organic matter with a low Oxide of Silver; and analysis indicates further
that the relative proportion of each constituent in these compounds may
vary. For instance, when Citrate of Silver is reduced by light, and acted
on with Ammonia, a black powder remains, which was found to contain as
much as 95 per cent, real Silver; but Albuminate of Silver treated in the
same way yields on analysis less of metallic Silver, and more volatile and
carbonaceous matter.
The use of _Ammonio_-Nitrate of Silver in preparing the salt tends also
to increase the relative quantity of metal left in the compound after
reduction and fixing. The length of time during which the light has acted,
has also a modifying effect of the same kind,--the product of reduction by
a powerful light being more nearly in the state of metal, and containing
less both of Oxygen and organic matter.
_Action of Light upon Chloride of Silver associated with organic
matter._--Photographs formed on Chloride of Silver alone, would, after
fixing, consist of metallic Silver, but such a process could not be
carried out in practice. The addition of organic matter is absolutely
necessary in order to increase the sensitiveness, and to prevent the
image from being dissolved in the Bath of Hyposulphite of Soda. The blue
Subchloride of Silver is decomposed by fixing, a very scanty proportion of
grey metallic Silver remaining insoluble; but the red compound of Suboxide
of Silver with organic matter is almost unaffected by Hyposulphite of
Soda, or Ammonia.
The increase of sensitiveness and intensity produced by the use of organic
matter is accompanied also by a change in the composition of the picture;
the image losing the metallic character which it possesses when formed on
pure Chloride of Silver, and resembling in every respect the product of
the action of light upon organic Salts of Silver.
There are certain characteristic tests which may usefully be employed in
distinguishing the metallic image from what may be termed the organic or
non-metallic image. One of these tests is Cyanide of Potassium. An image
formed upon pure Chloride of Silver, although pale and feeble, may, after
fixing, be immersed ill dilute solution of Cyanide of Potassium without
injury. But a photograph on Chloride of Silver supported by an organic
basis, is much acted upon by Cyanide of Potassium, quickly losing its
finer details.
A second test is the Hydrosulphate of Ammonia. If no organic matter be
employed, the image becomes darker and more intense by treatment with a
soluble Sulphuret; whilst the non-metallic image, formed on an organic
surface, is quickly bleached and faded. The action of Sulphur upon the
image is indeed a mode of determining the real quantity of Silver present.
When existing in a very finely divided layer, Sulphuret of Silver often
appears yellow; but in a thicker layer it is black. Hence the colour
of the Photograph, after treatment with Sulphuretted Hydrogen, is an
indication of the proportion of metal present, and the reason of the
organic image becoming so perfectly faded is because it contains a minimum
of Silver in relation to the intensity. We see, therefore, that the
addition of organic matter to Chloride of Silver does not so much increase
the actual quantity of Silver reduced by light, as it adds to its opacity
by associating other elements with the Silver, and altogether modifying
the composition of the image.
The employment of _oxidizing agents_ shows also that in an ordinary
Photographic process by the direct action of light, other elements besides
Silver assist in forming the image: the pictures being found to be easily
susceptible of oxidation, whereas the metallic image formed on pure
Chloride of Silver resists oxidation.
_Composition of_ developed _images._--By exposing sensitive
layers of the Iodide, the Bromide, and the Chloride of Silver to the light
for a short time only, and subsequently developing with Gallic Acid,
Pyrogallic Acid, and the protosalts of Iron, a variety of images may be
obtained, which differ from each other materially in every important
particular, and a comparison of which assists the determination of the
disputed point.
The appearance and properties of the developed Photograph are found to
vary with the existence of the following conditions.
1st. _The surface used to sustain the sensitive layer._--There is a
peculiarity in the image formed on _Collodion_. Collodion contains
Pyroxyline, a substance which behaves towards the salts of Silver in a
manner different from that of most organic bodies, exhibiting no tendency
to assist their reduction by light. Hence Chloride of Silver on Collodion
darkens far more slowly than the same salt upon Albumen, and the image,
after fixing, is feeble and metallic. Iodide of Silver on Collodion,
exposed and developed, gives usually a more metallic image, with less
intensity, than Iodide of Silver upon Albumen, or on paper sized with
Gelatine. By adding to the Collodion a body which has an affinity for low
oxides of Silver, such for instance as Glycyrrhizine, the opacity of the
developed image is increased.
2nd. _The nature of the sensitive salt._--When Iodide of Silver is used
to receive the latent impression, the image after development, although
lacking intensity of colour by reflected light, is more nearly in the
condition of metallic Silver than if Bromide or Chloride of Silver
be substituted; and of the three salts, the Chloride gives the most
intensity, with the least quantity of metallic Silver. This rule applies
especially when organic matters, Gelatine, Glycyrrhizine, etc., are
present.
3rd. _The developing agent employed._--An organic developing agent like
Pyrogallic Acid may be expected to produce a Collodion image more intense,
but less metallic, than an inorganic developer, such as the Protosulphate
of Iron.
4th. _The length of time during which the light has acted._--Over-action
of the light favours the production of an image which is dark by
reflection and brown or red by transmission, corresponding in these
particulars to what may be termed the non-metallic image containing an
oxide of Silver.
5th. _The stage of the development._--The red image first formed on the
application of the developer to a gelatinized or albuminized surface of
Iodide of Silver is less metallic, and more easily injured by destructive
tests, than the black image, which is the result of prolonging the action.
Developed photographs which are of a bright red colour after fixing,
correspond in properties to images obtained by the direct action of light
on paper prepared with Chloride of Silver, more nearly than to Collodion,
or even to fully developed Talbotype Negatives.
To conclude the Paper, the following may be offered in the way of
recapitulation:--An image consisting of metallic silver, as a rule,
reflects white light, and shows as a positive when laid on black velvet;
but a non-metallic organic image is dark, and represents the shadows of a
picture. Collodion positives developed with protosalts of Iron are nearly
or quite metallic. Photographs on Albumen or Gelatine less so than those
on Collodion. Developed Photographs contain more Silver than others, if
the development has been prolonged. The half shadows of the image in a
Positive Print are especially liable to suffer under injurious conditions,
since they contain the Silver in a less perfect state of reduction.[24]
[24] The Author omits, in this place, all mention of molecular conditions
affecting intensity, inasmuch as at the present time nothing positive
has been determined with regard to them. It is however known that in the
use of the protosalts of Iron as developing agents, the appearance of
the image is much influenced by the rapidity with which the reduction is
effected--the particles of Silver being larger and more metallic when the
development is conducted slowly. The process of electro-plating and other
chemical operations of a similar kind prove that the physical properties
of metals precipitated from solutions of their salts, vary greatly with
the degree of fineness and arrangement of their particles.
ON THE VARIOUS AGENCIES DESTRUCTIVE TO PHOTOGRAPHIC PRINTS.
_Action of Sulphuretting Compounds upon Positive Prints._--It was first
noticed by Mr. T. A. Malone, that the most intense Photograph might be
destroyed by acting upon it with solution of Sulphuretted Hydrogen or a
soluble Sulphuret, for a sufficient length of time.
The changes produced by a sulphuretting compound acting upon the red image
of a simply fixed print are these:--the colour is first darkened, and a
degree of brilliancy imparted to it; this is the effect termed "toning."
Then the warm tint by degrees alters to a colder shade, the _intensity_ of
the whole image is lessened, and the half-tones turn yellow. Lastly, the
full shadows pass also from black to yellow, and the print fades.
Now in this peculiar reaction we notice the following points of interest.
If at that particular stage at which the print has reached its maximum of
blackness, it be raised partially out of the liquid and allowed to project
into the air, the part so treated becomes yellow before that which remains
immersed. Again, if a print toned by Sulphur be placed in a pan of water
to wash, after the lapse of several hours it is apt to assume a faded
appearance in the half-tones. The full shadows, in which the reduced
Silver salt is thicker and more abundant, retain their black colour for
a longer time, but if the action of the sulphuretting Bath be continued,
every portion of the print becomes yellow.
These facts prove that _Oxygen_ has an influence in accelerating the
destructive action of the Sulphur compounds upon Positive prints; and this
idea is borne out by the results of further experiments, for it is found
that moist Sulphuretted Hydrogen has little or no effect in darkening the
colour when every trace of air is excluded. When prints are washed in
water they are exposed to the influence of the dissolved air which water
always contains, and hence the change from black to yellow is produced.[25]
[25] Further remarks upon the action of damp air upon Positives toned by
Sulphur are given at p. 153.
There are some substances which facilitate the yellow degeneration of
Positives toned by Sulphur, a knowledge of which will be useful: they
are--1st, powerful oxidizers, such as Chlorine, Permanganate of Potash,
and Chromic Acid; these, even when highly diluted, act with great
rapidity: 2nd, bodies which dissolve Oxide of Silver, as soluble Cyanides,
Hyposulphites, Ammonia; also _acids_ of various kinds, and hence the
frequency of yellow finger impressions upon old sulphuretted prints, which
are probably caused by a trace of organic (Lactic?) Acid left by contact
with the warm hand.
It was at one time supposed that the Photograph in the stage at which it
appears _blackened_ by Sulphur, consisted of Sulphuret of Silver, and that
this black Sulphuret became yellow by absorption of Oxygen and conversion
into Sulphate. MM. Davanne and Girard, who examined the subject, thought
that there might be two isomeric forms of Sulphuret of Silver, a black
and a yellow form; the former of which passing gradually into the latter
produced the fading of the impression. But neither of these views are
correct; for it is proved by careful experiment, that the Sulphuret of
Silver is a highly stable compound, not prone to oxidize, and, further,
that the change of colour from black to yellow has no reference to a
modification of this salt. The truth appears to be that the image whilst
in the black stage contains other elements besides Sulphur and Silver, but
when it has become yellow by the continued action of the sulphuretting
compound, it is then a true Sulphuret.
_Comparative permanence of Photographs under the action of
Sulphur._--_Developed_ Positives, as a rule, stand better than those
printed by direct exposure to light; but much depends upon the nature of
the negative process followed; and hence no general statement can be made
which will not be liable to many exceptions. The mode of conducting the
development must not be overlooked. The prints, which become very red in
the Hyposulphite fixing Bath from the action of the developer having been
stopped at too early a period, are often sulphuretted and destroyed even
more readily than a vigorous sun-print obtained by direct exposure to
light.
A point of even greater importance is _the nature of the sensitive
surface_ which receives the latent image. It is the print _developed upon
Iodide of Silver_ which especially resists sulphuration. In that case,
not only is the preliminary toning effect of the Sulphur more slow than
usual, but the impression cannot be made to fade by any continuance of the
action. It loses much of its brilliancy, and is reduced in intensity, but
it is not so completely destroyed as to be useless. The reason of this, as
shown in the last paper, depends upon the fact that the Talbotype proofs
contain the largest amount of Silver in the image.
The employment of Gold in toning does not render an ordinary sun-print
as permanent as a Positive developed upon Iodide of Silver. The deep
shadows of the picture are protected by the Gold, but the lighter shades
not so perfectly. Hence after the Sulphur has acted, in place of the
universal yellow and faded aspect presented by the simple untoned print,
the Positive fully toned by Gold has black shadows with yellow half-tones.
Therefore, whilst recommending the use of Gold as a toning agent, it does
not seem advisable to lay too much stress upon it as a preservative from
the destructive action of Sulphur.
