The Barnet Book of Photography: A Collection of Practical Articles. Various

Чтение книги онлайн.

СКАЧАТЬ [4]See foot-note to page 30.

      Mix equal parts of ortol solution and soda solution.

      This developer behaves in much the same way as pyro-soda and gives very similar results. It has the advantage, however, that it does not stain the fingers, and has practically no tendency to produce either fog or stain on the plates. Moreover the same quantity of solution can be used for several plates; when the action becomes perceptibly slower or weaker, part of the old solution is poured away and an equal quantity of freshly mixed ortol and soda solutions is added.

      The chief differences to be observed are (1) that the different parts of the image follow one another more rapidly than with pyro-soda, even though the plate may have been correctly exposed, and (2) the colour of the reduced silver is somewhat bluer than with pyro-soda, and therefore in order to obtain the same degree of printing opacity, as distinct from visual opacity, development must be carried a little further.

      Apart from these differences, what has been said of pyro-soda holds good for ortol soda and need not be repeated.

DEVELOPMENT WITH HYDROQUINONE (QUINOL). Quinol Solution.
Hydroquinone	90	grains or	2	parts
Sodium sulphite	1	oz. or	10	parts
Water to make up to	10	oz. or	100	parts
Alkali Solution.
Potassium carbonate (dry)	1	oz. or	10	parts
Potassium bromide	20	grains or	0.46	parts
Water to make up to	10	oz. or	100	parts

      Mix two parts of hydroquinone solution with one part of alkali solution and one part of water, or, if a more energetic developer is wanted, mix equal volumes of the hydroquinone and alkali solutions. Hydroquinone is not an advantageous developer for general purposes, but it is useful when negatives are required showing strong contrast between the highest lights and the deepest shadows, and especially when it is important that there should be no deposit at all in the deepest shadows. This is the case, for example, when copying line engravings, pen and ink drawings and similar subjects.

      DEVELOPMENT WITH FERROUS OXALATE.

      This method of development also is not well adapted for general work, but it is invaluable for certain purposes. The reduced silver has a pure grey-black colour and there is exceedingly little tendency to produce fog of any kind. On the other hand, the developer admits of little modification in its composition and therefore the exposure must be fairly correct. It is also important to avoid contamination with even minute quantities of hypo, since this substance very readily causes stains.

Ferrous Sulphate Solution.
Ferrous sulphate	½	oz. or	25	parts
Sulphuric acid	Small quantity
Water to make up to	10	oz. or	100	parts

      About three-quarters of the total quantity of water is mixed with a small quantity (not more than 50 minims per 10 ozs., or one part per 100) of sulphuric acid, and the ferrous sulphate (proto-sulphate of iron) which must be in clear pale green crystals without any yellowish incrustation, is dissolved in it with the aid of a gentle heat. After the solution has cooled, it is made up to the specified volume with water. This solution alters when exposed to air, and should, therefore, be kept in small (2 oz.) bottles, filled up to the neck and tightly corked.

Oxalate Solution.
Potassium oxalate	10	oz. or	25	parts
Potassium bromide	40	grains or	0.23	part
Water to make up to	40	oz. or	100	parts

      For use take four parts of oxalate solution and one part of ferrous sulphate solution, pouring the latter into the former and not vice versa. In order to obtain slower action with a rather softer image and a slightly browner deposit, the developer may be diluted with an equal volume of water. Slower action, with slightly increased printing contrasts, and clearer shadows, results from an increase in the proportion of bromide.

      FIXING.

      After development is finished, the dark-coloured reduced silver that forms the image remains mixed with a considerable quantity of semi-opaque, yellowish unaltered silver bromide, which would not only interfere with the printing, but would also gradually darken when exposed to light. The negative must therefore be "fixed" by dissolving out the unaltered silver bromide, and this is accomplished by immersing the plate in a fairly strong solution of sodium thiosulphate (formerly called sodium hyposulphite) commonly known as "hypo." The usual strength of the fixing is as follows:

Fixing Bath.
Hypo (sodium thiosulphate)	10	oz. or	25	parts
Water to make up to	40	oz. or	100	parts

      A solution of double this strength is, however, not unfrequently used, and acts more rapidly, especially in cold weather.

      The developed plate, after being well rinsed with water, is placed in the fixing bath and allowed to remain in it with frequent rocking until the silver bromide has all been dissolved out of the film. This is ascertained by lifting the plate out of the dish and looking at the back by reflected light, the plate being held in front of something dark. It is not difficult to see whether the silver bromide has all disappeared or not, but in order to СКАЧАТЬ