The use of hydro-quinone is becoming more extensive. Hydroquinone with sulphite and soda has recently been strongly recommended for gelatine plates, and Vogel says that he now uses hydroquinone to the exclusion of all other developers for paper prints, as well as for gelatino-bromide plates. No other developer, in this writer's opinion, permits of such easy, clean, and completely certain work. This is a very strong expression of opinion from an able worker. If in the hands of others it ensures a like favourable verdict, there will have to be an improvement in the method of using pyro, or that agent must in all probability encounter neglect similar to that which, for all save printing and reproduction processes, has come upon iron development.
Although now using hydroquinone only for the development of paper prints, Vogel encountered some difficulties and objections, which, having overcome, he describes for the guidance of those who may follow. To begin with, he mentions that with over-exposure, even in slight degree, the picture, especially if the film contain much silver iodide, readily acquires a disagreeable greenish hue. He has recently found that the tone of the picture depends essentially upon the amount of soda carbonate present in the developer. With a solution prepared according to the following formula:-
40 gr. crystallised soda sulphite, 5 gr. hydroquinone, 2 1/4 oz. water, 50-150 gr. crystallised soda carbonate, it was found that when the soda carbonate was in small proportion - i. e. up to 75 gr. - the tone of the print was always greenish; with a larger proportion of soda the tone improved, becoming eventually black or blackish brown.
With 75 gr. of soda carbonate good results were obtained, provided the exposure was exact, but not if there was over exposure. A very great increase of soda produced some blistering. Potash carbonate possesses the advantage, for the development of paper prints over soda, that a greenish tone is never obtained with it, and is therefore to be strongly recommended for positive work.
The following mixture was prepared:
40 gr. crystallised soda sulphite. 5 gr. hydroquinone. 2 1/2 dr. water.
When perfectly dissolved (hastened by standing the bottle containing the ingredients in warm water), 50-54 gr. of potash carbonate was added. A larger proportion of potash carbonate than this causes, especially in warm weather, considerable frilling. For use, 1 part of this solution is diluted with 5 parts of water. The developer in the concentrated form as above, keeps good for a long time; when diluted to the strength given for use, it remains good only for about 2-3 weeks. The strong solution is, on account of its keeping properties, very convenient for use on a journey. When using with gelatine plates, Vogel states that he obtained very good results with this formula, and that it may be especially recommended for instantaneous negatives on account of the very full amount of detail brought out by it. The development also proceeds quickly.
It is important to notice that the soda sulphite is in good condition - has not effloresced by keeping. On account of the greater keeping properties of a concentrated solution he now gives the formula in the following shape; - 160 gr. crystallised soda sulphite. 20 gr. hydroquinone.
300 gr. crystallised soda carbonate.
The precaution previously mentioned of ensuring that all the hydroquinone is dissolved before the addition of the alkali must of course be observed. For use, 2 parts are diluted with 5 parts of.
Both the potash and the soda developer can be varied so as to suit differing characteristics. For plates working with too much vigour, less water may be used in (he developer; while, on the other hand, plates yielding an image deficient in vigour may be treated with a solution to which more water has been added.
An item having an important bearing on the general extension of the use of hydroquinone is the much lower price at which it is now procurable. In Germany it is now less in price than pyrogallic acid, the cost being about 10d. per oz.