In twenty-five ounces of hot water dissolve half a pound of best neutral oxalate of potash, and keep this in a stoppered bottle as stock solution. What is known as neutral oxalate should be used, and in order to ensure having a suitable salt it had better be obtained from a recognised photographic chemist or dealer.
As the above solution becomes cool, a good deal of the oxalate will probably settle at the bottom in the form of solid crystals; of these no notice need be taken, for as long as there are undissolved crystals at the bottom of the bottle we know we have a saturated solution.
We shall now require a dish of porcelain or enamelled iron, and if we choose the latter great care must be taken to see that the enamel is not cracked or blistered, as it will have an injurious effect if the oxalate of potash solution obtain access to the iron under the enamel.
As it will be convenient to be able to alter the temperature of the solution when in this dish at will, a spirit lamp or stove or a small gas-stove will be a useful, if not an essential addition. Over such heating apparatus the dish should be supported on an iron tripod, or by any extemporized substitute.
If a porcelain dish be used, a thin sheet of iron should be placed first on the tripod stand, and then three or four scraps of iron, large common iron nails will serve very well, and on these the porcelain dish is allowed to rest so that it does not come into direct contact with the iron plate.
The purpose of this is to save the dish from cracking, moreover the iron plate becomes hot, and retaining a good deal of heat serves as a kind of accumulator which goes far to maintain the dish and the contained solution at a uniform temperature for at least a short time. Even better than this arrangement will be an iron dish filled with clean dry sand, the porcelain dish to rest on the sand which retains much heat.
If an enamelled iron dish be employed, these precautions are not so necessary, though they may still be used with advantage.
Next we shall require another dish or similar vessel into which we pour a weak solution of hydrochloric acid, the usual proportions being:—
| Water | 70 | parts. |
| Pure hydrochloric acid | 1 | part. |
This constitutes the whole of the very simple apparatus needed, and we may now proceed to develop our print, which as already described is exposed to light in a printing frame in the usual manner until the image appears rather less than half-printed.