The object of this second development is to darken the creamy white silver bromide, by reducing it to a metallic state. Daylight is important at this stage ; and if the darkening has to be done at night, the plate should be held up while three inches of magnesium ribbon are burned at about six inches distance. Three or four minutes must be allowed for this action of the developer, which must be carried on for some time after the high-lights are completely darkened.
The plate is then washed for about five minutes and immediately placed to dry ; fixing is unnecessary unless the plate has been intensified.
Such is the development of the autochrome plate and it will be found,providing the exposure is correct,and the development has been properly carried out, that when viewing the plate by transmitted light, it appears a brilliant transparency with the objects rendered in their true natural colours. All that remains to be done, when the plate is thoroughly dry, is to coat it with a special varnish, composed of 1 ounce of gum dammar dissolved in 10 ounces of pure benzol. An alternative varnish is one consisting of celluloid dissolved in amyl-acetate ; both of these varnishes are better bought ready-made, as they are messy to prepare and filter.
Varnishing adds greatly to the brilliancy and quality of the plates, and at the same time favours their permanence. Under no circumstances should ordinary varnish be used, as this would damage the colours.
Should the plate when varnished appear too dark, and all the colours degraded apparently with black, it is due either to underexposure or under-development, but in all probability the former. If the plate is under-exposed, sufficient of the silver is not reduced to a metallic state in the first development, and consequently too much is left on the plate to form the positive image, which is darkened in the second development. On the other hand, in cases of overexposure the image is too transparent and the colours will lack brilliancy, and in such cases the result is considerably improved by intensification. The plate may be intensified either immediately after the second development or, if preferred,at anytime after it has been dried, providing the varnish has not been applied.
Should it be desired to intensify the image in order to increase the brilliancy, the following solutions are necessary, and should be prepared in advance : Bath E. Oxidizing Solution.
Solution " C " (see page 177)
Bath F. Intensifier.
Citric acid ............
Bath G. Intensifier.
Bath H. Clearing Solution.
Potassium permanganate ......
Bath I. Fixing Bath.
Liquid sodium bisulphite
Liquid sodium bisulphite is easily obtainable in London, but should the photographer find himself unable to get this compound, saturated solution of potassium metabisulphite may be used instead,
To Intensify. The plate is immersed in solution E, given above, for 10 or 15 seconds. This oxidizes any trace of developer remaining in the film and allows of proper intensification. Should this oxidizing solution be left on for too long a time the colours will be weakened, and in cases of decided under-exposure this is an advantage, and the time in " E " may be increased to half a minute or even longer. The solutions labelled "F" and "G' form the intensifier, and two drams of "F" are added to three ounces of " G ' immediately before use. The plate as it comes from the oxidizing solution is rinsed for a few seconds in running water, and the inten-sifier poured over it. Action is rapid and the intensifier will be found to strengthen the colours considerably.
If the Autochromes are wanted for lantern slides, the intensification must be of the slightest character, or they will be made too dense. If they are to be seen in the hand the action may be carried further. The intensifier discolours in a minute or two, but may be used until fine black particles are discernible in it, when it must be thrown away. If the intensification by that time has not gone far enough, the plates may be rinsed, immersed in the " E " solution again, and the process repeated with fresh intensifier ; but such occasions are very rare. As a general rule, the intensifier will be found to do all that is wanted by the time it begins to discolour. As intensification is pushed the plate will be stained yellow, but this may be disregarded as the stain disappears in the after operations.