In order to create a resist matrix for the copper, the positive film image must be exposed onto the sensitized gelatin tissue. A grain or screen texture also must be applied over the image to create ink holding wells in the etched areas of the plate. A traditional aquatint grain can be applied directly to the copper plate, or more easily, a screen pattern can be exposed onto the gelatin tissue prior to the positive exposure.
When the sensitized gelatin is exposed to light, the resulting hardening effect, called tanning or insolubilization, is directly proportional to the amount of light hitting the gelatin. Exposure to light will affect the gelatin by raising its melting point from 32°C (90°F) to as high as ...
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