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Towler, John. The Silver Sunbeam.
Joseph H. Ladd, New York: 1864. Electronic edition prepared from
facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson,
New York. Second printing, Feb. 1974. ISBN 871000-005-9
Chapter XVIII.
INTENSIFIERS.
INTENSIFIERS are substances which, when applied in solution to
the developed image, increase the opacity of the shadows and middle
tints, rendering them more impermeable to light in direct positive
printing. With a proper adjustment of light and developer, and
especially in ordinary landscape-photography, an intensifier is
seldom needed; but many artists prefer the use of the intensifier on
every occasion; they maintain that a negative can always be
preserved as clear and transparent in the lights as a positive lay
this process, and yet the density of the shadows may be increased to
any extent without any fear of fogging. The intensifying process
becomes, therefore, a fixed part in the preparation of a negative.
The operation is partly physical and partly chemical; physical,
because whatever may have been the action of the light on those
parts in which the, image is now apparent, they seem still to be
endowed with properties of attraction of an intensity in proportion
to the development produced, just as they were at the commencement
of reduction; but the nitrate of silver, iodide; or bromide of
silver, having been exhausted, the application of any developer,
however sensitive or intense, could produce no more opacity on the
shadows for want of material to be reduced--but, mark it well, the
physical condition is there to institute this reduction the moment
material is supplied.
From my preceding remarks it is supposed that the developed image
consists of reduced silver, or in altered salt of silver very
different from any with which we are acquainted; there is no more
iodide or nitrate of silver; these have been removed
in the fining and washing Now in order to restore the partially
developed image to the chemical condition requisite for the
recommencement of the development, a solution of iodine in iodide or
potassium, or a dilute solution of tincture of iodine, is flowed
over the plate, and kept in motion over the image in order to
preserve uniformity of action. The iodine thus coming in contact
with the silver shallows enters into combination with this metal,
and forms a new and thicker deposit of iodide of silver with all the
gradations of opacity of the image, and not a uniform film of
deposit. The solution of iodine on the collodion loses color all the
while; but the collodion film assumes at first a grayish and then a
yellowish-gray hue. Even at this stage there is much more opacity in
the shadows of the picture than before, and the negative by this
proceeding may probably ire dense enough; if not, proceed to the
second stage. The first stage is the depositing stage; the
second, the reducing or developing stage proper; and yet this
deposit of the first stage is a chemical combination of iodine and
silver which is now soluble in the fixing solutions, and before it
was not. By this process of depositing and fixing, and by regulating
the quantity of the iodine solution, a negative which is too opaque
may be rendered more transparent and less dente ad libitum.
Osborne has availed himself of this property to clarify his
negatives for the photolithographic process; I would recommend it
also in the preparation of clear and sharp negatives for obtaining
enlarged positives in the solar camera. As soon as the depositing
stage is complete, and the film leas been washed, the collodion film
is ready for the reception of the next operation.
The second stage consists in communicating to the iodized image a
minute quantity of nitrate of silver, either alone and diluted, or
in connection with the developer; it is, in fact, a mere repetition
of the original process of development; the surface of the collodion
is in the same condition as at the commencement when it left the
camera; there are present iodide of silver, nitrate of silver,
iodide of potassium, the peculiar and unknown physical attraction
existing in the formed image where before the image as yet was
unformed, and the developing solution either of sulphate of iron or
pyrogallic acid. The second stage is then a system or process of
redevelopment. By this operation the intensity may be increased to
any extent; the shadows can be made quite opaque and utterly
impermeable to the actinic influence. The intensifying part of the
collodion process is very much in the power of the artist; success,
therefore, will depend principally on the artistic condition of what
I denominate the Foundation Negative If the foundation
negative, however thin the shadows may be, contain light, shades,
and middle tones in perfect detail, then the artist has it in his
power to raise these three conditions gradually and uniformly
higher, until the shadows become endowed with a proper opacity. At
the end of this stage fixing solutions have but little effect, which
seems to demonstrate that the yellowish-gray iodide has been
converted into an insoluble metallic film or an unknown insoluble
silver salt. It is not necessary to use the fixing solution. All
that is required is to wash the image well before it is dried and
varnished.
Other deposits and other metals may be introduced in the
intensifying operations, which will be found described below.
