This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Straight Perpendicular Lines. This may be caused by the bath becoming charged with alcohol liberated from the collodionized plates. Such a bath should be boiled down for an hour, when most of the alcohol will be driven off in vapor. After the bath becomes cool it may again be brought up to its normal bulk by the addition of more pure water, and after filtering it is again ready for use.
Scum On The Film. This is usually caused by too strong a silver bath. Test the bath with the hydrometer. If it registers over 45° reduce with pure water. Sometimes the collodion may be too strongly bromo-iodized. In such a case, add a little plain collodion to the iodized collodion, which will overcome the difficulty.
952. The scum may also be formed on the wet negative, from a scum that sometimes collects on the surface of the sensitizing bath. In such a case, float a strip of tissue-paper over the bath. The scum will collect on the tissue and can be withdrawn. It may require two or three such applications to remove it entirely. By careful use of the bath, keeping the dark-room clean and free from dust, and the sensitizing bath dish always covered, you will seldom experience any trouble. When the trouble does appear it is almost certain that the bath has become contaminated, and the best thing to do is to give it a good sun bath, by pouring it into a bottle and placing it in the strong sunlight for a day or two. Before sunning, however, the bath should be made alkaline with carbonate of soda or a few drops of ammonia. A small quantity only will be required, and if not certain that it is alkaline test with red litmus paper. When this paper turns blue the bath is alkaline.
Pin-Holes. These may be caused by dust on the plate while collodionizing it. They may also be caused by too much iodide in the collodion, and, sometimes, even an insufficient amount of iodide will produce pin-holes. If you are careful to prepare the collodion according to formula, and procure C. P. fresh chemicals, you will not experience any trouble.
Comet-Like Spots. These are sometimes caused by undissolved particles of gun-cotton in the collodion. They are also caused by rust from the water faucet. Always filter your fresh-made collodion, and it is also advisable to filter the water from the tap. A 1inen cloth placed over the mouth of the faucet will answer.
Round Black Spots. These are usually caused by dust in the air. Remedy: Keep the dark-room closet free from dust of any kind.
Contrasty Negatives. This may be due to too acid a silver bath. Either the bath when freshly made may have been too strongly acidified, or, if it works well when fresh, it may have become charged with acid by constant use. The iodide in the collodion will in time liberate the nitric acid in the film thus charging the bath with considerable of the acid, which may cause trouble. By occasionally testing the bath with litmus paper it may be kept at the right stage. If at any time it tests strongly acid, it may be slightly neutralized by the adding of a little carbonate of soda. It is advisable, after adding the soda, however, to sun the bath for a few hours, and again filter before using.