The hotter the water the greater its washing-off action, and hence in cases of over-exposure very hot water may go far to recover the print. When the desired result is secured, transfer the print to a dish of cold water, this instantly tends to slightly harden the film by cooling it, and after two or three minutes it is passed into a dish of alum and water, which further hardens it and also "clears" the print of any bichromate salts which may still remain. In the alum bath the print should remain until any sign of yellow stain has disappeared, when after a final rinse of a few minutes in cold water to remove the alum, the print may be hung up to dry.
It will be seen that there is no prolonged washing as with those processes in which hypo is employed, and the print is absolutely permanent.
It must be remembered, however, that in the finished picture we are looking at the back of the printed film as it received the light impressions from the negative, and hence the image is reversed, that is, the left is on the right and the right on the left. For landscape and views this reversed position will probably be of no importance, but if it is desired to have things right way round—in portraits it will be essential—we must either work from reversed negatives, or we must again transfer the film which will then constitute a double transfer. We shall now understand why previously we called the paper to which the film was transferred single transfer.
Inasmuch as it will be seen that the print is not on paper, but consists of a transferable film of pigmented gelatine, it will be understood that the paper employed is merely a support to that film, hence it is customary to speak of the paper as the support, whilst moreover it maybe, and as often as not is ivory, glass, textile fabrics, wood, or other substances.
If now we wish to again transfer the film so as to correct the lateral reversal, we substitute for the single transfer paper a "temporary support."
The temporary support which is to receive the film merely whilst it is being developed, and with the intention of its being subsequently transferred again to a final support, may be paper or many other things.
Moreover, remembering that the film is mainly gelatine, it should be clear that whatever the nature of the surface of the temporary support, the soft glutinous film will take that surface just as we may make the impression of a seal in sealing-wax.
The normal carbon print is shiny, due to the gelatine, and so, if as a temporary support we were to use ground glass or matt "opal," the carbon print film would receive the fine granulated surface and give a matted print as a result. This merely by the way as suggesting an additional advantage offered by the double transfer process as a set-off against the slight extra trouble.
If double transfer is determined upon, and it is not intended to experiment with ground glass, etc., then when purchasing the carbon tissue, some temporary support (sheets of paper coated with gelatine and shellac) should be procured, also some pieces of final support.
Whatever the temporary support, it must receive an application of waxing solution. This also may be bought, or can be made of:—