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.