If stripping negatives are to be prepared, it is advisable, with the lead intensified negatives, to give the glass plate a preliminary rub with vaseline oil, to well wash, and then to coat with raw collodion.
Husnik’s graphite intensification is still to be mentioned, which, in some cases, may be of great service, but requires practice and care.
The expert photo-mechanical worker will adopt one or the other methods for his work, and as soon as he can work safely and without hesitation he will adhere to that.
4. THE PREPARATION OF AUTOTYPE NEGATIVES OF BLACK AND COLOURED DRAWINGS.
As regards the preparation of autotype negatives from simple black originals—wash drawings—the above photographic processes are quite sufficient. For taking painted originals—such as oil paintings, water-colour drawings, fresco, and pastell drawings—an orthochromatic collodion must be used if a correct rendering of the tones in the negative is to be obtained.
In our studio we have obtained excellent results with two kinds of colour sensitive collodion. These are the isochromatic collodion emulsion prepared by Dr. E. Albert and that suggested by Major von Hübl. In using the former, the glass plates as soon as they are taken out of the acid bath and well washed must be given a substratum of the following solution:—
| White gelatine | 5 | parts |
| Water | 500 | parts |
to which is added—
| Glacial acetic acid | 15 | parts |
| Alcohol | 10 | parts |
The solution should be filtered and coated whilst warm, and the plate dried in a room free from dust at least 15° R.