_Exposure of Positive Prints to a Sulphuretting Atmosphere._--In testing
the action of a solution of Sulphuretted Hydrogen upon paper Positives,
it did not appear that the conditions under which the prints were placed
bore a sufficiently close resemblance to the case of Positives exposed
to an atmosphere contaminated with _minute traces_ of the gas; and this
more particularly because it is known that _dry_ Sulphuretted Hydrogen has
comparatively little effect upon Photographic Prints.
The experiments were therefore repeated in a somewhat different form.
A number of Positives (about three dozen) printed in various ways,
were suspended in a glass case, measuring 2-1/2 feet by 21 inches,
and containing 7-1/2 cubic feet of air; into which was introduced,
occasionally, a few bubbles of Sulphuretted Hydrogen, just sufficient to
keep the air of the chamber smelling perceptibly of the gas. A polished
Daguerreotype plate was hung up in the centre, to serve as a guide to the
progress of the sulphuretting action.
By the second day the metal plate had acquired a faint yellow hue,
not easily seen except in certain positions; but the Positives were
unaffected. At the expiration of three days the majority of the pictures
exhibited no signs of change, but a few untoned prints of a pale red
colour, some of which had been printed by development, and others by
direct exposure to light, had perceptibly darkened.
After the eighth day, the action, appearing to progress more slowly
than at first, was stopped, and the prints removed. The general results
obtained were as follows:--
The Daguerreotype plate had become strongly tarnished with a film of
Sulphuret of Silver, which appeared yellowish-brown in some parts and
steel-blue in others. The Positives were, as a rule, toned to a slightly
colder shade, but many of them had scarcely changed.
No obvious difference was observed between prints developed on paper
prepared with Chloride of Silver, and others printed by direct exposure to
light; but in all cases the prints obtained by those methods which give a
very red image after fixing, were the first to show the change of colour
due to sulphuration, the proofs submitted to the test having all been
previously toned with Gold.
Effect of Oxidizing Agents upon Positive Prints.--It appeared of
importance to ascertain to what extent Photographic Prints are susceptible
of oxidation; on account of the atmospheric influences to which they are
necessarily exposed. In experimenting upon this subject the following
results have been obtained.
Powerful oxidizers destroy Positive Prints rapidly; the action usually
commencing at the corners and edges of the paper, or at any isolated
point, such as a metallic speck or particle of extraneous matter, which
can serve as a centre of chemical action. This same fact is often noticed
in the fading of Positives by long keeping, and therefore since other
destructive actions (with the exception of that of Chlorine) do not appear
to follow the same rule, it is an argument in addition to others which can
be adduced, that Photographic Prints are frequently destroyed by oxidation.
Air which has been _Ozonized_ by Phosphorus, and in which blue
litmus-paper becomes reddened, quickly bleaches the Positive image. Oxygen
gas, obtained by voltaic decomposition of acidified water and which should
contain Ozone, did not appear to have an equal amount of effect, the
action being comparatively slight, or altogether wanting.
_Peroxide of Hydrogen_ obtained in solution, and in conjunction with
Acetate of Baryta, by adding Peroxide of Barium to dilute Acetic Acid,[26]
bleaches darkened Positive paper; but the effect is slow, and does not
take place to a very perceptible extent if the liquid be kept alkaline to
test-paper.
[26] Hydrochloric Acid, which is usually recommended in place of Acetic
Acid, cannot be employed in this experiment; it seems to cause a
liberation of free Chlorine, which bleaches the print instantly.
Nitric Acid applied in a concentrated form acts immediately upon the
darkened surface, bleaching every part of the print with the exception
of the bronzed shadows, which usually retain a slight residual colour. A
solution of Chromic Acid is still more active. This liquid may usefully be
applied to distinguish prints toned by Sulphur from others toned by Gold;
the presence of metallic Gold protecting the shadows of the picture in
some measure from the action of the acid. The solution should be prepared
as follows:--
Bichromate of Potash 6 grains.
Strong Sulphuric Acid 4 minims.
Water 12 ounces.
A solution of Permanganate of Potash is an energetic destroyer of paper
positives; and, as it is a neutral substance, may conveniently be employed
in testing the relative capability of withstanding oxidation possessed
by different Photographic Prints. The solution should be dilute, of a
pale pink hue, and the Positives must be moved occasionally, as the first
effect is to decolorize a great portion of the liquid, the Permanganate
oxidizing the size and organic tissue of the paper. After an immersion
of twenty minutes to half an hour, varying with the degree of dilution,
the half-tones of the picture begin to die out, and the full shadows
become darker in colour; the bronzed portions of the print withstand the
action longer, but at length the whole is changed to a yellow image much
resembling in appearance the Photograph faded by Sulphur.
_Comparative permanence of Photographs treated with Permanganate of
Potash._--Developed prints prepared by a Negative process withstand the
action better than others. But to this rule there are exceptions; much
depending upon the time of exposure to light, and the extent to which
the development is carried. Those prints which, being exposed for a
short time, and afterwards strongly developed, become dark in colour and
vigorous in outline, are more permanent than others which having been
over-exposed and under-developed, lose their dark colour and become red
and comparatively faint in the Hyposulphite fixing Bath.
Positives developed upon a surface of _Chloride_ of Silver on plain paper
do not resist the oxidizing action so perfectly as those on Iodide of
Silver. Prints developed upon paper prepared with Serum of Milk containing
Caseine stand better than those on plain paper.
Of prints obtained by the ordinary process of direct exposure to light,
those on plain paper are the first to fade, the oxidizing action being
most seen upon the _half-tones_. The use of _Albumen_ gives a great
advantage. Developed prints on Albumen stand far better than the same
upon plain paper; and even the Albuminized sun prints are less injured by
the Permanganate than the best of the Negative prints prepared without
Albumen. Caseine has the same effect, but to a less extent; and as Serum
of Milk almost invariably contains uncoagulated Caseine, its efficacy is
thus explained.
The manner of toning the print is a point of importance; previous
sulphuration in an old Hyposulphite Bath always facilitating the oxidizing
action.
_Action of Chlorine upon Positive Prints._--Aqueous solution of Chlorine
destroys the Photographic image, changing it first to a violet tint
(probably Subchloride), and subsequently obliterating it by conversion
into white Chloride of Silver. The impression, although invisible,
remains in the paper, and may be developed in the form of yellow or brown
Sulphuret of Silver by the action of Sulphuretted Hydrogen. It also
becomes visible on exposure to light, and assumes considerable intensity
if the paper be previously brushed with free Nitrate of Silver. Sulphate
of Iron produces no effect upon the invisible image of Chloride of Silver;
but Gallic or Pyrogallic Acid, rendered alkaline by Potash, converts it
into a black deposit.
The Action of Chlorine water usually commences at the edges and corners of
the print, in the same manner as that of oxidizing agents. The proofs upon
Albumen are the least readily injured, and next, those developed on Iodide
of Silver.
_Hydrochloric Acid._--The liquid acid of sp. gr. ·116, even when free from
Chlorine, acts immediately upon the half-tones of a positive print, and
destroys the full shadows in the course of a few hours; a slight residual
colour however usually remains in the darkest parts. The prints developed
on Iodide of Silver are the most permanent.
_Sulphuric, Acetic Acids, etc._--Acids of all kinds appear to exert an
injurious influence upon Positive prints, and especially so upon the
half-tones of the image, the effect varying with the strength of the acid
and the degree of dilution with water. Even a vegetable acid like Acetic
gradually darkens the colour and destroys partially or entirely the faint
outlines of the picture.
_Bichloride of Mercury._--The most important particulars relating to
the action of this test upon Photographs are well known. The image is
ultimately converted into a white powder, and hence, in the case of a
Positive print, it becomes invisible; immersion in Ammonia or Hyposulphite
of Soda however restores it in a form often resembling in tint the
original impression. A point worthy of note is the protective effect of a
deposit of Gold, which is very marked, the proof, after toning, resisting
the action of the Bichloride for comparatively a long time.
_Ammonia._--The effect of Ammonia upon a print is rather to _redden_ the
image than to destroy it; the half-tones become pale and faint, but they
do not disappear. Toning with Gold enables the proof to resist the action
of the strongest solution of Ammonia, and hence Ammonia may safely be
employed as a fixing agent after the use of the Sel d'or Bath.
_Hyposulphite of Soda._--A concentrated solution of Hyposulphite of Soda
exercises a gradual solvent action upon the image of Photographic Prints,
at the same time tending to communicate Sulphur and to darken the colour
of the impression. A faint yellow outline of Sulphuret of Silver usually
remains after the solution of the image is completed.
Developed prints of all kinds, but in particular the Talbotype proofs
upon Iodide of Silver, are less readily dissolved by Hyposulphite of Soda
than those obtained by the direct action of light. There is also a slight
difference between plain and Albuminized prints, which is in favour of the
former, the albuminized paper always losing somewhat more by immersion in
the Hyposulphite Bath than plain Chloride paper sensitized by Nitrate of
Silver.
_Cyanide of Potassium._--The solvent action of Cyanide of Potassium is
most energetic upon Photographs formed on paper. These images, whether
developed or not, do not withstand the test so well as the impressions on
Collodion. Albuminized proofs are also somewhat more easily affected than
prints on simple Chloride paper sensitized with Nitrate or Ammonio-Nitrate
of Silver.
_Heat, moist and dry._--Long-continued boiling in distilled water has
a reddening action upon Positive Prints. The image becomes at length
pale and faint, resembling a print treated with Ammonia before toning.
A deposit of Gold upon the image lessens, but does not altogether
neutralize, the effect of the hot water. If the boiling be long continued,
the violet-purple tone often imparted by the Gold invariably gives place
to a chocolate-brown, which appears to be the most permanent colour.
Prints _developed_ by Gallic Acid upon paper prepared with Serum of Milk
or with a Citrate, suffer as much as others obtained by direct action of
light. Ammonio-Nitrate prints on highly salted paper, which become nearly
black when toned with Gold, retain their original appearance the most
perfectly; a slight diminution of brightness being the only observable
difference after long boiling in water. Albumen proofs, and prints on
English papers, or foreign papers prepared with Serum of Milk, Citrates,
Tartrates, or any of those bodies which _redden_ the reduced Salt, are,
as a rule, rendered lighter in colour, and pass from purple to brown when
boiled in water.
Dry heat has an opposite effect to that of hot water, usually _darkening_
the colour of the image. On exposing a plain paper print simply fixed, and
thoroughly freed from Hyposulphite of Soda by washing, to a current of
heated air, it changes gradually from red to dark brown, in which state
it continues until the temperature rises to the point at which the paper
begins to char, when it resumes its original red tone, becoming at the
same time faint and indistinct.
_The Products of Combustion of Coal-gas a cause of Fading._--Coal-gas
contains Sulphur compounds, which in combustion are oxidized into
Sulphurous and Sulphuric Acids; other substances of a deleterious nature
may also be present. A plate of polished silver suspended in a glass tube,
through which was directed the current of heated air rising from a small
gas jet, became tarnished with a white film in the course of twenty-four
hours. Positive prints exposed to the same, absorbed moisture and faded;
the action resembling that of oxidation, in being preceded by a general
darkening in colour. Of four prints exposed, an Iodide-developed print was
the least injured, and next, a print upon Albuminized paper.
ON THE ACTION OF DAMP AIR UPON POSITIVE PRINTS.
