IMPROVEMENTS IN PREPARING PAPER FOR AND IN OBTAINING PHOTOGRAPHIC PROOFS OR IMPRESSIONS
Year: 1858

page_text: This invention received Provisional Protection, but notice to proceed with the
application for Letters Patent was not given within the time prescribed
by the Act.)
PROVISIONAL SPECIFICATION left by William Clark at the Office of the
Commissioners of Patents, with his Petition, on the 27th February 1858.
-A communication from M. Niepce de St. Victor, of Paris, France.
I, WILLIAM CLARK, of 53, Chancery Lane, in the County of Middlesex,
Engineer and Patent Agent, do hereby declare the nature of the said Invention
for "IMPROVEMENTS IN PREPARING PAPER FOR AND IN OBTAINING PHOTOGRAPHIC
PROOFS OR IMPRESSIONS," to be as follows:-
This process is based on the property that all bodies have of absorbing a
greater or less quantity of light. This new process of photography, which I
call "photography by absorption of light," consists in taking a sheet of paper
which has been kept in the dark during a certain number of days, and
immersing the same in a solution of salt of uranium, (this salt has the property
of absorbing a very large quantity of light,) but I prefer to use azotate of
oxide of uranium. This latter is produced either in treating oxide of uranium
with diluted azotic acid, or in dissolving in water crystals of azotate of oxide
of uranium in proportions of about ten per cent. The sheet of paper must be
impregnated with salt of oxide of uranium in.a sufficient quantity, that its tint
may be of a nice straw. yellow colour, and after it is dried it is to be kept in
the obscurity mentioned. Other salts of uranium will answer the purpose, and,
which I substitute for the bichromate of potassa usually employed in
photography

page_text: When it is desired to operate with this sheet of paper, it is covered by a
photographic negative impression or proof either on glass or on paper. This
sheet is then exposed to the action of the sun for about a quarter of an hour and
afterwards kept in the dark, the proof which covered it is withdrawn, and the
sheet is treated with a solution containing about six per cent. of azotate of
silver. The operator then sees the appearance of a very distinct positive
image of the chestnut coloured tint of the ordinary proofs. In order to fix
this image it suffices to immerse it in pure water which dissolves all that part
of salt of oxyd of uranium which by reason of the dark parts of the negative
proof have not received the action of light, after which the image or impres-
sion is fixed.
If, after having well washed the proof with pure water it is desired to
transform it into a black tint, the said proof must be treated with a solution of
commercial chloride of gold in proportions of about two-tenths of a per cent.,
and then washed again with pure water. All these operations do not require
more than half an hour, after which the photographic image is entirely
finished.
The proof obtained with azotate of silver may also be transformed into a
black tint by using the two following processes:-They consist in passing the
sheet of paper after it has been impregnated with salt of oxide of uranium, and
exposed to the sun, in a solution of bichloride of mercury, and in which it is
left only a few minutes according to the length of time it is exhibited to the
light, which time must be three times longer than in the first process above
specified. After the proof has been washed in pure water, it is introduced in
the solution of azotate of silver, and left in the same till the image is perfectly
obtained with a black tint like ink; it is afterwards washed again with pure
water, and then the proof is fixed. After the "passage of the proof in the
bichloride of mercury, the solution of azotate of silver may be replaced by a
solution of chloride of gold, which latter will give the proof a blue black tint
or colour.
After the sheet of paper impregnated with salt of oxide of uranium has been
exposed to the light, the image may be instantaneously obtained by treating
the said sheet with a solution of commercial chloride of gold; in this case
the proof has a very dark blue tint; it is at last washed with pure water, after
which the proof or image is fixed.
These photographic images being obtained, as before mentioned, with a
salt of uranium combined with a salt of gold or of silver and of mercury, are
capable of resisting and are not liable to be effaced by the energetic action of
a boiling solution of potassium; aqua regia alone alters them.

page_text: As no sulphur exists in these impressions or images in contradistinction to
those obtained with chloride of silver, it appears that such images will be
much more stable than the photographs obtained by the ordinary processes,
and that this new mode of obtaining positive proofs, which is very simple and
very rapid, is the desideratum long sought for in photography.
The solution of azotate of uranium may be replaced by a solution of tartric
acid, or of citric acid, or of oxalic acid, or of sulphate of alumina, or of citrate
of iron, or of arsenious acid, or of neutral tartrate of potash, and of lactic
acid, all the above substances having much the same property as the salt of
uranium, but they will not produce an indelible impression, as does the salt
of uranium.
Negative proofs or impressions may be produced by placing in the dark a
sheet of paper impregnated with salt of uranium, but, as this process is of
long duration, it would serve only for obtaining views of inanimate objects.
Very fine and beautiful negative proofs are obtained in the dark room by
putting on a sheet of glass a solution of azotate of uranium mixed with gelatine,
and preferably in gum diluted. A salt of uranium mixed with gelatine or
gum give to these matters the property of being insoluble like the bichromate
of potash when these substances have been exposed to the light, which would
allow of substituting them instead of the bichromate of potash in the processes
of engraving on steel or of lithophotography on stone.
Positive proofs on glass for the stereoscope may also be obtained by replacing
the albumen by a coating of gum containing azotate of oxide of uranium, and
in developing or producing the image, either with chloride of gold or with
azotate of silver. For attaining this result the subjection or exhibition to the
light must be sufficiently long to allow the instantaneous development of the
impressions or images, and in order that the gum shall have no time to dissolve
where it is not acted on by the light.
This process has the advantage of giving the proofs blue, red, black, or
chestnut-coloured tints, according to the combination of the salts of gold and
of silver.