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ON THE WHITENING OF COLLODION PICTURES AS POSITIVES,
AND SUBSEQUENT BLACKENING FOR NEGATIVES.
UNDER this head I shall endeavour to give a description of the best method to accomplish these two objects. Before commencing the process of whitening a picture, it should be considered whether the collodion with which it is made is strong enough to bear without injury the application of the very corrosive compound employed.
The acid solution of corrosive sublimate would very soon destroy the little remaining tenacity of a weak collodion, and the operator would have the mortification of finding that his picture, although able to bear the application of the whitening solution, would fail altogether to resist the action of water in the after washing. To resist, therefore, the rough usage the picture will have to undergo, it should be made with tolerably strong collodion. Prepare, first, a saturated acid solution of corrosive sublimate (bi-chloride of mercury).
|Bi-chloride of mercury||2 drs.|
This will form the normal solution, to be diluted when occasion may require. After thoroughly washing the picture, to free it from fixing solution, and whilst still moist, cover it rapidly, at one motion, with a weak solution of bi-chloride of mercury, made with:
|Saturated solution (bi-chloride of mercury)||1 part|
After the plate is fairly and evenly covered the greater portion of the liquid is drained back into the cup, and a drachm of the saturated solution is added to it and applied, after which the picture will rapidly whiten.
This operation may occupy from one to three minutes, and sometimes longer. When the whitest effect is produced, the picture is carefully washed with a stream of water; afterwards put by to drain, protected from dust, to be varnished with white lac varnish. The reason for first washing the plate with a weak solution is, to prepare it for the stronger dose, which latter, if applied at first, would be likely to cause an unequal effect.
When the picture is clearly and distinctly whitened, the process has an excellent effect; oftentimes minute markings are brought out, before invisible. In bleaching a picture very beautiful tones are oftentimes produced by stopping the action of the chloride before the whole picture is whitened; pictures thus treated have somewhat the appearance of drawings, in which the high lights and brightest parts have been put in; thus taking off the sameness which the drawing will assume if the action is carried further.
Copies of engravings have a charming appearance when thus treated, and are superior even to the original in general tone. The process does not stop here; the bleaching of the picture may be considered the first step towards rendering a feebly-developed picture a densely black negative for printing.
Of a saturated solution of:
|Hypo-sulphite, of soda||2 drops|
mix thoroughly, and pour it rapidly on the plate ; do not be sparing of the liquid; two ounces, for instance, will not be too much for a plate 6 X 5, as the more quickly it is poured over the less likely will it be to stain.
It should be poured over from one comer, and not down upon the middle of the plate. This liquid will immediately discolour the whole picture, giving it a light brown colour, which can be turned to black by the application of a stronger dose of hypo-sulphite of soda. When the picture has attained its full depth, which it does almost immediately, it should be washed carefully, the water being applied in a gentle stream, in a slanting direction, on to the plate, so as to drain off quickly.
After washing, apply a gentle heat to the plate; when dry, and still warm, pour over it a white lac varnish. Often sufficient blackness can be given to a drawing with the first application of the bi-chloride of mercury solution, if the action is stopped in time ; that is, before the whitening effect comes on. Also the blackening can be effected by a weak solution of cyanide of potassium, applied immediately after the bi-chloride.
|Saturated solution cyan. potassium||1 drop|
This solution is very rapid in its action, and gives even more blackness to a picture than hyposulphite soda, but it requires greater care in the management, for if the solution is at all too strong the blackening at first produced disappears, and the picture becomes bleached before water can be applied to stop its action. As has been mentioned in a previous division, a bath of the bleaching liquid can be employed.
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