In order to ascertain this point, more than six dozen Positives, printed
on every variety of paper, were mounted in new and perfectly clean
stoppered glass bottles, at the bottom of each of which was placed a
little distilled water, to keep the contained air always moist. They were
removed at the expiration of three months, having been kept during that
time, some in the dark, and others exposed to the light. As the prints
were prepared by various methods, toned in different ways, and mounted
with or without substances likely to exercise a deleterious action, this
series of experiments will possess considerable value in determining some
of the intrinsic causes of fading of Positives.[27]
[27] For a more detailed account of the experiments, see the original
paper in the 'Photographic Journal,' vol. iii.
The general results obtained were as follows:--Positives which had been
_simply fixed_ in Hyposulphite of Soda remained quite uninjured. Whether
developed by Gallic Acid on either of the three Salts of Silver usually
employed, or printed by direct action of light, the result was the same.
Hence we may infer that the darkened material which forms the image of
Photographic Prints does not readily oxidize in a damp atmosphere.
_Toned_ Positives were found in many cases to be less permanent than
Positives simply fixed. This was especially the case when the toning had
been effected by _Sulphur_; all the sulphuretted prints, fixed in solution
of Hyposulphite which had been long used, became yellow in the half-tones
when exposed to moisture. Positives fixed and toned in Hyposulphite
containing Gold were variously affected; some prepared when the solution
was in an active state being unchanged, others losing a little half-tone,
and others, again, fading badly. These latter were prepared in a Bath
which had lost Gold and acquired sulphuretting properties; and it was
noticed that they were more injured by the action of boiling water than
those Positives which proved to be permanent under the influence of the
moisture.
Toning by means of Chloride of Gold appeared to be highly satisfactory,
but the number of prints operated upon was small. The Sel d'or process
also did not injure the integrity of the image, no commencing yellowness
or bleaching of half-tones being visible after exposure to the moist air.
This series of experiments confirmed the statement made in a former paper,
that some tints obtained in Positive printing are more permanent than
others. Violet tones produced by Sulphur invariably passed into a dull
brown by the action of the moist air; and even when Gold was employed
in toning, these same purple colours were usually _reddened_. This was
especially the case when English papers were used, or foreign papers
re-sized with Serum of Milk containing Caseine. The chocolate-brown tints
which best stand the action of boiling water, and in particular those upon
Ammonio-Nitrate paper, were least affected by the damp air; and indeed it
was evident that the two agents, viz. moist air and hot water, acted alike
in tending to _redden_ the print, although the latter did so in the most
marked manner.
It seemed also, from the results of these experiments, to be a point of
great importance that the size should be removed from the print in order
to render it indestructible by damp air. This was evidently seen in two
cases where Positives, toned in an old Hyposulphite and Gold Bath, were
divided into halves, one of which was treated with a strong solution of
Ammonia. The result was that the halves in which the size was allowed
to remain, faded, whilst the others were comparatively uninjured. The
Albumen proofs especially suffered when the size was left in the paper, a
destructive mouldiness forming, and fading the picture. The use of boiling
water obviated this, and the prints so treated remained clean and bright.
A partial decomposition of Albumen however occurred in some cases even
when hot water was used, the gloss disappearing from the paper in isolated
patches. With _Caseine_ substituted for Albumen there was also a loss of
half-tone; thus seeming to indicate that both these animal principles,
although stable under ordinary conditions, will, even when coagulated by
Nitrate of Silver, decompose if kept long in a moist state.
The use of improper substances for mounting proved to be another
determining cause of fading by oxidation. Those bodies which combine
with Oxide of Silver, are likely upon theoretical grounds to destroy the
half-tones of the image; and it was found, that if the picture were left
in contact with Alum, Acetic Acid, etc., or with the substances which
generate an acid by fermentation, such as paste or starch, it invariably
faded.
The supposed accelerating influence of _Light_ upon the fading of
Positives was not confirmed by these experiments, as far as they extended.
Many of the bottles containing the Photographs were placed outside the
window of a house with a southern aspect during the whole of the three
months with the exception of two or three weeks, but no difference
whatever could be detected between Positives so treated and others kept
in total darkness. It will be proper however that this part of the
investigation should be repeated, allowing a longer time.
An examination of the various modes employed for coating Positives, in
order to exclude the atmosphere, showed that many of them were not fitted
to fulfil the purpose intended. Waxed prints faded quite as much when
exposed to moisture as others not waxed. White wax is a substance often
adulterated, and Oil of Turpentine has been shown to contain a body
resembling Ozone in properties, and possessing the power of bleaching a
dilute solution of Sulphate of Indigo. Spirit varnish applied to the
surface of the picture after re-sizing with Gelatine was plainly superior
to white wax, but nevertheless it did not obviate the fading effect of the
moisture upon an unstable Positive which had been toned by sulphuration.
Its protective influence is therefore limited.
ON THE CHANGE IN COMPOSITION WHICH HYPOSULPHITE OF SODA EXPERIENCES BY USE
IN FIXING PAPER PROOFS.[28]
[28] These observations are condensed and re-arranged from the papers
published by the Author in the 'Photographic Journal' for September and
October, 1854.
It was remarked by Photographers at an early period that the properties of
the Fixing Bath of Hyposulphite of Soda became altered by constant use;
that it gradually acquired the power of _darkening_ the colour of the
Positive image. This change was at first referred to the accumulation of
_Salts of Silver_ in the Bath, and hence directions were given to dissolve
a portion of blackened Chloride of Silver in the Hyposulphite in preparing
a new solution.
Careful experiments performed by the Author convinced him that an error
had been entertained; since it was found that the simple solution of
Chloride of Silver in Hyposulphite of Soda had no power of yielding
the black tones. But it afterwards appeared that if the fixing Bath,
containing dissolved Silver Salts, were set aside for a few weeks, a
_decomposition_ occurred in it, evidenced by the formation of a black
deposit of Sulphuret of Silver; and _then_ it became active in toning the
proofs.
The presence of this deposit of Sulphuret of Silver indicated that a
portion of Hyposulphite of Silver had spontaneously decomposed, and,
knowing the products which are generated by the spontaneous decomposition
of this salt, a clue to the difficulty was afforded. One atom of
Hyposulphite of Silver includes the elements of one of Sulphuret of Silver
and one of Sulphuric Acid. Sulphuric Acid in contact with Hyposulphite of
Soda produces _Sulphurous Acid_ by a process of displacement; and Plessy
has shown that Sulphurous Acid reacts upon an excess of Hyposulphite
of Soda, forming two of that interesting series of Sulphur compounds
designated by Berzelius the "Polythionic Acids."
It appeared therefore probable, upon theoretical grounds, that the Penta-,
Tetra-, and Trithionates might produce some effect in the Hyposulphite
fixing Bath. Upon making the trial these expectations were verified; and
it was found that Tetrathionate of Soda added to Hyposulphite of Soda
yielded a fixing and toning Bath quite equal in activity to that produced
by means of Chloride of Gold.
It may be useful to review for an instant the composition of the
Polythionic series of acids; it is thus represented:--
Sulphur. Oxygen. Formulæ.
Dithionic or Hyposulphuric Acid 2 atoms 5 atoms S{2}O{5}
Trithionic Acid 3 " 5 " S{3}O{5}
Tetrathionic Acid 4 " 5 " S{4}O{5}
Pentathionic Acid 5 " 5 " S{5}O{5}
The amount of _Oxygen_ in all is the same, that of the other element
increases progressively; hence it is at once evident that the highest
member of the series might _by losing Sulphur_ descend gradually until it
reached the condition of the lowest.
This transition is not only theoretically possible, but there is an actual
tendency to it, all the acids being unstable with the exception of the
Hyposulphuric. The Alkaline Salts of these acids are more unstable than
the acids themselves; a solution of Tetrathionate of Soda becomes milky in
the course of a few days from deposition of Sulphur, and, if tested, is
then found to contain _Tri_thionate and eventually _Di_thionate of Soda.
The cause of the change in properties of the fixing Bath being thus
clearly traced to a decomposition of Hyposulphite of Silver, and a
consequent generation of unstable principles capable of imparting Sulphur
to the immersed proofs, it seemed desirable to continue the experiments.--
There is a peculiar _acid condition_ commonly assumed by old fixing
Baths, which could not be satisfactorily explained, since it was known
that acids do not exist long in a free state in solution of Hyposulphite
of Soda, but tend to neutralize themselves by displacing _Hyposulphurous
Acid_ spontaneously decomposable into Sulphurous Acid and Sulphur. This
point is set at rest by the discovery of a peculiar reaction which takes
place between certain salts of the Polythionic acids and Hyposulphite of
Soda. A solution of Tetrathionate of Soda may be preserved for many hours
unchanged; but if a few crystals of Hyposulphite of Soda be dropped in, it
begins very shortly to deposit Sulphur, and continues to do so for several
days. At the same time the liquid acquires an acid reaction to test-paper,
and produces effervescence on the addition of Carbonate of Lime.
It is evident that a Sulphur acid exists which has not hitherto been
described, and that this acid is formed as one of the products of the
decomposition of the Hyposulphite of Silver contained in the fixing Bath.
The subject is an important one to Photographers, because it is found that
Hyposulphite Baths which have acquired the acid reaction, although toning
quickly, yield Positives which fade on keeping. The acid may perhaps
combine with the reduced Silver Salt, which, if the image be allowed to
contain Suboxide of Silver, is theoretically probable.
The experiments were next directed towards ascertaining more carefully the
effect of the acid fixing Bath upon the Positive proofs. Tetrathionate of
Soda added to solution of Hyposulphite of Soda produces, at the expiration
of twelve hours, a liquid which, when filtered from the deposited Sulphur,
reddens blue litmus-paper slowly. Positive prints immersed in the Bath
pass from red to black, dissolving in the half-tones, and becoming yellow
and faded if the action be too long continued. On adding Carbonate of Soda
in quantity sufficient to remove the acid reaction, the power of toning
is much diminished, but dark colours can still be obtained by continuing
the action. The solvent effect upon the half-tones, evidently caused in
great measure by the acid, is lessened; whilst the tendency to yellowness
in the white parts of the proof, almost disappears. These effects are
more particularly manifested when the prints are immersed in the Bath
immediately on their removal from the printing frame; and it is found
almost impossible to preserve the whites of the impression clear, in the
acid Bath, unless the Nitrate of Silver has been washed away.
Solution of half-tones and yellowness in the lights, both a source of
annoyance to the operator, are thus traced in great measure to an acid
condition of the fixing and toning Bath; and the remedy is obvious.
The Author's experiments upon the Tetrathionates and their reaction with
Hyposulphite of Soda likewise elicited the important fact that _alkalies_
decompose the unstable sulphuretted principle. If the Bath be treated
with Potash or Carbonate of Soda, an alkaline _Sulphuret_ appears to be
gradually formed, which precipitates Sulphuret of Silver, and in the
course of a few days the liquid returns to its original condition and
ceases to act as a toning agent upon the proof. The same effect takes
place to a great extent when the solution is set aside for several weeks
or months; a process of spontaneous change going forward, which issues in
a deposition of Sulphur and Sulphuret of Silver, and a partial loss of
sulphuretting properties in the liquid.
It may be interesting to the scientific investigator to describe the mode
of preparing a fixing and toning Bath, illustrating the above remarks:--
Take of Nitrate of Silver 3 drachms.
Hyposulphite of Soda 4 ounces.
Water 8 ounces.
