This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
In blocking out some figures from a glass with oil or water colour, the positive should be mounted with its glass side outwards, otherwise the image would be reversed. If the positive is so mounted, there will be no danger in painting over it with oil or water colour, The figure may, of course, be painted out on the film side of the positive; but in such a case, care must be taken in cleaning off, as a collodion film is exceedingly tender, ami a good plan would be to soak the positive in turps, and then stroke the paint gently with a tuft of cottonwool. Another plan for getting rid of a figure is to cut for it a mask in tissue or tracing paper; or the blocking-out may be done on a glass cover placed over the positive. In each of these methods the work is out of focus, and a hard, sharp blocking-out line around the figure is avoided; but if the outline is very intricate, and the tone of the background differs considerably from that of the figure, the painting-out method is best. Figures are sometimes blocked out with a No. 1 retouching pencil after rubbing over the glass with retouching medium in the usual way.
Fancy backgrounds, etc., then can be introduced.
 
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