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 ensure complete fixing the plate must not be taken out of the bath as soon as this has happened, but should be left in for a few minutes longer, the dish being rocked so that the dissolved silver salt may diffuse out of the film into the fixing bath.
When removed from the fixing bath the plate should be allowed to drain into the bath for a few moments and should then be washed for five or ten minutes in running water under the tap. It is best to put the plate in a dish standing on the sink and have a piece of flexible indiarubber tubing reaching from the tap to within a couple of inches or so of the top of the dish, so that the water may not splash too much. After washing in this way, the plate is placed in a grooved zinc rack, which is immersed in a tank (preferably of zinc), containing sufficient water to completely cover the plates, and here it remains until the whole batch of plates in hand has been developed and they can all receive their final washing together. The plates stand upright in the rack, and the entrance and exit of the water must be so arranged that the water enters at the bottom and overflows at the top, or, what is perhaps better, enters at the top and is drawn off from the bottom, the waste pipe opening at the bottom of the tank and being bent and carried upwards until its mouth is at the level at which the water is to stand in the tank.
When running water is not available the plates may be washed in dishes. After being well rinsed to remove the adhering hypo solution, the plate is covered with water (about 3½ oz. for a half plate or 5 oz. for a whole plate) and allowed to remain with frequent rocking for five or six minutes. The water is then well drained off, a second quantity added and allowed to remain for the same time as before, with frequent rocking, when it is poured off in its turn. Treatment in this way with six successive quantities of water will remove all the hypo, provided that the film has not been treated with alum.
Another plan, rather less troublesome, but also less expeditious, is to place the rack containing the plates in a tank not much more than big enough to hold it, taking care that there is not less than two inches between the lower edges of the plates and the bottom of the tank. After standing for some time the rack and the plates are slowly and carefully lifted out and allowed to drain, the tank emptied and filled with fresh water, and the rack and plates then replaced. Eight or ten successive quantities of water applied in this way should remove all the hypo, but if there is any doubt on this point the plates, after they are supposed to be washed and have been removed from the tank, should be allowed to drain into a measuring glass or into a dish, the contents of which are afterwards transferred to a measuring glass and mixed with a small quantity of a solution of silver nitrate. If the plates are really completely washed nothing will happen, or at most a white precipitate will be produced which will remain white if not exposed to daylight. If, on the other hand, the plates still retain hypo, the silver nitrate will produce a precipitate which will gradually become orange and eventually dark brown. Should this happen, the washing must be continued.