London, April 9, 1853.
Economical Iodizing Process.—Mr. Maxwell Lyte is probably as good a judge as myself, as to where any weak point or difficulty is found in iodizing paper with the carbonate of potass: if any chemical is likely to be the cause of unusual activity, it is the carbonic acid, and not the cyanide of potash. I still continue to use that formula, and have not iodized paper with any other: though I have made some variations which may perhaps be of use. I found that the nitrate of potash is almost the same in its effects as the carbonate. I would as soon use the one as the other; but the state I conceive to be the most effective, is the diluted liquor potassæ: that would be with iodine about the same state as the iodide of potash, but hitherto I have not tried it, though mean to do so.
I am not quite certain as to whether, theoretically, this position is right; but I find in iodide of potash, and in the above formula, that the iodine is absorbed in greater quantities by the silver, than the alkaline potash by the nitric acid. Thus, by using a solution for some time, it will at last contain but very little iodine at all, and not enough for the purpose of the photographer; hence it requires renewing. And I have lately observed that paper is much more effective, in every way, if it is floated on free iodine twice before it is used in the camera, viz. once when it is made, and again when it is dry: the last time containing a little bromine water and glacial acetic acid. It appears to me that the paper will absorb its proper dose of iodine better when dry, and the glacial acetic acid will set free any small amount of alkaline potash there may be on the surface; so that it will not embrown on applying gallic acid. By using the ammonio-nitrate of silver in iodizing, and proceeding as above, I find it all I can wish as far as regards the power of my camera. With this paper I can use an aperture of half an inch diameter, and take anything in the shade and open air in five or six minutes, in the sun in less time. The yellow colour also comes off better in the hypo. sulph.
I think Mr. Maxwell Lyte has made a mistake as to the price he quotes: about here I cannot get any iodide of potash under 2s. per ounce, and the five grains to the ounce added to the common dose of nitrate of silver is hardly worth speaking of; it would amount, in fact, to about fifteen grains in a quire of Whatman's paper,—no great hardship, because many use much higher doses of silver for iodizing; forty grains to the ounce is not uncommonly used, but I believe twenty-five grains quite enough.
I presume, in Sir Wm. Newton's mode of treating positives, the acid of the alum decomposes the alkali of the hypo. sulph. And it would be, I suppose, better for the picture, if its state were entirely neutral when put away or framed; but if alum is added, acid must remain, since Sir Wm. says it combines with the size. What I should imagine is, that the idea is good; but experience can only decide if the picture is better put away in an acid condition. I should think there are more available acids for the purpose, for alum has an injurious effect upon colour; and a positive is nothing but colour, the organic matter of the paper stained as it were by the silver: for, after all its washings and application of re-agents, no silver can possibly remain in the paper. The safest state therefore of putting away ought to be ascertained and decided upon; as it is no use doing them if they fade, or even lose their tones.
Weld Taylor.
N.B.—The iodized ammonio-nitrate paper will not bear exposure to the sun; it will keep any length of time, but should be kept in a paper, and away from any considerable degree of light.
[Replies to Minor Queries.]
Bishop Juxon's Account of Vendible Books in England (Vol. vi., pp. 515. 592.).—The following note in Wilson's History of the Merchant Taylors' School, p. 783., solves the Query respecting the authorship of this bibliographical work.