(10) One of the most powerful - if not, indeed, the most powerful - detergents for refractory plates is the mixture of sulphuric acid and bichromate of potash recommended by Carey Lea some years ago. It is especially useful with glasses which have been frequently used, or which from the nature of the treatment they have undergone resist the action of both acids and alkalies completely. Its utility is dependent upon the powerful action of chromic acid upon organic matter, and we have never yet met with a plate which did not succumb to its treatment. One precaution is necessary in using it, however; it must be carefully removed from the glass by copious washing as soon as possible after it has done its duty. If allowed to soak for some time, as is frequently the practice, the plates appear to absorb the solution (the penetrating power of which is extraordinary), or an insoluble compound becomes firmly attached to the surface and stedfastly refuses to be displaced. Though generally invisible, it results in a peculiar mottled appearance between the glass and the developed film which entirely ruins the picture.
We recently treated a number of plates which had become useless from this cause with various detergents, including acids as well as alkalies, but to no purpose; friction with various abrading powders failed to remove the defect, and we were well-nigh compelled to give it up. Remembering, however, that cyanide of potassium has been utilized by carbon printers for the purpose of reducing the strength of overprinted proofs - which it does by virtue of its action upon the insoluble compounds of chromium - we resolved to try its efficacy on our refractory plates, when all the mottling disappeared as if by magic. Those amongst our readers who dare to fly in face of all that has been lately written upon the dangers attending cyanide and bichromate of potash have here a " wrinkle." Surely those who have dared bichromate will not fear the minor dangers of cyanide. (Brit. Jl, Phot) (11) A cream of tripoli powder and spirits of wine, with a little ammonia added, is a very good solution for cleaning glass plates. Old collodion is also very good; it should be thinned down with an equal bulk of spirits of wine; add an excess of iodide of potassium, and shake till the solution is saturated. Caustic potash is very good; so is carbonate of soda.
If the plates be new, and covered with little gritty particles which do not come off on the application of potash, they may be removed with nitric acid. (12) Methylated spirits, washleather, and plenty of " elbow-grease." (13) At a recent meeting of the American Lyceum of Natural History, Dr. Walz suggested a method for cleaning greasy beakers and photographic glass plates, which must at once commend itself to all practical chemists and photographic operators. He takes a dilute solution of permanganate of potash, and pours in enough to wet the sides of the vessel to be cleaned. A film of hydrated manganic oxide is deposited, which is then rinsed with hydrochloric acid. Chlorine is formed, which acts in the nascent state on the organic matter, which becomes readily soluble. The permanganate solution can be used again and again till its oxidizing power is exhausted. (16) Dissolve 15 gr. of iodide of potassium in 5 oz. of water and 5 oz. of alcohol, afterwards adding 3 gr. iodine and enough whiting or rottenstone to make a creamy paste.
Rub a little of this on the glass with a rag until clean, then polish with a cloth. (J. Hughes.)