Wet Printing Principles
More effective than the scratch treatments normally used and mentioned in a previous chapter, the technique of wet printing can provide clean copies from originals scratched on the base and to a lesser degree on the emulsion. The effect of a scratch is shown in figure A. Light transmitted by film is refracted and scattered by surface scratches and thus the scratch is seen as a dark line.
If the film surface - in
figure B - is coated with the liquid of the same (or almost the same)
refractive index as the film (dotted lines) light passes through it undeviated
and a dark line is not seen.
Wet printing is based upon this phenomenon. It depends for its success on the selection of a liquid with two qualities. Firstly, it must have the same or nearly the same refractive index of the film base (cellulose acetate has a refractive index of 1.490). Secondly, it must not contain water so that the liquid will not be absorbed by the film emulsion during printing, primarily so that the liquid can be dried off quickly before the film is wound up. Trichloroethylene (index 1.478) has the nearest optical properties to those of the film base and emulsion.
However, this liquid is too volatile for pre-wet printing so that the evaporation from the surface of the film, before reaching the exposure aperture, is too high. Trichloroethylene is going to be proscribed in most countries because of its toxicity. It also removes the plasticiser from the film base and after several printing passes makes film brittle.
Perchlorethylene (index 1.504) is less volatile and therefore preferred.
There are three basic methods of wet printing:
