The negative should be developed with the above-mentioned iron solution:—
| (A.) Metol | 15 | parts |
| Citric acid | 10 | parts |
| Distilled water | 1000 | parts |
| (B.) Silver nitrate | 10 | parts |
| Distilled water | 100 | parts |
The negative must be fixed before intensification with hypo and well washed.
A small quantity of solution A is poured over the negative to drive off the water, and then it is intensified in the ordinary way with 10 parts of solution A and 1 part of solution B.
Further intensification is effected, as described, with mercury, the blackening with thiosulphate of gold.
When the negative is finished and dried it is coated with a solution of gelatine 1 : 50, or a 1 : 15 solution of gum, in order to protect the film, and the negative should be levelled and allowed to dry.
Good opacity in the ground and clear glass in the lines of the drawing are the fundamental requirements of a negative for photo-lithography. But drawings are not always prepared in such a manner that this can be completely obtained. With flat drawings which are drawn in grey instead of black the first intensification is only continued till the finest lines appear clear. For further intensification the negative after drying is given a coating of a light sensitive solution of—
| Gum arabic | 17·5 | parts by weight |
| White sugar | 17·5 | parts by weight |
| Potassium bichromate | 5·8 | parts by weight |
| Glycerine | 1·8 | parts by weight |
| Water | 35 | parts by weight |
Ammonia is now added till a light colour is formed. After drying it is exposed from the glass side to 15° Vogel’s photometer, washed for about a quarter of an hour and then soaked in warm water till a relief is formed. By this operation a transparent relief is formed on the lines of the drawing, which absolutely protects the same from fogging. The intensification is then effected with mercuric chloride and chloride of gold, or an intensifier of {40} solutions of uranium and ferridcyanide is used, which is composed of two solutions—
| (A.) Uranium nitrate | 8 | parts by weight |
| Sugar | 8 | parts by weight |
| Water | 350 | parts by weight |
| (B.) Potassium ferridcyanide | 8 | parts by weight |
| Sugar | 8 | parts by weight |
| Water | 350 | parts by weight |