STAINS.
Many photographic experts have made this the subject of research, and the ex periments of Mr. C. H. Bothamley a few years ago did a very great deal towards clearing up some disputed points. Among other things, he pointed out the absolute necessity of excluding hypo. from all opera tions before fixing. Until this stage is reached, the hypo. should be placed en tirely on one side. A certain part of the working room should be set apart for the hypo. dishes, and used for nothing else. Do not make up solutions of hypo. near the toning bench ; in fact, no trace of solu tions or crystals should be allowed where other work is done. See that the hands are well washed after manipulating the prints while fixing, and not merely dried with a towel. This is a frequent cause of bad stains ; the towel appears clean, but is really affected sufficiently to contamin ate the fingers instead of cleansing them. Even though great care is used to thoroughly wash down a bench that has been splashed with hypo., some part will probably be absorbed in the wood, and at some future time will wcrk mischief.
17 How TO TREAT HYPO. STAINS.
To the novice it is always a cause of con siderable wonderment why a print may be placed entirely in a bath of hypo. without
staining, and yet if touched by the hand which has been in contact with the merest trace of the salt a bad stain is produced. This is easily explained by a little experi ment. Take a small quantity of silver chloride and divide it into two parts, placing each portion in a test tube. Pour over one a strong hypo. bath, say 3 in 20, and a weak one, 1 in 200, over the other. That in the strong hypo. bath will be immediately dissolvci:. but the other will form a dirty yeljw 3“etallic-looking pre cipitate. This is the insoluble silver thiosulphate, which rapidly decomposes into the dirty brown silver sulphide, con stituting the stain on the paper. In most cases the stain is complicated by the silver organate present. Such stains are to some extent soluble or removable by strong potassium cyanide, but it needs to be used with extreme care, and, generally •speaking. it is easier and better to make a new print. The method adopted is to moisten the corner of a silk handker chief and rub it on a lump of potassium cyanide, and then lightly touch the stain until it shows signs of disappearing, when it may be plunged into a bath of clean water.