Dissolve the Nitrate of Silver in 2 ounces of the water, then from the
total quantity of Hyposulphite of Soda, weigh out
Hyposulphite of Soda 2 drachms;
dissolve this likewise in 2 ounces of water, and the remainder of the
Hyposulphite in the other 4 ounces. Then, having the three solutions in
separate vessels, pour the Nitrate of Silver at once into the 2-ounce
solution of Hyposulphite, agitating the precipitated Hyposulphite of
Silver rapidly. In a short time it will begin to decompose, passing from
white to canary-yellow, and then to orange-yellow. _When the orange-yellow
begins to verge towards brown_, add the 4-ounce concentrated solution of
Hyposulphite, which will at once complete the decomposition, a part of the
precipitate dissolving and the remainder becoming perfectly black. After
filtering out the black Sulphuret of Silver, the solution is ready for use.
A Bath prepared by this formula is not usually very active, but it shows
clearly the process by which an ordinary fixing Bath may be converted into
a toning Bath by the immersion of positives having free Nitrate of Silver
upon the surface.
The following formula is more economical and gives a better result, but
it cannot be used for "Ammonio-Nitrate" prints; the addition of an alkali
precipitating Sulphuret of Iron.
Strong solution of Perchloride of Iron 6 fluid drachms.
Hyposulphite of Soda 4 ounces.
Water 8 ounces.
Nitrate of Silver 30 grains.
Dissolve the Hyposulphite of Soda in seven ounces of the water, the
Nitrate of Silver in the remaining one ounce; then pour the Perchloride of
Iron into the solution of Hyposulphite, by degrees, stirring all the time.
The addition of the Iron Salt strikes a fine purple colour, but this soon
disappears. When the liquid has become again colourless, which it does
in a few minutes, add the Nitrate of Silver, stirring briskly. Perfect
solution will take place without any formation of black Sulphuret.
A toning Bath prepared with Chloride of Iron will be ready for use twelve
hours after mixing, but it will be more active at the expiration of a
week. The solution is acid to test-paper, and _milky_ from a deposit of
Sulphur, which must be filtered out.
The Perchloride of Iron should be prepared by boiling Peroxide of
Iron with Hydrochloric Acid, in preference to dissolving Iron wire in
Aqua-Regia.
The addition of the Nitrate of Silver is made in order to produce a
portion of Hyposulphite of Silver in the bath; the presence of a Silver
Salt having been found to modify the tint of the Positives, and to prevent
their quickly turning yellow.
SECTION IV.
_On the Fading of Photographic Prints._
For many years subsequent to the discovery of the process of Photographic
Printing by Mr. Fox Talbot, it was not generally known that pictures so
produced were easily susceptible of injury from various causes, and in
particular from traces of the _fixing-agent_ remaining in the paper.
Hence, due care not being taken in the proper cleansing and preservation
of the proofs, the majority of them faded.
This matter became at last one of such importance that the Council of the
Photographic Society decided upon forming a Committee for the purpose of
examining the subject. The Author was honoured by being placed upon this
Committee, and the researches of which an abstract has been given in the
previous Section, were undertaken at the request of the Society.
The present Section is intended to explain practically and in a
concise manner the causes of the fading of Photographic Prints, and
the precautions which should be taken to ensure their permanency. The
chemistry of the subject having been fully explained in the last Section,
it will suffice to refer the reader to its pages for more detailed
information.
_Historical evidence of the permanence of Photographs._--It is a point of
interest to collect information as to the existence of old Photographs
which have remained many years unchanged. There are numerous instances
of Positives printed more than ten years ago, which have not perceptibly
altered up to the present time. These prints are mostly on plain paper,
Albumen not having come into use at so early a date. The general
impression of practical operators however is, that fading has occurred
less frequently since the introduction of Albuminized paper.
Positives printed by development on paper prepared by Talbot's method
seem as a rule to have stood remarkably well, and instances of Talbotype
Negatives having faded are rare.
Of the prints which have proved to be permanent, some are red or brown in
colour, but many, being of a dark or purple shade, have evidently been
toned, although not with Gold, the use of which was unknown to the earlier
Photographers.
It is plain from data thus collected, that Photographs do not necessarily
fade by time; and the fact that in one and the same portfolio are
constantly seen prints which appear permanent, and others in an advanced
state of change, cannot but lead to the inference that the main causes of
deterioration are intrinsic, depending upon some injurious matters left in
the paper; which is confirmed by experiment.
_Causes of fading._--The Author believes that the fading of Photographic
Prints may almost invariably be referred to one or other of the following
conditions:--
a. _Imperfect washing._--This is perhaps the most important of all,
and the most frequent. When Hyposulphite of Soda is allowed to remain
in the paper, even in minute quantity, it gradually decomposes, with
liberation of Sulphur, and destroys the print in the same way and quite
as effectually as a solution of Sulphuretted Hydrogen or an alkaline
sulphuret.
Imperfect washing may be suspected, if the Photograph, within a few months
from the date of its preparation, _begins to get darker in colour_: the
_half-tints_, which are the first to show the action, afterwards passing
into the yellow stage, whilst the dark shadows remain black or brown for a
longer time.
The proper mode of washing Photographs is sometimes misunderstood. The
length of time during which the print lies in the water is a point of
less importance, than that the water should be continually changed. When
a number of Positives are placed together in a pan, and a tap turned upon
them, the circulation of fluid does not necessarily extend to the bottom.
This is proved by the addition of a little colouring matter, which shows
that the stream flows actively above, but at the lower part of the vessel,
and between the prints, there is a stationary layer of water which is of
little use in washing out the Hyposulphite. Care should therefore be taken
that the pictures are kept as far as possible separate from each other,
and when running water cannot be had, that they are frequently moved and
turned over, fresh water being constantly added. When this is done, and
especially if the _size_ be removed from the paper in the manner presently
to be advised, _four or five hours_ washing will be sufficient. It is a
mistake to allow the pictures to remain in the water for several days;
which produces no good effect, and may tend to encourage a putrefactive
fermentation, or the formation of a white deposit upon the image when the
water contains Carbonate of Lime.
b. _Acid matters left in the Paper._--Upon examining collections of old
Photographs, it is not uncommon to find prints which are stated to have
remained unaltered for a long time after their first production, but
in the course of time to have lost their brilliancy, and become pale
and indistinct. This kind of fading often commences at the corners and
edges of the paper, and works inwards towards the centre. The Author's
experiments have shown that it is principally caused by a slow process of
_oxidation_.
The Photographic Image does not appear readily susceptible of oxidation
unless it be previously darkened by the action of Sulphur, or placed in
contact with acids or bodies which act as solvents of Oxide of Silver
(p. 146). The materials often used in sizing papers, such as Alum and
Resin, being of an acid nature, are directly injurious to the image;
and the removal of the size, which may easily be effected by means of a
dilute alkali or an alkaline carbonate, without injury to the tint, has
the additional advantage of carrying out the last traces of Hyposulphite
of Soda, and also the germs of _fungi_, which if allowed to remain would
vegetate and produce a destructive mouldiness on exposure to damp (Chap.
III. Part II.).
The fact that acids facilitate oxidation of the image is likewise a hint
that Photographic Prints should not be handled too frequently, or touched
with the finger more than is necessary; the warm hand may leave behind a
trace of acid[29] which would tend in time to produce a yellow mark.
[29] The writer has seen blue litmus-paper immediately reddened by being
laid upon the arm of a person suffering from acute Rheumatism. This acid
is probably Lactic Acid!
c. _Moisture as a cause of fading._--Although. Photographs properly
printed are not readily injured by damp air (p. 153), yet as there are
_impurities_ of various kinds constantly floating in the atmosphere,
a state of comparative dryness may be said to be essential to the
preservation of all Photographs. In collecting evidence upon the subject,
"wet" and "damp" are frequently alleged as having been causes of
fading--the prints were hung against a damp wall during frosty weather,
in a room without a fire: or the rain had been allowed to penetrate the
frame! No pictures will long survive such treatment, and Photographs, like
engravings and water-colour paintings, require common care to be exercised
in their preservation.
d. _The modes of Mounting the Proof._--This subject has been alluded to in
the abstract of the Author's papers at p. 155. All cements which are of an
acid nature, or which are liable to become _sour_ by acetous fermentation,
should be avoided. Flour paste is especially injurious, and many cases
of fading have been traced to this cause. The addition of Bichloride of
Mercury, which is often made to prevent the paste from becoming mouldy,
would still more unfit it for Photographic use (p. 151). Starch is not
much preferable. No substance appears better than Gelatine, which does not
readily decompose, and shows no tendency to absorb atmospheric moisture.
The _deliquescent_ nature of many bodies is a point which should be
borne in mind in mounting Photographs, and hence the use of a salt like
_Carbonate of Potash_, which the writer has known to be added to paste to
prevent the formation of acid, would be unadvisable.
e. _The effect of Imperfect Fixation._--The earlier Photographers did not
always succeed in properly fixing their prints, since old Photographs are
often found thickly studded with spots and blotches in the tissue of the
paper. These prints however are not invariably faded upon the surface,
and hence it cannot be said that imperfect fixation will certainly end
in the total destruction of the picture. Still a notice of the subject
may properly be introduced in this place, and the attention of the reader
be once more drawn to the importance of washing the print in water on
removing it from the printing frame; a decomposition invariably occurring
when paper Positives _saturated with free Nitrate of Silver_ are plunged
in a dilute solution of Hyposulphite of Soda, containing an insufficient
quantity of the salt to dissolve away the Hyposulphite of Silver before it
begins to undergo spontaneous change.
f. _Exposure to an impure Atmosphere as a cause of Fading._--The five
causes of fading which precede, have mostly reference to an intrinsically
faulty condition of the print. This, the sixth, explains the mode in which
a Photograph carefully prepared may yet suffer injury from deleterious
matters often present in the atmosphere. The air of large cities, and
particularly that emanating from sewers and drains, contains Sulphuretted
Hydrogen, and hence articles of silver-plate become tarnished unless
placed beneath glass. The injury which a print sustains by exposure to
air contaminated with Sulphuretted Hydrogen, is less than the tarnish
produced upon the bright surface of a silver plate (see p. 148); but it
is recommended as a precautionary measure, that Photographic Pictures be
protected by glass or kept in a portfolio, and that they be not exposed
too freely to the air.
The products of the combustion of coal-gas are probably more likely than
the cause last named, to be a source of injury to Photographs suspended
without any covering. The sulphur compounds in gas burn into Sulphurous
and Sulphuric Acids, the latter of which, in combination with Ammonia,
produces the sparkling crystals often observed upon the shop windows.
The question as to the manner in which the Photographic Image may best be
protected from these extraneous causes of fading has been mooted, and many
plans of coating prints with some impervious material have been devised.
If the pictures are to be glazed or kept in a portfolio, this of itself
will be sufficient, but in other cases it may perhaps be useful to apply a
layer of spirit or gutta-percha varnish. The use of wax, resin, and such
bodies is likely, by introducing impurities, to act injuriously rather
than otherwise.
g. _Decomposition of Pyroxyline a source of Injury to Collodion
Photographs._--Collodion Positives and Negatives are usually esteemed
permanent; but some have been exhibited which, having been put away in a
damp place, gradually became pale and indistinct. The change commences at
rough edges and isolated points, leaving the centre, as a rule, the last
affected. On examination, numerous cracks are often visible, thus seeming
to indicate that the Collodion film has undergone decomposition. The
result of this would be the liberation of corrosive Oxides of Nitrogen,
which destroy the image. Substitution compounds containing Peroxide of
Nitrogen are known to be liable to spontaneous change. The bitter resin
produced by acting upon white sugar with Nitro-Sulphuric Acid, if not kept
perfectly dry, will sometimes evolve enough gas to destroy the cork of the
bottle in which it is kept; the solution of the resin has then a strong
acid reaction, and rapidly fades an ordinary Positive Print.
