The best rule is to print the lightest shade on the positive very decidedly darker than it would be wished that it should remain permanently.
_To fix the Positive._--On removing it from the pressure frame, place it in a bath made as follows:
Water 6 oz. Hyposulphite of soda 1 oz. Nitrate of silver solution, 50 grs. to oz. 15 minims. Iodide of silver, dissolved in a saturated solution of hypo. 10 minims. Chloride of gold 2 grains. Chloride of silver (blackened by light) 5 grains. Acetic acid 2 drops.
Mix these: let them stand some hours; and filter before use. If the chloride of silver is omitted, the bath will do very well, but will very much improve with age, as it will acquire chloride of silver from the positives placed in it. { 582}
The time to leave the positive in the fixing bath varies from one hour to twelve. To get good black and white tints, the average time is five or six hours. When the desired tint is obtained, remove it into a bath composed of
Leave in this for half an hour, and then keep it in running water for several hours. If the water is hot, the time of soaking may be lessened: boiling water is objectionable. Nearly dry the positive between sheets of clean blotting-paper, and finish it by passing a very hot iron over it.
_General Remarks._--The albumenised paper will keep any length of time in a dry place.
When made sensitive, as directed, it will keep three days, always supposing that it is both prepared and kept most carefully excluded from white light. If, instead of a solution of nitrate of silver of ninety grains to the ounce, a weaker one be used, to make the paper sensitive, it will keep when sensitive a much longer time,--with a thirty-grain solution, a fortnight, or sometimes even a month; but then it does not give a positive of the same force and tone as that obtained with the stronger solution.
After the fixing bath has done its day's work, it should be poured back into the bottle from which it came, and the bottle be filled up from the finishing bath; and so the bath is kept always of the same quantity; and by adding from time to time chloride of gold, it is kept of the same quality.
The nitrate of silver and chloride of silver will never have to be renewed. The iodide of silver should be added as at first, viz. ten drops for about every two hundred positives fixed; and the acetic acid, viz. two drops for about every four hundred.