Fe(OH)3 + 3HCl =FeCl3 + 3H20.
These printing papers are coated with gelatine containing bichromate. The action of light on potassium bichromate in the presence of easily oxidisable organic matter, such as gelatine, may be summarised in the equation:
K2Cr207 = K20 + Cr203 + 3O.
Potassium bichromate is not sensitive to light when alone, and, in contact with organic matter the action takes place, curiously enough, more rapidly in the dry state than when moist. The effect of this chemical change is twofold. The Cr203 reacts with the gelatine to form a dark opaque substance.1 The gelatine in contact with this dark substance becomes insoluble. The developer is hot water, which dissolves out the unchanged gelatine, leaving the insoluble portions as a raised image.
In conclusion, a word of warning to the reader whose knowledge of chemistry is still in the elementary stages, may not come amiss. Many of the leading operations in photography are delicate, and require conscientiousness and care. They are performed by very minute quantities of the reagents. An average developing formula contains only two to three grains of pyrogallol to the ounce of water; with some other developers the proportion is finer still: for instance, effective work may be done with a solution containing only 1/4 grain of paramidophenol to the ounce. A toning solution contains very frequently 1/8 to 1/10 of a grain of gold in each ounce of water, together with two or three grains of a moderately alkaline salt. Needless to say how completely the desired effect may be rendered impossible if these are allowed to come in contact with the saturated solutions in the fixing bath, or the ferrous oxalate developer. A few grains of amidol, blowing as dust about the room, may settle on printing-out paper and make whole sheets worthless. Dishes and measures must be kept scrupulously clean, and rinsed after use; and the fingers that have been dipped in fixing baths or bichromate sensitisers must not handle plates, silver papers, or toning prints until they have been very carefully washed and dried. Ordinary tap-water often contains sufficient alkaline matter or other impurities to entirely alter the action of a toning bath, and should, therefore, be avoided for such purposes.
1 It was formerly supposed that oxygen liberated in the nascent state, had a charring action on the gelatine. Dr. Eder, as well as MM. Lumiere and Seyewetz, has shown by convincing experiments that the gelatine is not oxidised, and may be recovered by digestion either with very dilute sulphuric acid (1 in 1,000) or an alkali in the case of chrome alum.
Among works dealing more fully with the chemistry of photography may be mentioned those of Dr. Eder and Prof. Meldola; also Chapman Jones' Science and Practice of Photography; and on the special subject of dry plates and development, Sheppard and Mees' Investigations on the Theory of the Photographic Process.