The Exposure

The positive film is placed, right side up, emulsion side down, on top of the sensitized gelatin.

The positive film and gelatin tissue are placed in a vacuum frame.

The ultraviolet sensitive gelatin is exposed to ultraviolet light in an amount determined by the densities of the film and will become a permeable resist to the etchant (ferric chloride) when transferred onto the copper plate later. Once transferred to the copper plate, this gel will regulate the rate at which the ferric chloride seeps through to start etching the copper underneath. In line etching, the resist either blocks the etchant completely or admits it where incised.

In aquatint gravure however, the resist is permeable, in proportion to its thickness. The thinner resist will allow the etchant to seep through first and etch deepest in areas corresponding to the darkest areas of the image. Thicker resist will hold the etchant back longer, so that only light etching occurs. By moving the plate through etching baths of various concentrations (ferric chloride and water), we can adjust how deeply each tone is etched in relation to other tones.

How does the resist get thicker or thinner? Ultraviolet light sets off a chemical reaction that hardens or crystallizes the gelatin. When exposed in contact with a transparency, the shadows block most of the ultraviolet light, limiting the extend of crystallization in the underlying gelatin. Under the transparency’s highlights which admit more ultraviolet light, corresponding greater crystallization occurs. Molecules of gelatin activated by ultraviolet light bind to one another in \long chains through a mechanism known as cross-linking. Such light-sensitive reactions involving the conversion of energy to matter are the basis of all plant life. Cross-linking makes the resist more or less insoluble depending on how far it extends through the gelatin. One of the many remarkable things about the technique is that the amount of crystallization tracks the amount of ultraviolet light exactly, even at the extremes of light and darkness, registering the finest gradations of tone.

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