Perhaps of all the various arts discovered by man, this is the
most beautiful, producing a perfect likeness of any being or object
in a few moments, and with a truth not to be equalled by years of
study and practice in other ways. One of the earliest discovered
and simplest modes of forming a sun picture consists in using
the object itself (where this is possible) to form the negative.
This can only be done when the object is flat, as a manuscript, a
picture, a leaf, or a piece of lace-work; a piece of paper washed
over with a solution of common salt, dried, and again washed over
with a solution of nitrate of silver, will answer for this purpose;
it should be prepared by candle-light, as daylight blackens it, and
it should be kept from the light till the moment for use. If this
paper, having the object to be copied placed on it and kept flat by
a piece of glass pressed firmly on by a contrivance shown in fig. 4,
be exposed to the sunshine, it will be blackened in all places but
those kept from the light by the darker parts of the object, and the
lights and shadows will vary accordingly as the various parts of the
object itself are more or less transparent. When the effect has been
produced to a sufficient degree, the paper should be removed and
at once washed in a solution of hyposulphite of soda, which removes
all the superfluous silver (which otherwise would be darkened by the
light); when removed from the hyposulphite of soda it should be well
soaked in clean water. This when finished is a true negative picture
of the object, and can be used to produce the positive. The negative
should first be made partly transparent by brushing it over with some
melted white wax, then placing it between folds of blotting paper
and passing a hot iron over it, the blotting paper will absorb all
the superfluous wax, and the negative will be fit for use. A piece
of paper prepared like the first with salt and nitrate of silver,
should be placed against the front of the negative, kept smooth by
a piece of glass, and exposed to the sunshine for a quarter of an
hour or more, taking care that the back of the negative is exposed
to the light and the prepared paper behind; a positive picture
will be produced, which is to be at once treated with a solution
of hyposulphite of soda and washed with clean water, as was the
negative. Any number of positive impressions may thus be produced.
The “Calotype” or “Talbotype” process may be performed as follows,
recollecting that all these processes must be conducted by
candlelight, or in a room having the window covered with yellow
silk, or some other substance excluding white light. Select a piece
of paper of an even surface and structure, such as Whatman’s paper,
brush it over with a solution of nitrate of silver--about sixteen
grains to an ounce of water--and let it dry, then fold up the edge
of the paper all round and dip the surface to which the solution
of nitrate of silver has been applied into a solution of iodide
of potassium--an ounce to a pint of water--contained in a shallow
vessel, and let it partly dry; float it for a quarter of an hour in
a vessel of clean water, moving it gently about so as to remove all
soluble matter, and hang it up by a corner to dry, taking care to
let nothing touch the surface. When about to be used, this paper must
be brushed over (using a soft camel-hair brush) with a mixture, in
equal quantities, of a solution of nitrate of silver--fifty grains to
an ounce of water--and a saturated solution of gallic acid, forming
what is called the “gallo-nitrate” of silver. Leave this solution on
the surface for ten minutes, and again immerse the paper in two or
three successive portions of pure water, moving it to and fro; this,
when nearly dry, is in a fit state to place in the camera, and is an
exceedingly sensitive paper, which in a good light will be affected
in a few seconds. When the image has been impressed by the camera,
the paper should again be brushed over with the gallo-nitrate of
silver, and held either near a fire or a plate of hot iron, so as
to receive a gentle warmth; the impression will now come out, of
a deep rich colour. Dip it into warm water for a few minutes, and
put it into the hyposulphite of soda bath--an ounce to a quart of
water--from which it is finally removed and placed in a large vessel
of pure water for several hours. When dry, it is complete, forming a
good negative, which, when waxed, may be used to print positives as
before described.
The “Collodion” process is conducted as follows. Having selected a
plate of glass of the required size and of good quality (patent plate
answers well), clean it thoroughly with rottenstone and spirits of
wine, and polish it with a clean dry linen cloth; then hold it on
the tips of the fingers and thumb in a horizontal position, pour
upon it a small quantity of the “iodized collodion,” and so incline
it from side to side, that all the surface shall be covered with the
collodion, that which is superfluous being returned to the bottle
by pouring it from one corner of the glass. In a few minutes the
coating of collodion will be dry, and should form a perfectly even
surface, without air spots or uneven markings. The iodized collodion
is made by dissolving gun-cotton in ether, and adding spirit of wine
and iodide of ammonia. The glass plate coated with collodion is now
dipped into a solution of nitrate of silver--thirty grains to an
ounce of water--and allowed to remain two or three minutes; when all
the superfluous fluid has drained off, the plate is ready for use,
and is extremely sensitive to light. When the impression has been
produced by the camera, a solution of pyro-gallic acid, mixed with
a little acetic acid, is poured over the surface, which brings up
the image; when this is developed to a proper extent, the surface
must be washed with clean water and afterwards with a solution of
hyposulphite of soda, which dissolves out all the silver from those
parts not darkened by the light in the camera, leaving the glass in
those parts quite transparent. A negative is thus obtained fit to
print from in the usual way, but glass being more transparent than
the waxed paper, it produces a more perfect positive. The surface
thus prepared should be coated with a varnish, made by dissolving
amber in chloroform, which is done by pouring it on and decanting the
superfluous varnish similarly to the mode of coating with collodion;
this varnish dries in a few minutes, and preserves the surface from
injury. Sometimes these negatives are converted into positives,
instead of being used to print from; in these cases the surface is
coated with black japan, and the collodion, having a whitish surface,
owing to reduced silver, shows up in comparison with the black
varnish which is seen through the transparent parts. These positives
are extremely sharp and well defined.
The “Daguerreotype” process consists in forming the image on the
surface of a metallic plate (copper coated with silver), which
is first polished to a perfect surface like a looking-glass, then
exposed to the vapour of iodine in a properly-constructed box,
afterwards to the vapour of chloride of bromine, and again to the
vapour of iodine; this produces a thin film of a mixture of bromide
and iodide of silver, which covers the surface of the plate, and is
very sensitive to light, which reduces the iodide and bromide to the
metallic state in a few seconds. The plate is then exposed to the
action of the light in the camera, and when sufficiently affected,
exposed to the vapour of mercury, the mercury being put into a box,
and heated by a spirit lamp; the mercurial vapour adheres to all the
parts of the silver reduced by the light, and forms a light-coloured
amalgam, corresponding to the lights of the objects represented. The
plate is then washed with a solution of hyposulphite of soda, which
removes all the bromide and iodide of silver not reduced by the
light, and exposes the highly-polished surface of the silver, which
forms the shadows of the picture; this is now “fixed” with a very
weak solution of chloride of gold and hyposulphite of soda, which is
poured over the surface and heated while there by applying the flame
of the spirit lamp to the back of the plate for a minute or two; the
solution is then poured off, and the picture when washed with water
is complete.