These facts are interesting, and indicate that Collodion Pictures,
containing in themselves the elements of their destruction, should be
protected from moisture by a coating of varnish.
_Comparative Permanence of Photographic Prints._--There is every reason
to think that the Photographic Image, however formed, is permanent, if
certain injurious conditions are avoided;--in other words, that prints do
not necessarily fade, in the same manner as fugitive colours, by a simple
exposure to light and air. But supposing a case, which is the common
one, of injurious influences which cannot altogether be removed, it may
be useful to inquire what mode of printing gives the greatest amount of
stability.
Positives produced by a short exposure to light and subsequent
development with Gallic Acid, may be expected to be more permanent than
ordinary sun-prints; not that there is any reason to suppose the chemical
composition of a developed image to be peculiar, but that the use of the
Gallic Acid enables us to increase the intensity of the red picture first
formed, and to add to its stability by precipitating fresh Silver upon
it. This point has not always been attended to. It has been recommended
to remove the print from the developing solution whilst in the _red_ and
early stage of development, and to produce the dark tones subsequently by
means of gold; but this plan, although giving very good results as regards
colour and gradation of tone, appears to lessen the advantage which would
otherwise accrue from the adoption of a Negative process, and to leave the
picture, as regards permanency, much in the condition of an ordinary print
obtained by direct action of light.
The original Talbotype process, in which the latent image is formed upon
Iodide of Silver, produces, next to Collodion, the most stable image; but
the difficulty of obtaining bright and warm tints on Iodide of Silver,
will stand in the way of its adoption.
The _toning_ of Paper Positives is the part of the process which is likely
to injure their stability; inasmuch as the finest results cannot easily be
obtained without incurring _sulphuration_, and the action of Sulphur, if
carried to any extent, has been shown to be detrimental. The point to be
kept in view, is to alter the original structure of the image as little as
possible in toning; and it is best to use Gold in preference to Sulphur
as the colouring agent. On theoretical grounds, toning by an alkaline
solution of Chloride of Gold (p. 132), and fixing by Ammonia, is the best
process; but the employment of Sel d'or, which gives a more agreeable
colour and has not been found practically to injure the image, will be
generally preferred. In using _a single fixing and toning Bath_ the same
object of working by Gold rather than by Sulphur may be best attained by
maintaining the activity of the Bath by constant additions of Chloride of
Gold.
The prints which are _least stable_ are such as have been toned in _acid
Hyposulphite Baths, without Gold;_ and the difficulty of preserving such
pictures from becoming yellow in the half-tones is very great. Possibly
a portion of the Sulphuretted Acid may unite with the Suboxide of Silver
and cannot be removed by washing (see p. 158); but even if this be not
the case, it is certain that no ordinary amount of care will obviate the
occasional occurrence of fading, unless the Hyposulphite Bath be kept
_neutral to test-paper_. And all those plans of toning in which Acetic or
Hydrochloric Acid is mixed with Hyposulphite of Soda, and the Positive
immersed whilst the liquid is in a milky state from precipitation of
Sulphur, ought studiously to be avoided.
It will be well also to avoid pushing the action of the fixing and toning
Bath to its utmost limits, since practice and theory both teach us that
the Positives which have been long in the Hyposulphite, and consequently
show a tendency to yellowness in the light parts, are most liable to
lose their half-tones on keeping. Photographic Prints are found often to
_darken_ slightly in the course of years; and therefore by suspending the
toning action at an earlier stage a margin is left for what some have
termed "an improvement by time."
The use of _Albuminized_ in preference to plain paper gives an advantage
in protecting the image from oxidation; but if constantly exposed to
moisture, a putrefactive decomposition of the animal matter may occur. The
proper colour of the Albumen image being a _pale red_, the black tones
should not be sought for on that variety of paper: their production,
if Hyposulphite of Soda were used in toning, would probably imply an
amount of Sulphuration which would more than counterbalance any advantage
otherwise derivable from the Albumen.
Permanent Positives of a black colour may easily be obtained by
sensitizing plain paper, free from animal matters, with Oxide of Silver
in place of Nitrate. The simply fixed image being in that case of a
_sepia tint_, requires a less amount of toning to change it to black.
An impression was at one time prevalent that Ammonio-Nitrate prints
were unstable; but so far from such being the case, they are proved
to withstand the action of all destructive tests better than pictures
prepared upon the same kind of paper sensitized with plain Nitrate of
Silver.
_Mode of testing the permanence of Positives._--The tests for Hyposulphite
of Soda are not sufficiently delicate to indicate with certainty when the
process of washing has been properly performed. The quantity of that salt
left in the paper is usually so small and so much mixed up with organic
matter, that the application of Protonitrate of Mercury or of Nitrate of
Silver to the liquid which drains from the corner of the print, would
probably mislead the operator.
A dilute solution of Permanganate of Potash, prepared by dissolving from
half a grain or two grains of the salt, according to its purity, in one
gallon of distilled water, affords a convenient mode of testing Positives
as regards their power of resisting oxidation; and to an experienced
eye it will prove the presence or absence of Hyposulphite of Soda, the
smallest trace of which is sufficient to remove the pink colour of the
Permanganate.
The most available and simple plan of testing permanence is to enclose
the pictures in a stoppered glass bottle with a small quantity of water.
If they retain their half-tones after a course of three months of this
treatment, and do not become mouldy, the mode of printing followed is
satisfactory.
Boiling water will also be found useful in distinguishing the unstable
colours produced by Sulphur from those following the judicious employment
of Gold; in all cases the image will at first be reddened by the hot
water, but if toned without Sulphur it will, as a rule, recover much of
its dark colour on drying.
The characteristic appearance of prints which have been I much
sulphuretted in the toning Bath, and are very liable to fade, should be
known. A yellow colour in the lights is a bad sign; and if the half-tones
are at all faint and indistinct, with an aspect of commencing yellowness,
it is almost certain that the Positive will not last for any considerable
length of time.
CHAPTER IX.
ON THE THEORY OF THE DAGUERREOTYPE AND TALBOTYPE PROCESSES, ETC.
SECTION I.
_The Daguerreotype._
It was not the original intention of the Author to include a description
of the Daguerreotype Process within the limits of the present Work. The
Daguerreotype is a branch of the Photographic Art so distinct from the
others, that, in manipulatory details, it bears very little analogy to
them; a slight sketch of the theory of the process may not however be
unacceptable.
All necessary remarks will fall under three heads:--The preparation
of the Daguerreotype film;--the means by which the latent image is
developed;--and the strengthening of the image by Hyposulphite of Gold.
_The Preparation of the Daguerreotype Film._--The sensitive film of the
Daguerreotypist is in many respects different from that of the Calotype or
Collodiotype. The latter may be termed wet processes, in contradistinction
to the former, where aqueous solutions are not employed. The Daguerreotype
film is a pure and isolated Iodide of Silver, formed by the direct action
of Iodine upon the metal. Hence it lacks one element of sensitiveness
possessed by the others, viz. the presence of soluble Nitrate of Silver in
contact with the particles of Iodide of Silver.
It is important to remember that the Iodide of Silver prepared by
acting with vapour of Iodine upon metallic Silver, is different in its
Photographic action from the yellow salt obtained by double decomposition
between Iodide of Potassium and Nitrate of Silver. A Daguerreotype film,
when exposed to a bright light, first darkens to an ash-grey colour and
then becomes nearly white; the solubility in Hyposulphite of Soda being
at the same time lessened. A Collodion film, on the other hand, if the
excess of Nitrate of Silver be washed off, although it is still capable of
receiving the radiant impression in the Camera, does not alter either in
colour or in solubility, by exposure even to the sun's rays.
_Details of the process for preparing a Daguerreotype Plate._--A copper
plate of moderate thickness is coated upon the surface with a layer of
pure Silver, either by the electrotype or in any other convenient manner.
It is then polished with great care, until the surface assumes a brilliant
metallic lustre. This preliminary operation of polishing is one of great
practical importance, and the troublesome details attending it constitute
one of the main difficulties to be overcome.
After the polishing is complete, the plate is ready to receive the
sensitive coating. This part of the process is conducted in a peculiar
manner. A simple piece of cardboard or a thin sheet of wood, previously
soaked in solution of Iodine, evolves enough of the vapour to attack the
silver plate; which being placed immediately above, and allowed to remain
for a short time, acquires a pale violet hue, due to the formation of
_an excessively delicate layer_ of Iodide of Silver. By prolonging the
action of the Iodine the violet tint disappears and a variety of prismatic
colours are produced, much in the same way as when light is decomposed
by thin plates of mica or the surface of mother-of-pearl. From violet
the plate becomes of a straw-yellow, then rose-colour, and afterwards
steel-grey. By continuing the exposure, the same sequence of tints is
repeated; the steel-grey disappears, and the yellow and rose-colours
recur. The deposit of Iodide of Silver gradually increases in thickness
during these changes; but to the end it remains excessively thin and
delicate. In this respect it contrasts strongly with the dense and creamy
layer often employed in the Collodion process, and shows that a large
proportion of the Iodide of Silver must in such a case be superfluous, as
far as any influence produced by the light is concerned. An inspection
of a sensitive Daguerreotype plate reveals the microscopic nature of the
actinic changes involved in the Photographic Art, and teaches a useful
lesson.
_Increase of sensibility obtained by combining the joint action of Bromine
and Iodine._--The original process of Daguerre was conducted with the
vapour of Iodine only; but in the year 1840 it was discovered by Mr. John
Goddard that the sensibility of the plate was greatly promoted by exposing
it to the vapours of Iodine and Bromine in succession,--the proper time
for each being regulated by the tints assumed.
The composition of this Bromo-Iodide of Silver, so called, is uncertain,
and has not been proved to bear any analogy to that of the mixed salt
obtained by decomposing a solution of Iodide and Bromide of Potassium with
Nitrate of Silver. Observe also that the Bromo-Iodide of Silver is more
sensitive than the simple Iodide _only token the vapour of Mercury is
employed as a developer_. M. Claudet proves that if the image be formed by
the direct action of light alone (see page 174), the usual condition is
reversed, and that the use of Bromine under such circumstances retards the
effect.
_The Development and Properties of the Image._--The latent image of the
Daguerreotype is developed in a manner different from that of the humid
processes generally,--viz. by the action of Mercurial vapour. Mercury,
or Quicksilver, is a metallic fluid which boils at 662° Fahrenheit. We
are not however to suppose that the iodized plate is subjected to the
vapour of Mercury at a temperature at all approaching to 662°. The cup
containing the Quicksilver is previously heated by means of a spirit-lamp
to about 140°, a temperature easily borne by the hand, in most cases,
without inconvenience. The amount of Mercurial vapour evolved at 140° is
very small, but it is sufficient for the purpose, and after continuing the
action for a short time the image is perfectly developed.
There are few questions which have given rise to greater discussion
amongst chemists than the nature of the Daguerreotype image.
Unfortunately, the quantity of material to be operated on is so small,
that it becomes almost impossible to ascertain its composition by direct
analysis. Some suppose it to consist of Mercury alone. Others have thought
that the Mercury is in combination with metallic Silver. The presence
of the former metal is certain, since M. Claudet shows that, by the
application of a strong heat, it can actually be volatilized from the
image in sufficient quantity to develope a second impression immediately
superimposed.
