For very fine line drawings or reproductions of copper-plate engravings the developer is used somewhat weaker, and one-fifth of its volume of water added. The development takes somewhat longer; the fine lines remain, however, very clear.
After developing the plate is well washed and then intensified first with a solution of—
| Water | 5000 | ccm. |
| Ferrous sulphate | 180 | g. |
| Citric acid | 90 | g. |
{38}
to which a few drops of a 1 : 12 silver nitrate solution have been added. Care should be taken that in the first intensification the glassy places remain clear. When, therefore, the desired opacity is attained the plate should be immediately washed. Too long intensification fogs the finer lines.
The plate should be fixed with 5 per cent. solution of potassium cyanide. For this, however, a concentrated solution of hyposulphite of soda can be used.
After well washing, the negative will show, when examined by transmitted light, whether the ground is opaque enough, or whether a further intensification is necessary. With correct manipulation this first intensification will be sufficient for fine, well-prepared drawings.
If, however, the opacity proves to be too little, after thoroughly well washing, the second intensification or blackening is effected with a concentrated solution of mercuric chloride in distilled water, which is allowed to act till there is a distinct precipitate and the film has become grey. After well washing the plate is flowed over with solution of thiosulphate of gold till the grey quicksilver precipitate has become absolutely black. The gold solution consists of—
| (A.) Distilled water | 1000 | parts |
| Chloride of gold | 1 | part |
| (B.) Distilled water | 1000 | parts |
| Hypo-sulphite of soda | 10 | parts |
Immediately before use solution A is added with constant stirring to solution B, and a few drops of ammonia added. If the desired blackening is not effected with one application the operation must be completed.