints should be washed under the conditions (§ 607) described for all other papers.