It is a remarkable fact that an image more or less resembling that
developed by Mercury can be obtained by _the prolonged action_ of light
alone upon the iodized plate. The substance so formed is a white powder,
insoluble in solution of Hyposulphite of Soda; amorphous to the eye,
but presenting the appearance of minute reflecting crystals when highly
magnified. Its composition is uncertain.
For all practical purposes the production of the Daguerreotype image by
light alone is useless, on account of the length of time required to
effect it. This was alluded to in the third Chapter, where it was shown
that in the case of the Bromo-Iodide of Silver an intensity of light 3000
times greater is required, if the use of the Mercurial vapour be omitted.
_M. Ed. Becquerel's discovery of the continuing action of rays of
yellow light._--Pure homogeneous yellow light has no action upon the
Daguerreotype plate; but if the iodized surface be first exposed to
white light for a sufficient time to impress a latent image, and then
_afterwards_ to the yellow light, the action already commenced is
_continued_, and even to the extent of forming the peculiar white deposit,
insoluble in Hyposulphite of Soda, already alluded to.
Yellow light may therefore in this sense be spoken of as a _developing_
agent, since it produces the same effect as the Mercurial vapour in
bringing out to view the latent image.
A singular anomaly however requires notice, viz. that if the plate be
prepared with the mixed vapours of Bromine and Iodine, in place of Iodine
alone, then the yellow light cannot be made to develope the image. In
fact, the same coloured ray which continues the action of white light upon
a surface of _Iodide_ of Silver, actually _destroys_ it, and restores the
particles to their original condition, with a surface of _Bromo_-Iodide of
Silver.
These facts, although not of great practical importance, are interesting
in illustration of the delicate and complex nature of the chemical changes
produced by light.
_The Strengthening of the Daguerreotype Image by means of Hyposulphite of
Gold._--The use of the Hyposulphite of Gold to whiten the Daguerreotype
image, and render it more lasting and indestructible, was introduced by M.
Fizeau, subsequent to the original discovery of the process.
After removal of the unaltered Iodide of Silver by means of Hyposulphite
of Soda, the plate is placed upon a levelling stand and covered with a
solution of Hyposulphite of Gold, containing about one part of the salt
dissolved in 500 parts of water. The flame of a spirit-lamp is then
applied until the liquid begins to boil. Shortly a change is seen to take
place in the appearance of the image; it becomes whiter than before, and
acquires great force. This fact seems to prove conclusively that metallic
Mercury enters into its composition, since a surface of Silver--such, for
instance, as that of the Collodion image--is _darkened_ by Hyposulphite of
Gold.
The difference in the action of the gilding solution upon the image and
the pure Silver surrounding it illustrates the same fact. This Silver,
which appears of a dark colour, and forms the shadows of the image, is
rendered still darker; a very delicate crust of metallic Gold _gradually_
forming upon it, whereas with the image the whitening effect is immediate
and striking.
SECTION II.
_Theory of the Talbotype and Albumen Processes._
_The Talbotype or Calotype._--This process, as practiced by many at the
present time, is almost identical with that originally described by Mr.
Fox Talbot. The object is to obtain an even and finely divided layer of
Iodide of Silver upon the surface of a sheet of paper; the particles of
the Iodide being left in contact with an excess of Nitrate of Silver,
and usually with a small proportion of Gallic Acid, to heighten, still
further, the sensibility to light.
The English papers sized with Gelatine are commonly used for the Calotype
process; they retain the film more perfectly at the surface, and the
Gelatine in all probability assists in forming the image. With a foreign
starch-paper, unless it be re-sized with some organic substance, the
solutions sink in too deeply, and the picture is wanting in clearness and
definition.
There are two modes of iodizing and sensitizing the sheets: first, by
floating alternately upon Iodide of Potassium and Nitrate of Silver, in
the same manner as in the preparation of papers for Positive Printing;
and second, by what is termed "the single wash," which is thought by many
to give superior results as regards sensitiveness and intensity of image.
To iodize by this mode, the yellow Iodide of Silver, prepared by mixing
solutions of Iodide of Potassium and Nitrate of Silver, is dissolved in
a strong solution of Iodide of Potassium; the sheets are floated for an
instant upon this liquid and dried; they are then removed to a dish of
water, by the action of which the Iodide of Silver is precipitated upon
the surface of the paper in a finely divided state.
The properties of a solution of Iodide of Silver in Iodide of Potassium,
or of the double Iodide of Potassium and Silver, are described at page 43,
a reference to which will show that the double salt is decomposed by a
large quantity of water, with precipitation of the Iodide of Silver, this
substance being insoluble in a dilute solution of Iodide of Potassium,
although soluble in a strong solution.
Paper coated with Iodide of Silver by this mode, after proper washing in
water to remove soluble salts (which if allowed to remain would attract
damp), will keep good for a long time. The layer of Iodide appears of a
pale primrose colour, and is perfectly insensitive to light. Even exposure
to the sun's rays produces no change, thus indicating that an excess of
Nitrate of Silver is essential to the visible darkening of Iodide of
Silver by light. The paper is also insensitive to the reception of an
invisible image, differing in this respect from the washed Collodion
plate, which receives an impression in the Camera, although apparently
freed from Nitrate of Silver.
To render Calotype paper sensitive to light, it is brushed with a solution
of Nitrate of Silver containing both Acetic and Gallic Acids, termed
"Aceto-Nitrate" and "Gallo-Nitrate" solution. The Gallic Acid lessens
the keeping qualities of the paper, but increases the sensitiveness.
The Acetic Acid prevents the paper from blackening all over during
the development, and preserves the clearness of the white parts; its
employment is indispensable.
The paper is commonly excited upon the morning of the day upon which it
is intended to be used; and the longer it is kept, the less active and
certain it becomes. An exposure of five to eight minutes in the Camera is
the average time with an ordinary view lens.
The picture is developed with a saturated solution of Gallic Acid, to
which a portion of Aceto-Nitrate of Silver is added to heighten the
intensity. Both Sulphate of Iron, and Pyrogallic Acid have also been used,
but they are unnecessarily strong, the invisible image being more easily
developed upon paper than upon Collodion (see page 143).
After fixing the Negative by removing the unaltered Iodide of Silver with
Hyposulphite of Soda, it is well washed and dried. White wax is then
melted in with a hot iron, so as to render the paper transparent, and to
facilitate the after-process of printing.
The Calotype cannot be compared with the Collodion process for
sensitiveness and delicacy of detail, but it possesses advantages for
tourists and those who do not wish to be encumbered with large glass
plates. The principal difficulty appears to be in obtaining a uniformly
good paper, many samples giving a speckled appearance in the black parts
of the Negative.
_The Waxed Paper process of Le Grey._--This is a useful modification
of the Talbotype introduced by M. Le Grey. The paper is waxed before
iodizing, by which, without involving any additional operation, a very
fine surface layer of Iodide of Silver can be obtained. The Waxed Paper
Process is well adapted for tourists, from its extreme simplicity and the
length of time which the film may be kept in a sensitive condition.
Both English and foreign papers are employed: but the former take the
wax with difficulty. Mr. Crookes, who has devoted his attention to this
process, gives clear directions for waxing paper; it is essential that
pure white wax should be obtained direct from the bleachers, since the
flat cakes sold in the shops are commonly adulterated. The _temperature_
must also be carefully kept below that point at which decomposition of
the wax takes place; the use of too hot an iron being a common source of
failure (see 'Photographic Journal,' vol. ii. p. 231).
The sheets of paper, having been properly waxed, are soaked for _two
hours_ in a solution containing Iodide and Bromide of Potassium, with,
enough free Iodine to tinge the liquid of a port-wine colour. The greasy
nature of wax impedes the entry of liquids, and hence a long immersion
is required. The iodizing formulæ of the French Photographers have been
encumbered by the addition of a variety of substances which appear to
introduce complications without giving proportional advantage, and Mr.
Townshend has done the art a service by proving that the Iodide and
Bromide of Potassium, with free Iodine, are sufficient. This latter
ingredient was first used by Mr. Crookes; it seems to add to the clearness
and sharpness of the Negatives; and as the papers are _coloured_ by the
Iodine, air-bubbles cannot escape detection. The process of exciting with
Nitrate of Silver is also rendered more certain by the employment of free
Iodine, the action of the Bath being continued until the purple colour
gives place to the characteristic yellow tint of the Iodide of Silver.
Waxed Paper is rendered sensitive by immersion in a Bath of Nitrate of
Silver containing Acetic Acid; the quantity of which latter ingredient
should be increased when the papers are to be long kept. As the excess of
Nitrate is subsequently removed, the solution may be used weaker than in
the Calotype or Collodion process.
After exciting, the papers are washed with water to reduce the amount of
free Nitrate of Silver to a minimum. This lessens the sensitiveness, but
greatly increases the keeping qualities, and the paper will often remain
good for ten days or longer.
It is a very important point, in operating with Waxed Paper, to keep
the developing dishes clean. The development is conducted by immersion
in a Bath of Gallic Acid containing Acetic Acid and Nitrate of Silver;
and being retarded by the superficial coating of wax, there is always a
tendency to an irregular reduction of Silver upon the white portions of
the Negative. When the developer becomes brown and discoloured, this is
almost sure to happen; and it is well known to chemists that the length
of time during which Gallic Acid and Nitrate of Silver may remain mixed
without decomposing, is much lessened by using vessels which are dirty
from having been before employed for a similar purpose. The black deposit
of Silver exercises a _catalytic_ (καταλυσις, decomposition by contact)
action upon the freshly-mixed portion, and hastens its discoloration.
The Waxed Paper process is exceedingly simple and inexpensive,--very
suitable for tourists, as requiring but little experience, and a minimum
of apparatus. It is however slow and tedious in all its stages, the
sensitive papers frequently taking an exposure of twenty minutes in the
Camera, and the development extending over an hour or an hour and a
half. Several Negatives however may be developed at the same time; and
as the removal of the free Nitrate of Silver gives the process a great
advantage during hot weather, it will in all probability continue to be
extensively followed. The prints which have been sent to the Exhibition
of the Photographic Society, show that waxed paper in the hands of a
skilful operator may be made to delineate architectural subjects with
great fidelity, and also to give the details of foliage and landscape
Photography with distinctness.
_The Albumen process upon Glass._--The process with Albumen originated in
a desire to obtain a more even surface layer of Iodide of Silver than the
coarse structure of the tissue of paper will allow. It is conducted with
simple Albumen, or "white of eggs," diluted with a convenient quantity of
water. In this glutinous liquid Iodide of Potassium is dissolved; and the
solution, having been thoroughly shaken, is set aside, the upper portion
being drawn off for use, in the same manner as in the preparation of
Albuminized paper for printing.
The glasses are coated with the Iodized Albumen, and are then placed
horizontally in a box to dry. This part of the process is considered the
most troublesome, the moist Albumen easily attracting particles of dust,
and being apt to blister and separate from the glass. If an even layer of
the dried and Iodized material can be obtained, the chief difficulty of
the process has been overcome.
The plates are rendered sensitive by immersion in a Bath of Nitrate of
Silver with Acetic Acid added, and are then washed in water and dried.
They may be kept for a long time in an excited state.
The exposure in the Camera must be unusually long; the free Nitrate of
Silver having been removed by washing, and the Albumen exercising a direct
retarding influence upon the sensitiveness of Iodide of Silver.
The development is conducted in the ordinary way by a mixture of Gallic
Acid and Nitrate of Silver, with Acetic Acid added to preserve the
clearness of the lights. It usually requires one hour or more, but may be
accelerated by the gentle application of heat.
