The following working method, described by H. Kuhn in 1921, under the name of " Leim druck " (glue printing), is employed by some German artists.
Sheets of strong glue (Lyons glue or Cologne glue) are broken with a hammer into small pieces, and soaked in cold water for about 48 hours. The excess of water is poured off, and this glue is dissolved in a water-bath in the water which it has absorbed. Then, in a mortar or on a polished stone slab, are mixed, with a grinding muller, about iso minims of this solution,' 75 minims of a 10 per cent solution of ammonium bichromate, and 15 gr. of a very finely-ground black pigment (soft drawing crayon), the tone of which may be rendered warmer by a little red chalk ; if the mixture is too thick it may be diluted with a very little water.
A white or slightly-tinted paper, of smooth surface and thin substance, and larger than the negative, is fixed to a board with drawing pins, and covered rapidly with the pigmented glue, using a large soft brush. The coating must be sufficiently thick, but not too thick, or it will crack 'n drying. Inequality of coating has no harmful effects so long as the coating is suffi ciently thick in all parts. After drying, a very glossy coating indicates that the proportion of gelatine to pigment is too great ; a too matt coating points to an excess of pigment.
When drying is quite complete, the gelatine side of the sheet is laid down on a glass slab, and the back is covered over with paraffin oil, two applications at least, with a tuft of cotton-wool, the operation being done in a somewhat dark corner. A few minutes are allowed to elapse after each treatment, and any excess of paraffin is removed with blotting-paper before exposure.
Exposure to light being made through the support, the final image will be reversed unless a reversed negative or a film negative is em. ployed. With many subjects, however, reversal does not matter.
The printing is best timed by means of an actinometer covered with a piece of the same paper employed as support, and similarly treated with paraffin.
When taken from the printing frame, the print is washed in cold water for about five minutes, then " developed "in warm water at about 85° F.
When drying is complete, the paper is cleaned by immersion for about three minutes in a grease solvent, e.g. petrol, benzine, or, preferably, a non-inflammable solvent such as carbon tetra chloride. As the paper, however, still retains a slight translucency, prints should be mounted with starch paste on to white card.