From the recent experiments and observations of Blanquart
Evrard,* it appears that a
negative may be intensified by a second exposure to light before
fixing. Thus, supposing a negative be developed as far as it seems
possible to carry on the reduction, in this condition let it be
exposed for a short time to diffused light. This physical force, it
is said, again acts actinically, but now only upon the parts which
contain the image, communicating to these new vigor, and a fresh
impulse, which, on the application of the developer, again will
assist in the formation of further reduction.
As soon as the image has been fixed, as in the first example, it
is sometimes flowed with a saturated solution of bichloride of
mercury, by which probably the bichloride is reduced to the
protochloride, and the liberated chlorine goes over to the silver,
and forms chloride of silver. This application communicates a
whiteness to the image, and thickens the deposit. When the negative
has been washed, it is flowed with an iodizing solution, containing
five per cent of iodide of ammonium in water. In this way the image
becomes converted into a double iodide of silver and mercury, which,
when washed, is treated with the iron or pyrogallic developer,
containing a few drops of nitrate of silver, as before. It
frequently happens in this, as in the preceding case, that the film
at the end of the first stage is opaque enough. In this case it
maybe rendered black by flowing it with ammonia, hyposulphite of
soda, or cyanide of potassium.
A third method of strengthening the dark parts of a negative
takes advantage of the alkaline sulphides, which convert the
developed film into a sulphide. By this operation, however, the film
as a rule is not increased in thickness, its color alone being
changed, which is frequently more agreeable to look at, and
apparently more dense, because it is black, or bluish-black. These
alkaline sulphides may be used with advantage at the end of the
first stage or deposit, in order to blacken this deposit; but by
this mode of intensifying there is a great liability to unequal
action, to decomposition after the negative is varnished, to
contraction of the collodion film, and its separation from the
glass; besides this, sulphur seems to be precipitated sometimes in
very irregular patches, giving a speckled appearance to the
negative.
Preparation of Bichloride of
Mercury--Corrosive Sublimate.
Symbol, Hg. Cl. Combining Proportion, 136.9. Spec.
grav., 5.4.
Dissolve red oxide of mercury in hydrochloric acid; evaporate and
crystallize; or sublime a mixture of equal weights of sulphate of
mercury and common salt in a stoneware retort by heating to redness
in a sand-bath. The bichloride, being volatile, passes out, whilst
sulphate of soda remains behind in the retort. This substance melts
at 509°, and boils at 563°; it dissolves in twenty parts
of cold water, in two parts of boiling water, in two and one third
of cold alcohol, and in three of cold ether. When hydrosulphuric
acid is passed through a solution of this salt, a brownish
precipitate is first formed, which eventually becomes quite white.
This is a chlorosulphide.
Preparation of Sulphide of
Potassium.--Hepar Sulphuris.
Symbol, K S3.
Fuse together, at a low red heat, one part of sulphur, and two of
carbonate of potash, as long as effervescence takes place; then pour
on to a marble slab, and when cool, break up the mass, and keep it
in well-closed bottles. This sulphide has a liver-brown appearance.
By the addition of an acid to a solution of the sulphide,
hydrosulphuric acid is liberated, a soluble salt formed, and sulphur
precipitated of a milk-white color. The alkaline sulphides have the
same reaction on metallic salts as hydrosulphuric acid, forming
precipitates of different colors, by which frequently the metals can
be recognized, as, for instance, antimony, cadmium, etc.
Preparation of Sulphide o f
Ammonium.
Symbol, N H4 S. H S.
Let a current of hydrosulphuric acid pass through concentrated
ammonia to saturation; then add an equal bulk of ammonia. This is
one of the most important reagents in chemistry. Hydrosulphuric acid
produces precipitates in metallic salts, Some of which are soluble
in sulphide of ammonium and others not; from this fact we can
distinguish one metal from another, thus the sulphide of arsenic is
yellow, and so is that of cadmium; but the former is soluble in
sulphide of ammonium, the latter is insoluble. The alkaline
sulphides precipitate silver black from its solutions; thus nitrate
of silver, as a dye for the hair, is turned of an intense black if
followed up with sulphide of ammonium.
Notes
* Vide Humphrey's Journal.
Vol. XV. No. 1.
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