Albumen pictures are remarkable for elaborate distinctness in the
shadows and minor details, and are admirably adapted for viewing in the
Stereoscope; but they do not often possess the peculiar and characteristic
_softness_ of the Photograph upon Collodion. The process is well adapted
for hot climates, being very little prone to the cloudiness and irregular
reduction of Silver which are often complained of with moist Collodion
under such circumstances.
_M. Taupenot's Collodio-Albumen process._--This is a recent discovery
which seems to involve a new principle in the Art, and gives promise of
great utility.
One of the greatest objections to the Albumen process has been its want
of sensitiveness; but M. Taupenot found that this was obviated to a great
extent by pouring the Albumen upon a plate _previously coated with Iodide
of Silver_. In this way two layers of that sensitive salt are formed, and
the sensibility of the surface layer, which alone receives the image, is
promoted by its resting upon a substratum of Iodide rather than upon the
inert surface of the glass. In this view, if the theory be correct, the
lower particle of Iodide of Silver promotes the molecular disturbance of
the upper, itself remaining unchanged.
Other experimenters, pursuing the subject further, have asserted that
a successful result may be obtained by coating the plate with plain
Collodion and subsequently with Iodized Albumen. If this observation
should prove correct, the process will be simplified and its utility
increased.
In the sixth Chapter of Part II. the practical details of the
Collodio-Albumen process will be described.
END OF PART I.
PART II.
PRACTICAL DETAILS OF THE COLLODION PROCESS.
PRACTICAL DETAILS OF THE COLLODION PROCESS.
[Illustration]
CHAPTER I.
PREPARATION OF COLLODION.
This includes--the soluble Paper;--the Alcohol and Ether;--and the
iodizing compounds.
The formulæ for Negative and Positive Collodion, and for the Nitrate Bath
and developing fluids, are given in the second Chapter.
THE SOLUBLE PAPER.
Pyroxyline may be prepared either from cotton wool or from Swedish
Filtering-paper. Most operators prefer the latter, from its giving a
product of constant solubility, and yielding a fluid solution.[30] The
Cotton Wool however is better adapted for use with the Sulphuric Acid and
Nitre, since the Paper, from its closeness of texture, requires a longer
immersion in the mixture.
[30] Swedish filtering-paper may be procured at the operative chemists',
at about five shillings the quire. Each half-sheet has the water-mark "J.
H. Munktell."
Preparation of a Nitro-Sulphuric Acid of the proper strength.--There are
two modes of preparing the Nitro-Sulphuric Acid: first, by mixing the
acids; second, by the Oil of Vitriol and Nitre Process. The former is the
best in cases where large quantities of the material are operated on, but
the amateur is recommended to begin by trying the Nitre Process (p. 190)
as the most simple.
PREPARATION OF NITRO-SULPHURIC ACID BY THE MIXED ACIDS.
The operator may proceed in either of two ways; first, by taking the
strength of each sample of acid, and mixing according to fixed rule;
second, by a more ready plan, which may be used when the exact strength of
the acids is not known. Each of these will be described in succession.
a. _Directions for mixing according to fixed rule._--This process is given
from Mr. Hadow's original paper in the 'Quarterly Journal of the Chemical
Society.' It is certain in its results if the strength of both acids be
accurately determined.
A very perfect process for taking the strength of Nitric Acid is by means
of powdered Marble or Carbonate of Lime, as described in various works on
practical Chemistry. Sulphuric Acid may be estimated by precipitating with
Nitrate of Baryta, and weighing the insoluble Sulphate with the proper
precautions.
The specific gravity is not a criterion of strength to be perfectly relied
on, but if it be adopted as a test, the following points must be attended
to.
1st. That the temperature of the acid be at or near 60° Fahrenheit; the
density of Sulphuric Acid especially is, from its small specific heat,
greatly influenced a change of temperature.
2nd. The sample of Nitric Acid must be free from Peroxide of Nitrogen, or
only slightly coloured by it. This substance, when present, increases the
specific gravity of the acid without adding to its available properties.
A yellow sample of Nitric Acid will therefore be somewhat weaker than is
indicated by the specific gravity.
3rd. The Oil of Vitriol should yield no solid residue on evaporation.
Sulphate of Lead and Bisulphate of Potash are often found in the
commercial acid, and add much to its density. Oil of Vitriol containing
Sulphate of Lead becomes milky on dilution.
The formula for a definite Nitro-Sulphuric Acid, of the proper strength
for making the soluble Pyroxyline, may be stated thus:--
HO NO{5}, 2 (HO SO{3}) + 3-1/2 HO
or
Atoms. Atomic weight.
Nitric Acid 1 | 54
Sulphuric Acid 2 | 80
Water 6-1/2 | 58
| ---
| 192
Having found the percentage of real acid which is present,[31] the
following calculation will give the relative weights of the ingredients
required to produce the formula:--
Let { _a_ = percentage of real Nitric Acid,
{ _b_ = " " Sulphuric Acid,
then 5400/_a_ = quantity of Nitric Acid,
8000/_b_ = " Sulphuric Acid,
192 - 5400/_a_ - 8000/_b_ = " Water.
[31] Tables are given in the Appendix for calculation by specific gravity;
but direct analysis of the acids is the most certain.
Observe that the numbers in the calculation correspond to the atomic
weights recently given; and that the amount of water is derived from the
_total atomic weight_, viz. 192, _minus_ the sum of the weights of both
acids.
Hence if the samples of acid employed are too weak for the purpose, the
formula for the water gives a negative quantity.
The weight of mixed acids produced by the formula is 192 grains, which
would measure somewhere about two fluid drachms. Ten times this quantity
forms a convenient bulk of liquid, in which about 50 or 60 grains of Paper
may be immersed.
In weighing corrosive liquids, such as Sulphuric and Nitric Acid, a small
glass may be counterbalanced in the scale-pan, and the acid poured in
carefully. If too much is added, the excess can be removed by a glass rod,
or by "the pipette" commonly employed for such a purpose.
The following example of a calculation similar to the above may be given:--
100 parts of the Oil of Vitriol = 76·65 real acid.
" " Nitric Acid = 65·4 real acid.
therefore 8000/76·65 = 104·3 grains of Oil of Vitriol.
5400/65·4 = 82·5 " Nitric Acid
192 - 104·3 - 82·5 = 5·2 " Water.
Multiplying these weights ten times, we have
Oil of Vitriol 1043 grains.
Nitric Acid 825 "
Water 52 "
----
Total weight of the } 1920 grains.
Nitro-Sulphuric Acid }
Having prepared the acid mixture of a definite strength by the above
formula, the paper must be immersed according to directions given at page
191.
b. _Process for mixing Nitro-Sulphuric Acid, the strength of the two acids
not having been previously determined._--Take a strong sample of Nitric
Acid (the yellow Nitrous acid, so called, succeeds well), and mix it with
Oil of Vitriol as follows:--
Sulphuric Acid 10 fluid drachms,
Nitric Acid 10 "
Now immerse a thermometer and note the temperature;[32] it should be
from 130° Fahr. to 150°. If it sinks below 120°, place the mixture in a
capsule, and float upon boiling water for a few minutes.
[32] In the preparation of soluble cotton, and indeed in all Photographic
manipulations, a thermometer is almost indispensable. Instruments of
sufficient delicacy for common purposes are sold in Hatton Garden and
elsewhere, at a low price. The bulb should be uncovered, to admit of being
dipped in acids, etc., without injury to the scale.
A preliminary experiment with a small tuft of Cotton Wool (cotton shows
it better than paper) will then indicate the actual strength of the
Nitro-Sulphuric Acid. Stir the tuft in the mixture for five minutes.
Remove with a glass rod, and wash with water for a short time, until no
acid taste can be perceived. If the Wool becomes _matted_, and gelatinizes
slightly on its first immersion in the acid, or if, in the subsequent
washing, the fibres appear to adhere and to be disintegrated by the action
of the water, _the Nitro-Sulphuric Acid is too weak_. In that case add to
the acid mixture.
Oil of Vitriol, 3 drachms.
If the cotton was actually _dissolved_ in the first trial, an addition of
half of a fluid ounce of Oil of Vitriol may be required.
Supposing the cotton not to be gelatinized and to wash well, then wring
it out very dry, pull out the fibres, and treat it in a test-tube with
rectified Ether,[33] to which a few drops of Alcohol have been added. If
it be _insoluble_, dry it by a gentle heat and apply a flame: a brisk
explosion indicates that the Nitro-Sulphuric Acid employed is _too
strong_. In that case, add to the twenty drachms of mixed acids, one
drachm of water, and test again, repeating the process until a soluble
product is obtained.
[33] Observe that the Ether be pure; if it contains too much water and
Alcohol, it will not dissolve the Pyroxyline, or will yield an opalescent
solution.
There is a third condition of Pyroxyline, different from either of the
above, which may be puzzling:--the fibres of the Cotton mat together very
slightly or not at all on immersion, and the washing proceeds tolerably
well; the compound formed is scarcely explosive, and dissolves imperfectly
in Ether, leaving little nodules or hard lumps. The ethereal solution
yields, on evaporation, a film which is opaque instead of transparent. In
this case (presuming the Ether to be good) the acid mixture is slightly
too weak, or the temperature is too low, being probably about 90°, instead
of 130° to 140° (?).
When the acid mixture has been brought to the proper strength by a few
preliminary trials, proceed according to the directions given at the next
page.
PREPARATION OF NITRO-SULPHURIC ACID BY OIL OF VITRIOL AND NITRE.
This process is recommended, in preference to the other, to the amateur
who is unable to obtain Nitric Acid of convenient strength. The common Oil
of Vitriol sold in the shops is often very good for Photographic purposes;
but it is best, if possible, to take the specific gravity, when any doubt
exists of its genuineness. At a temperature of 58° to 60°, specific
gravity 1·833 is the usual strength, and if it falls below this, it should
be rejected. (See Part III. for 'Impurities of Commercial Sulphuric Acid.')
The Nitre must be the purest sample which can be obtained. Commercial
Nitre often contains a large quantity of _Chloride of Potassium_, detected
on dissolving the Nitre in distilled water, and adding a drop or two of
solution of Nitrate of Silver. If a milkiness and subsequent curdy deposit
is formed. Chlorides are present. These Chlorides are injurious; after
the Oil of Vitriol is added, they destroy a portion of Nitric Acid by
converting it into brown fumes of Peroxide of Nitrogen, and so alter the
strength of the solution.
_Nitrate of Potash is an anhydrous salt_,--it contains simply Nitric Acid
and Potash, without any water of crystallization; still, in many cases,
a little water is retained mechanically between the interstices of the
crystals, and therefore it is better to dry it before use. This may be
done by laying it in a state of fine powder upon blotting-paper, close to
a fire, or upon a heated metallic plate.
The sample must also be reduced to a fine powder before adding the Oil of
Vitriol; otherwise portions of the salt escape decomposition.
These preliminaries having been properly observed, weigh out
Pure Nitre, powdered and dried, 600 grains.
This quantity is equivalent to 1-1/4 ounce Troy or Apothecaries'
weight;--and to 1-1/4 ounce Avoirdupois weight _plus_ 54 grains. Place
this in a teacup or any other convenient vessel, and pour upon it.
Water 1-1/2 fluid drachms
mixed with Oil of Vitriol 12 "
Stir well with a glass rod for two or three minutes, until all
effervescence has ceased, and an even, pasty mixture, free from lumps, is
obtained.
