Sheets shims under blanket
Although the phototypie is a flat process, in the parts sunstroke of the image, the gelatin inflates less that the parts which are not it. As a consequence, the plate of gelatin, wet, being ready for the edition presents more or less hollow parts. It is hardly perceptible, but if the pressure takes place uniformly, there will be an excessive compression of the gelatin on the light areas. The consequence will quickly be smelt at the end of some prints. A grey veil will arise in the clear zones of the image because of the drying out of the gelatin which at first was the most swollen. Driest, this one will take slightly the ink, while it should not.
Thus, for not to compress the gelatin where she has to remain very wet, we place shim between the blanket and the cylinder of pressure. So during the edition the paper sheet fetches the ink in hollows, without practicing of pressure exaggerated on the gelatin which does not contain image (and the clear parts of the image). The gelatin remaining wet in the parts clear will continue to refuse the ink, avoiding greyed itself.
The incomprehension of reasons of rise of grey veil is one of usual causes of abandoned of this technique by beginners.
Shim is established by one or by several paper sheets, more or less fine, correctly positioned, compared to the image and to its darkest parts. These sheets are partially glued between them, as on the cylinder or the support of blanket for the manual press.http://www.phototypie.fr/wordpress/les-mises/mise_sous_blanchet/