Unlike the alkaline developer, the ferrous oxalate may be used to develop several plates in succession without the mixing of a fresh solution, if more than an hour or two do not intervene. By long exposure to the air, however, the strength of the developer is lost.
A more concentrated and consequently more ener getic oxalate developer may be made in the following manner : A bottle—holding, say, a pint—is nearly filled with a saturated solution of oxalate of potash. The whole is slightly warmed, and protosulphate of iron is dropped in in crystals. The bottle is shaken to dissolve these, and more crystals are added till no more will dissolve. The solution will keep for some weeks in a tightly-stoppered bottle.
The operation of development with ferrous oxalate is, as we indicated, much cleanlier than that with alkaline gyro. For this reason it is likely to be a favourite with many, and especially with ladies. We shall therefore briefly describe how it may be used so as to secure the greatest latitude of exposure which it is possible to have with it.
For the method of working which we are about to describe, three "dipping baths," large enough to hold the plates to be developed, are required, and one dipper. We illustrate a dipping bath. It will be seen that it is simply a vertical vessel so shaped as to hold a plate surrounded with solution. The best material is glass. The dipper is a little piece of apparatus for lowering a plate into the bath.
We shall call the baths Nos. 1, 2, and 3.
Into No. 1 there is poured concentrated ferrous oxalate solution, made as just described.
Into No. 2 is poured the ordinary ferrous oxalate developer, made as described first of all.
Into No. 3 is poured the same developer as goes into No. 2, but there is added ten minims of the ten per cent solution of bromide of ammonium to each ounce of this developer.
When we have a plate to develop and do not feel certain whether the exposure is correct or not, we place it on the clipper and lower it into bath No. 2 or 3, according to the amount of our uncertainty. If we know that our exposure cannot be very much wrong it goes into No. 2. If all proceeds satisfactorily it remains
there till development is complete. If exposure appears to have been too great it goes into No. 3 to be finished. If it appears to have been too little it goes into No. 1.
If we are very uncertain of our exposure, and think that it may have been very much too great, we com mence with No. 3. It is less necessary to use the alum solution after ferrous oxalate development than after alkaline pyro. The fixing is performed in exactly the same manner in the one case as in the other.
Sodie Sulphite Developer.
The brownish or yellowish colour which a gelatine plate negative, which has been developed with the ordinary alkaline pyro developer, exhibits is considered objectional by many.
To avoid it an addition to the developer was proposed by Mr. Herbert Berkeley some time ago, and the result obtained by his modification is certainly very excellent.
The addition consists of sodic sulphite or sulphite of soda, which is used in the proportion of four parts to every one part of pyroganic acid.
The stock solutions may be mixed in the following manner No. 1. Pyrogallic acid . 1 ounce.
Bromide of ammonium . . 600 grains.
Sulphite of soda . 4 ounces.
.Citric acid . . 60 grains.
Water, up to . 12 ounces. No. 2.
Strongest liquid ammonia •880) . 3 drachms.
Water . 1 pint.
No. 3.
No. 1 . 2 ounces.
Water . . 18 ounces.
No. 1 is mixed in the following manner : The sul phite (not sulphate) of soda is dissolved in eight ounces of water, a little heat being used. The citric acid is added. The solution is poured over the pyrogallic acid and bromide of ammonium, and is then made up to twelve ounces with cold water.
Solutions Nos. 2 and 3 are used precisely as the solutions of the ordinary developer which bear the same numbers.
Development will be slightly slower, but the result will be a negative of a very fine black colour.
Soda and Potash, Developers.
Some operators prefer to use common washing-soda or caustic potash instead of ammonia. It would appear that thereby a slightly shorter exposure may be given. For this developer solutions may be mixed as fol lows :— No. 1.