If we have an ordinary gelatine negative, say, of half-plate size, and
require to enlarge it to a whole plate, the simplest plan is to
thoroughly wash it and immerse in a solution composed of citric acid,
2 ounces; hydrofluoric acid, 1 ounce; acetic acid (glacial), 1 ounce;
glycerine, 1/2 ounce; water, 20 ounces. The action of the hydrofluoric
acid will be to detach the film from the glass, while the other acids
will cause the film to spread out considerably; the action being even
all over, the image is completely enlarged. It is then carefully
removed and washed in plenty of clean water, after which it can be
transferred to a larger piece of glass. The action is sometimes to
weaken the negative in density; it is therefore occasionally necessary
to intensify it.