The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Bitumen
Blacking
Bitumen. See Asphalt.
Bitumen Process. See Asphalt Process.
Black Glass. Glass deeply coloured with manganese and iron. Sometimes used as a reflector in photographing clouds.
Black Varnish. See Varnish.
Blacking. The interior of all cameras, dark slides, and lens tubes should be coated with a dead black to prevent the reflec-tion of light and consequent fog on the plate. A good black can be made by grinding lamp or ivory black into a paste with japanners' gold size. Another method is by coating the wood, etc., with a solution of sulphate of iron, and, when dry, applying a solution of tannin or decoction of logwood, two or three suc-cessive applications being sometimes necessary. For blackening lens mounts, two or three different methods are employed. Where the mount will not be touched by the fingers, drop-black ground up with weak glue and water may serve, but a better method and a more lasting one can be made by mixing fine lampblack with lacquer, and applying it in two or three successive coats to the heated mount; but where the mount will be fingered, it is obvious that some other method must be employed. We have the choice of two, one of which results in a bronze colour, the other is a non-reflecting dead black. For the former colour, solution of perchloride of platinum acidulated with nitric acid is used. This method is especially useful where any soldered joints exist; where solder is not used, an absolute dead black can be obtained by dipping the article, heated fairly hot, into a solution of nitrate of copper, made by dissolving copper wire in dilute nitric acid ; it is then heated over a Bunsen burner or spirit lamp, the green colour of the copper first showing, and at the proper temperature a fine dead black appears. Another method is to dissolve I part of carbonate of copper in 8 parts of solution of ammonia and adding 18 parts of water. Clean the brass well and immerse in the solution till black, rinse with water, dry in bran or sawdust, and give a rub over with thin oil varnish. Another formula to blacken wood is as follows : -
Extract of logwood ... Chromate of potash
Water            ......
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