YELLOW STAIN.—A yellow stain is often produced upon negatives caused by insufficient fixing. This is very often difficult to remove. One method recommended by Mr. Drake is as follows : Soak the plate for five minutes in clean water, meanwhile make a solution of iodide of potassium, twenty grains to the ounce of water, now put the plate in this solution and let it stay for ten minutes. If the stain is very old keep it in for half an hour. Now dissolve half a drachm of cyanide of potassium in one ounce of water. Take the plate and put into this, and gently rub the stains with a tuft of cotton wool (absorbent filtering cotton will do), free from grit, until they are quite gone. If the stains are very old, make the solutions stronger and soak for a longer time.
Yellow stains are also often produced by pyro-development. These can usually be removed by immersing the plate in a clearing solution composed of Alum 4 ounces Citric acid 2 ounces water 20 ounces for several times. In this solution the plate should be soaked for about ten minutes in water
before immersing in the above.
Yellow silver stains produced in printing from ordinary silver paper can be removed with the following : For use, mix equal parts of A and B fresh for each negative, and apply to stained portions, or the negative can be immersed in the solution. After the stain has disappeared the negative should be well washed, and treated with a saturated solution of chrome alum and well washed.
ZINC (Symbol, Zn ; atomic weight, metallic element never found in nature. Its chief sources are calamine or zinc carbonate, blende or zinc sulphide, and red zinc ore in which zinc oxide is associated with the oxides of iron and manganese.