Falkland, Torquay.
_The Black Tints on Photographic Positives._--A correspondent having
inquired how these were obtained, and another replying that it was caused
by starch, I beg to offer a process to your readers as to how they may
obtain those carbonic tints; though I must premise that the process
requires some skill, and is not always successful, though always sure to
make them black: but on occasions of failure the lights sink, and the
brilliancy of the picture is lost. That it is not starch in the French
process, unless that vehicle contains some preparation, I am tolerably
certain; the chloride of barium will often produce black images, though
very uncertain; and the black process as given by Le Gray is uncertain
also. For myself, I generally prefer the colour given by ammoniac salt; it
is artistical and sufficient for any purpose. The present process, which I
use myself when I require a black colour, with its imperfections, I offer
to the photographic readers of "N. & Q.," and here it is.
Take a two-ounce vial, and have some powdered litharge of lead, by some
called gold or scale litharge; pound it fine in a Wedgewood mortar, and put
in the vial about one scruple; pour on it about half an ounce of Beaufoy's
acetic acid, but do not replace the cork or stopper, as the gas evolved is
very active, and will burst the vial, placing the operator's eyes in
jeopardy; agitate and allow it to stand some hours to settle, or leave it
till next day, when it will be better for the purpose: then decant the
clear part and throw the fæces away, return the solution into the bottle,
and fill up with distilled water. The positive paper being now prepared
with the ammonio-nitrate of silver, and placed as usual in the sun, the
artist must remove it when a tolerably distinct image is visible, but not
altogether up: this is _one_ of the niceties of the process; if it is too
much done the blacks will be too black, and if not enough they will be
feeble and want richness; it is when a visible image of the whole is
developed: at this point put the positive into cold water; this will remove
a great deal of the silver that has not been acted upon by the light: let
it soak three or four minutes; take it out and blot off the water, laying a
clean piece of paper below. Now pour a small quantity of the solution of
lead on one end, and with a glass rod pass it carefully over every part;
blot it off, and giving the paper a little time to dry partially, pass over
a solution of newly made gallic acid; the shadows will rapidly become
perfectly blank, and the picture will come up. But _another_ nicety in the
process is the point at which it must be plunged into hyposulphite of soda
solution; if plunged in too soon the black will be mingled with the sepia
tints, and if too late the whole tint will be too black. I offer it,
however, because I know its capabilities of improvement, and the intensity
of the black is sometimes beautiful: it is better suited for architectural
subjects, where there is but little sky, as it will lay a faint tint over
it; but if a sky is attempted, it must be kept under by a brush with a
little hyposulphite of soda solution, touching it {316} carefully. The time
it will take in becoming black will not exceed one minute; but as the
eyesight is the guide, the moment the tints have changed from red to black
is the proper time to arrest its further progress: the combination thus
obtained will not change, nor, I believe, become faint by time; but I
repeat it may be much improved, and if any abler hand, or one with better
means at his disposal, will take the trouble to examine its capabilities, I
shall be very thankful for his notes on the subject.
N.B. The solution of lead must contain acid; and if by keeping it does not
change litmus-paper, acid must be added till it does.