275. A few further "indications" as to the imperfections in prints, may not bo out of place. The same faults wen - found, the same tribula-tions troubled the printer, in the early days of silver printing as now - as then. With care they can be generally avoided, but as we are not always careful, it will be of services to know the cause of any evil when it presents itself, and thus have a suggestion, too, as to the means of working out a cure.
876. One of the first perplexities which the printer novice meets, is the discoloration of his nitrate bath. One of the greatest causes of this annoyance is leaving the solution uncovered to dust and air, when not in use. Constant use also causes discoloration, and careless filtration will do the same. Knowing the causes, it is easier to prevent them; but when the trouble occurs, the bath may be rectified by the addition of kaolin, or some neutralizing ingredient - say permanganate of potash.
276. Be sure that the hack of the negative is clean, and that the contact is perfect before the negative is exposed to the light. When you wish to examine the print, go slow and sure. Do not allow the negative or paper to slip from their position, as that is sure to spoil the print, and result sometimes in spoiling the negative. Over- or under-printing is equally bad. The first results in an unpleasant heaviness, while the latter loses individuality. Watch the bleaching effects of the toning - bath and fixing solution, and print to it. Anticipate the tone before you reach it, and print to conform to it. - I. B. Webster.
276. The various methods in practice for decolorizing the silver bath for printing are the addition of kaolin; carbonate of soda producing carbonate of silver, or carbonate of silver direct; chloride of sodium, or chloride of silver; citrate of soda, and other like salts; Mr.
England's late method of boiling the bath; and, lastly, the addition of a very dilute solution pf permanganate of potash. Having given a good trial to most of these, I can speak of their various capabilities, and which, after a long practice, I greatly prefer for decolorizing the printing bath.
There is nothing that answers more perfectly than the old-fashioned stirring up with kaolin, but it is a very dirty process. It must always be done at the close of work, as it takes a long time to settle; a much larger quantity of solution must be always kept than is required; and although, by treating it, when done with, with nitric acid, a good quantity of silver may be recovered, still I believe it entails a great loss. Of the various alkaline salts which, added to the bath, produce salts of silver which unite with the coloring matter, and are either precipitated together or are filtered out in combination with each other, the carbonate and citrate of soda answer the best, but filtering is necessary, as they are so long settling.
277. A mottled effect sometimes appears on the paper after coming from the printing-frame. This is known as "measles," and is generally caused by the paper being insufficiently silvered, but sometimes arises from an inferior quality of paper, which imbibes the silver unevenly. If it comes from a lack of silver, the paper will print better on the end of the sheet which is lowest when hung up to dry, because the silver running down concentrates in drying, thereby leaving a larger quantity of
A plan I tried, to avoid the loss of precipitating the silver from the bath, and so reducing its strength and the consequent loss for the time of the silver, was to wash one batch of prints in as small a quantity of rain-water for their first wash as I could, pour off into a jug or large measure-glass, and precipitate the silver roughly with carbonate or citrate of soda; without waiting for it entirely to settle, I poured the liquid into the residue tub, and then added the precipitate to the printing - bath. By this means I avoided throwing nitrate of silver out of use for some time.
Mr. England's method of heating the bath is founded on the same principle as sunning the negative bath, heat in the former roughly doing what the actinic rays have more delicately to do for the latter, in both cases the solution being made alkaline. The silver is partially reduced and precipitated in combination with the organic matters. This process is troublesome, and takes time for its accomplishment. By far the best of all the methods I have put in practice is that of the addition of a dilute solution of permanganate of potash; it can be done on the instant; in fact, is best added to the measure just before pouring into the floating-dish. I have never found filtering at all necessary. One great advantage it particularly possesses is, that you can decolorize in the middle of work without stopping. It is well known that, after floating a dozen sheets in a bath in the heat of summer, the bath begins, with some papers, to darken, and will injure the purity of the whites of vignettes, and it is usual with printers to reserve the first sheets off the bath especially for them; but if you should have a large run of vignettes, or get used up with your reserved sheets, you must give up, or get out a fresh bath; but with the permanganate you have only to add a little more to the dish without removing, stir up, and you are as clean as at first starting. About a drachm a day of a ten-grain solution is quite enough for a gallon of bath. There is only one thing I have found necessary to take care of, which is, not to add more than is just necessary to take out the color. I fancy that more than this somewhat reduces the vigor of the proofs. I only hope that those who have not done so will give it a fair trial, and they will soon give up every other method for it. - F. G. Eliot.
277. For the information of those who do not understand the chemistry of nitrate of silver and alkaline salts, I will try to explain the action of neutralizing agents. If ammonia or carbonate of soda be used to neutralize a bath, a reaction takes place, - a simple one with ammonia, - in which the nitric acid combines with the ammonia to form nitrate of ammonium, which remains in solution in the bath as a neutral salt. With carbonate of soda a double reaction, in which the base, the metal sodium oxide, is converted into nitrate of soda, and free nitrate in that part of the sheet. If the paper itself is in fault, the mottled apperance will be evenly distributed all over. The remedy is to float your paper longer, if it is under - silvered, providing you are floating less than two minutes; if you are floating it that length of time or more, strengthen your solution about ten grains to the ounce, and try it again. When the paper itself is in fault, it is generally caused by the sizing; lay it in a .lamp place for some time, until the sizing gets softened: it will then absorb the silver solution better.