MANIPULATION OF PRINT-OUT PAPERS 2 532. The Printing-room. The loading of the printing frames, the control of the exposure, and the unloading of the frames should be done in a room lighted with yellow light or in any place where the illumination is weak.
On the commercial scale, one side of the printing-room is usually lighted by yellow windows and the other by plain glazing ; the dry paper is handled and the first operations are done by yellow light, whilst other treatments, from fixation onwards, can be carried out in white light. Frequently the same dark-room is used for washing prints made on development papers. When it is necessary to work by arti ficial light, the ordinary lamps are used; for judging toning, a lamp with a blue glass bulb, giving a light similar to daylight, should prefer ably be used in order that the tone may be more exactly judged.
If printing is by daylight the room should communicate with the outside or with the glazed enclosure in which the negatives will be exposed ; when artificial light is to be used, the lamps and the bench for the printing frames should be installed in the printing-room, to save coming and going.
533. Gold Toning in Alkaline Baths. The immersion of a photographic print in a solution. of pure gold chloride' would result in the sub stitution of gold for some of the silver consti tuting the image, but the resulting image would be very weak, particularly in the lighter tones. This weakness does not occur with solutions in which the yellow a-uric salt has been converted into the colourless aurous salt which, when replacing the silver of the image, deposits three times as much gold per unit amount of silver as the auric salt. Now, in an alkaline medium, auric compounds change progressively into aurous compounds ; these, however, may pass over into the inactive aurite state in the absence of soluble chlorides or if the bath is too alkaline..
The colour of the toned image depends essen tially on the speed of the toning, which, in its turn, depends on the alkalinity of the bath.
An acid bath gives images which tend to be red (finely divided gold), while a bath which is alkaline results in images which tend to a purplish brown, which may even approach black (gold in a less finely divided condition). Toning baths of a moderate temperature should be used. A bath which is either too hot or too cold does not give such good tones as one which is at a temperature round about 65° F.
534. Toning baths can be made alkaline by using one of a number of salts which have a feebly alkaline reaction ; fused sodium acetate, 1 borax, disodium hydrogen phosphate, etc., may be used. These salts give the same results when they arc employed in quantity sufficient for the bath to be equally alkaline in each case. The following solution, for example, may be used— Sodium acetate, fused . 90 gr. (ao grm.) Borax . . . mgr. (Po grm.) Gold chloride (r% solution) 4 drm. (25 c.c.) Water, to make . . 20 oz. (1,000 c.c.) (A i per cent solution of gold chloride is made by dissolving the contents of a r5-gr. tube, as sold, in 3 oz. of distilled water). The solution may be used after standing for about one hour. The used bath can be kept, but it is necessary to add small quantities of gold chloride from time to time.
The most definite means of ensuring the constancy of the composition of the bath is to use chalk toning (H. de Molard, 1851) ; the insoluble calcium carbonate is kept in the bath and does not react except as required. The following method of working is recommended (E. Lamy, 1897)— First of all, the following mixture is prepared with shaking The bath will become colourless on cooling ; it should not be filtered. When required for use, decant the necessary volume of this liquid, and for each roo volumes of the inactive bath add two volumes of i per cent gold chloride solution. When the toning is seen to be slowing down, more gold chloride should be added. The used bath is replaced in the inactive bath stock bottle and shaken up with the chalk.