7. The film does not stick to the glass.—Clean the plates very carefully, and make the Collodion a little thinner if required. Allow a longer time before dipping in the Bath. A very effectual plan is to roughen the surface of the plates, about an eighth of an inch round the edges.
IMPERFECTIONS IN COLLODION POSITIVES.
The principal difficulty in the production of Negatives is to ascertain the right time of exposure to light and the proper point to which to carry the development of the image. A minor amount of fogging, stains, etc., is of less consequence, and will scarcely be noticed in the printing.
With direct Positives however the case is different. The beauty of these pictures depends entirely upon their being clean and brilliant, without fogging, specks, or imperfections of any kind. On the other hand, the exposure and development of Positives is comparatively simple and easily ascertained.
1. The shadows dark and heavy.—The plate has not received sufficient exposure in the Camera;—or the film being very transparent and the Silver solution weak, Nitric Acid is present in the Bath, or the Collodion is brown from free Iodine; in the latter case make the Collodion a little thicker, and develope with Sulphate of Iron in preference to Pyrogallic Acid.
2. The shadows good, but the lights overdone.—The developing fluid may have been kept on too long; or the object is not properly illuminated ([p. 220]); or the Collodion is not adapted for Positives.
3. The high lights pale and flat, the shadows misty.—The plate is over-exposed. Indistinctness of outline caused by over-exposure is distinguished from that produced by fogging by holding the plate up to the light; in the former case the image shows as a Negative.
If the Collodion is colourless, clearer shadows will probably be obtained by dropping in Tincture of Iodine until a yellow colour is produced.
4. The picture developes slowly; spangles of metallic Silver are formed.—Too much Nitric Acid is present in proportion to the strength of the Bath, to the amount of Iodide in the film, and to the quantity of Protosalt of Iron in the developer ([p. 112]).
5. Circular spots of a black colour after hacking up with the varnish.—These are often caused by lifting the plate too quickly out of the Bath; or by pouring on the developer at one spot, so as to wash away the Nitrate of Silver; or by the use of glasses imperfectly cleaned.