Photographic images can be etched on glass by using a gum bichromate print as an intermediate. The process gives a "print" from a positive transparency, the image appearing as a positive when it is viewed against a dark background.
Sized paper is first coated with a solution of one part each of gum arable, sugar, and ammonium bichromate in eight parts of water and allowed to dry in the dark. The prepared paper is then exposed to daylight behind a positive transparency.
The sensitized layer of gum is hardened and made less sticky by the action of light, so, at the end of exposure, the areas behind the clear parts of the transparency are not as sticky as the areas behind the denser parts. While the surface is in this condition, it is dusted with fine bitumen powder; the powder sticks to the lightly-exposed areas more readily than to the heavily-exposed areas. The result is a direct positive of the transparency in bitumen dust on the sensitized paper.
This image is pressed into contact with the glass plate which has been previously warmed. The warmth softens the bitumen dust and makes it stick to the glass surface. When the paper support is soaked with water and peeled off, the bitumen powder is left and forms a positive image resist of the original transparency.
The glass is then exposed to the etching action of hydrofluoric vapour. Where the surface is protected by the bitumen dust, no etching takes place; elsewhere the glass is attacked by the acid and the result is a photographic image. The bitumen dust is finally washed off with petrol.
This is a reversal-negative process; the gum-bichromate-bitumen image being a reversal stage. The etched image is negative but acts as a positive since the dense areas scatter light and appear bright instead of dark. Hence the need for a positive original.
This method produces images which are an integral part of the glass. Image layers can be produced on top of the glass by any of the methods described for printing on metals.
Photographs may also be printed on a glass surface by any of the regular methods of printing on solid supports, i.e: the glass may be coated with a sensitized emulsion and printed under a negative by contact or enlarging; or it may have an image transferred to it by the transfer coating process; or it may take the place of the final support in the carbon or carbro processes.
Pictures in Glass
By an entirely different process photographic images can be formed inside the glass. This process makes use of a special type of photosensitive glass containing colloidal silver or other metal in suspension. The glass is exposed through a negative to a strong source of ultra-violet light. This converts the colloidal metals in the glass into a form in which they can be precipitated and rendered visible. The actual precipitation is brought about by heating the glass to just below its annealing temperature. The result is a visible image embedded in the glass.