CHROMATYPE. A name given to that class of photogenic de compositions in which chromic acid is deoxidized : the sesquioxide of chromium does not appear to undergo reduction to the state of protoxide, and in this it differs from the persalts of iron in ferrotype. The following are the modes adopted for getting photographs by the chromium salts : — (a) Soak the paper in a saturated solution of bichromate of pot ash, and dry it by rapid agitation in front of a brisk fire out of the light. It is now of a bright yellow colour, but exposure to the sun under a negative, will produce a positive, by darkening the exposed parts to a deep orange colour. Washing well in water removes the unchanged yellow salt from the lights, but the reduced sesquioxide in the shadows remains combined with the paper. The paper should be well sized, or the bichromate will be only feebly decomposed.
(b) Brush a sizing of starch very uniformly over the paper, and then steep it in a weak alcoholic solution of iodine, and if the coat ing of blue iodide of starch be not uniform repeat the operation. Steep it in bichromate and dry it as before. The print will be ne gative, even, from a negative, and positive from a positive, if after and washing it is again steeped in the solution of iodine -,nders the unexposed parts of a dark violet colour.
o a saturated solution of bichromate of potassa, add a lution of sulphate of nickel, in quantity more than sufficient npose the whole of the bichromate, or 1 drachm of the latter (the potassa) in crystals to 2 drachms of the former, also in crystals.
Apply this to the surface of the paper and expose under a negative., The exposed parts become brown and the rest remains yellow, and if now washed in water the result is a positive picture. But if the exposure be continued under the negative beyond the dark stage the browning disappears and the exposed parts are white, a -colourless double salt being formed there. When nitrate of silver is applied as a developer the chromate of silver is deposited in the unsunned por tions, where the original solution remains unchanged, but not on the whitened parts. Pure water will remove nearly all except the precipitated chromate, which, if thought desirable, may be converted into chlmide, and exposed again, and developed as in the usual silver processes. Sometimes the colourless double salt formed will de. compose the nitrate of silve,r, owing to its imperfect formation. The chromate of silver, by which the shadows are represented, is itself decomposed by light, and, therefore, must be changed in some way, before permanence is attained.
(d) Dissolve neutral chromate of copper in ammonia. The solu tion, which is now a mixture of chromate of ammonia and of ammo niacal solution of oxide of copper, is of a green colour, formed by the mixed yellow and blue of the separate solutions. Papers prepared with this behave like those soaked in the chromate of copper.