During the whole process, abundance of dense fumes of Nitric Acid will be
given off, which must be allowed to escape up the flue or into the open
air.
_A modification of the formula._--The above formula will invariably
succeed with a good sample of acid and pure Nitre. When tried however with
Oil of Vitriol rather weaker than ordinary, and _commercial_ Nitre, it may
fail, the cotton being gelatinized and dissolved. When such is the case,
the addition of water must be omitted or the quantity reduced from one
drachm and a half to half a drachm.
GENERAL DIRECTIONS FOR IMMERSING, WASHING, AND DRYING THE PYROXYLINE.
The mixture of Sulphuric Acid and Nitre requires to be used immediately
after its preparation, as it solidifies into a stiff mass on cooling; but
the mixed acids may be kept for any length of time in a stoppered bottle.
When Cotton is used, the fibres should be well pulled out, and small tufts
added one by one to the acid mixture, stirring with a glass rod in order
to keep up a constant change of particles. The Paper is cut into squares
or strips, which are introduced singly.
In either case the quantity must not be too great, or some portions will
be imperfectly acted upon; about 20 grains to each fluid ounce of the
mixture will be sufficient.
The _time of immersion required_ varies from ten minutes with Cotton, to
twenty minutes or even half an hour with the Paper. When an unusually
large proportion of Sulphuric Acid is used, as in the case of a weak
sample of Nitric Acid, the Cotton should be removed at the expiration of
six or seven minutes, as there is a tendency to partial solution of the
Pyroxyline in the acid mixture under those circumstances.
It is an advantage in some cases to prepare the material at a high
temperature, but unless the proportions of the Acids are strictly
according to Mr. Hadow's formula, solution of the Cotton may take place if
the thermometer indicates more than 140°.
After the action is complete, the Nitro-Sulphuric Acid is left weaker than
before, from addition of various atoms of water necessarily formed during
the change. Hence, if the same portion be used more than once, an addition
of Sulphuric Acid will be required.
_Directions for Washing._--In removing the Pyroxyline from the
Nitro-Sulphuric Acid, press out as much of the liquid as possible, and
wash it rapidly in a large quantity of cold water, using a glass rod to
preserve the fingers from injury. If it were simply thrown into a small
quantity of water and allowed to remain, the rise in temperature and
weakening of the acid mixture might do mischief.
The washing should be continued for at least a quarter of an hour, or
longer in the case of Paper, as it is essential to get rid of every
trace of acid. When the Nitre plan has been adopted, a portion of the
_Bisulphate of Potash_ formed adheres to the fibres, and if not carefully
washed out, an opalescent appearance is seen in the Collodion, resulting
from the insolubility of this salt in the ethereal mixture.
If no acid taste can be perceived, and a piece of blue litmus-paper
remains in contact with the fibres for five minutes without changing in
colour, the product is thoroughly washed. It is however a safe plan to
place the Pyroxyline in running water and allow it to remain for several
hours.
Lastly, wring it out in a cloth, pull out the fibres, and dry slowly, by
a moderate heat. After drying, it may be kept for any length of time in a
stoppered bottle.
RECAPITULATION OF THE GENERAL CHARACTERS OF PYROXYLINE PREPARED IN
NITRO-SULPHURIC ACID OF VARIOUS DEGREES OF CONCENTRATION.
_The acid mixture too strong._--The appearance of the cotton is not much
altered on its first immersion in the mixture. It washes well, without
any disintegration. On drying, it is found to be strong in texture, and
produces a peculiar crackling sensation between the fingers, like starch.
It explodes on the application of flame, without leaving any ash. It is
insoluble in the mixture of Ether and Alcohol, but dissolves if treated
with Acetic Ether.
_The acid mixture of the proper strength._--No agglutination of the fibres
of the cotton on immersion, and the product washes well; soluble in the
ethereal mixture, and yields a _transparent_ film on evaporation.
_The acid mixture too weak._--The fibres of the cotton agglutinate, and
the Pyroxyline is washed with difficulty. On drying, the texture is found
to be short and rotten. It does not explode on being heated, but either
burns quietly with a flame, leaving behind a black ash--in which case it
consists simply of unaltered cotton,--or is only slightly combustible,
and not explosive. It dissolves more or less perfectly in glacial Acetic
Acid. When treated with the ethereal mixture, it is acted on _partially_,
leaving behind lumps of unchanged cotton; the solution does not form an
even transparent layer on evaporation, but becomes opaque and cloudy as it
dries. This opacity however may be seen to a small extent with any sample
of Pyroxyline, if the solvents contain too much water.
In using Swedish Paper in place of Cotton, the Pyroxyline formed in too
weak a Nitro-Sulphuric Acid is usually insoluble in Ether and Alcohol, and
burns slowly like unchanged paper.
By studying these characters, and at the same time bearing in mind that _a
drachm and a half of water_ in the quantities of acid given in the formula
(p. 188) will suffice to cause the difference, the operator will overcome
all difficulties.
PURIFICATION OF THE SOLVENTS REQUIRED FOR COLLODION.
The purity of the Ether employed is a matter of as much importance in the
manufacture of a good Collodion as that of any other ingredient; this
point must be attended to in order to secure a good result.
There are four kinds of Ether sold by manufacturing chemists; first,
ordinary rectified Sulphuric Ether, containing a certain percentage of
Alcohol and of water; specific gravity about ·750. Second, the washed
Ether, which is the same agitated with an equal bulk of water, to remove
the Alcohol: by this proceeding the specific gravity of the fluid is
reduced considerably. Third, Ether both washed and re-rectified from a
caustic alkali, so as to contain neither Alcohol nor water; in this case
the specific gravity should not be higher than ·720. Fourth, "Methylated"
Ether, manufactured at a lower price than the others.
Rectified Ether of 750° is not to be depended on, inasmuch as the
specific gravity is often made up by adding water instead of alcohol.
Methylated Ether should be used only when economy is an object, as it is
prone to acidity and less certain in its properties.
Some of the qualities which render Ether unfit for Photographic purposes,
are as follows:--a peculiar and disagreeable smell, either of some
essential oil, or of Acetic Ether; an acid reaction to test-paper; a
property of turning alcoholic solution of Iodide of Potassium brown with
unusual rapidity; an alkaline reaction to test-paper; a high specific
gravity, from superabundance of Alcohol and water.
The Ether which has been both washed and redistilled is always the most
uniform in composition, and especially so if the second distillation be
conducted from Quicklime, Carbonate of Potash, or Caustic Potash. These
Alkaline substances retain the impurities, which are often of an acid
nature, and leave the Ether in a fit state for use.
The redistillation of Ether is a simple process: in dealing with this
fluid however the greatest caution must be exercised, on account of its
inflammable nature. Even in pouring Ether from one bottle into another, if
a light of any kind be near, the vapour is apt to take fire; and severe
injuries have been occasioned from this cause.
_Purification of Ether by redistillation from a caustic or carbonated
alkali._--Take ordinary rectified Sulphuric Ether, and agitate it with an
equal bulk of water to wash out the Alcohol; stand for a few minutes until
the contents of the bottle separate into two distinct strata, the lower of
which--_id est_, the watery stratum--is to be drawn off and rejected. Then
introduce Caustic Potash, finely powdered, in the proportion of about one
ounce to a pint of the washed Ether; shake the bottle again many times,
in order that the water--a small portion of which is still present in
solution in the Ether--may be thoroughly absorbed. Afterwards set aside
for twenty-four hours (not longer, or the Potash may begin to decompose
the Ether), when it will probably be observed that the liquid has become
yellow, and that a flocculent deposit has formed in small quantity.
Transfer to a retort of moderate capacity, supported in a saucepan of warm
water, and properly connected with a condenser. On applying a gentle heat,
the Ether distils over quietly, and condenses with very little loss; care
must be taken that none of the alkaline liquid contained in the body of
the retort finds its way, by projection or otherwise, into the neck, so as
to run down and contaminate the distilled fluid.
A more economical plan of purifying Ether is, without previous washing
with water, to agitate with Carbonate of Potash or with Quicklime, and
redistil at a moderate temperature.
In order to preserve Ether from decomposition, it must be kept in
stoppered bottles, nearly full, and in a dark place. The stoppers
should be tied over with bladder and luted, or a considerable amount of
evaporation will take place, unless the neck of the bottle has been ground
with unusual care. After the lapse of some months, probably a certain
amount of decomposition, evidenced by the liberation of Iodine on adding
Iodide of Potassium, will be found to have taken place. This however is
small in amount, and not of a character to injure the fluid.
Rectification of Spirits of Wine from Carbonate of Potash.--The object of
this operation is to remove a portion of water from the spirit, and so to
increase its strength. Alcohol thus purified may be added to Collodion
almost to any extent, without producing glutinosity and rottenness of film.
The salt termed Carbonate of Potash is a deliquescent salt,--that is, it
has a great attraction for water; consequently when Spirits of Wine are
agitated with Carbonate of Potash, a portion of water is removed, the salt
dissolving in it and forming a dense liquid, which refuses to mix with the
Alcohol, and sinks to the bottom. At the expiration of two or three days,
if the bottle has been shaken frequently, the action is complete, and the
lower stratum of fluid may be drawn off and rejected. _Pure_ Carbonate
of Potash is an expensive salt, and a commoner variety may be taken. It
should be well dried on a heated metal plate, and powdered, before use.
The quantity may be about two ounces to a pint of spirit; or more, if an
unusually concentrated Alcohol is required.
After the distillation is complete, a fluid is obtained containing about
90 per cent, of absolute Alcohol, the remaining 10 per cent, being water.
The specific gravity at 60° Fahrenheit should be from ·815 to ·825;
commercial Spirit of Wine being ·836 to ·840.
PREPARATION OF THE IODIZING COMPOUNDS IN A STATE OF PURITY.
These are the Iodides of Potassium, Ammonium, and Cadmium. The properties
of each are more fully described in Part III.
a. _The Iodide of Potassium._--Iodide of Potassium, as sold in the
shops, is often contaminated with various impurities. The first and most
remarkable is _Carbonate of Potash_. When a sample of Iodide of Potassium
contains much Carbonate of Potash, it forms small and imperfect crystals,
which are strongly alkaline to test-paper, and become moist on exposure to
the air, from the deliquescent nature of the Alkaline Carbonate. _Sulphate
of Potash_ is also a common impurity; it may be detected by Chloride of
Barium.
A third impurity of Iodide of Potassium is _Chloride_ of Potassium; it is
detected as follows:--Precipitate the salt by an equal weight of Nitrate
of Silver, and treat the yellow mass with solution of Ammonia; if any
Chloride of Silver is present, it dissolves in the Ammonia, and, after
filtration, is precipitated in white curds by the addition of an excess of
pure Nitric Acid. If the Nitric Acid employed is not pure, but contains
traces of free Chlorine, the Iodide of Silver must be well washed with
distilled water before treating it with Ammonia, or the excess of free
Nitrate of Silver dissolving in the Ammonia would, on neutralizing,
produce Chloride of Silver, and so cause an error.
_Iodate of Potash_ is a fourth impurity often found in Iodide of
Potassium: to detect it, add a drop of dilute Sulphuric Acid, or a crystal
of Citric Acid, to the solution of the Iodide; when, if much Iodate be
present, the liquid will become yellow from liberation of free Iodine. The
rationale of this reaction is as follows:--The Sulphuric Acid unites with
the base of the salt, and liberates Hydriodic Acid (HI), _a colourless
compound;_ but if Iodic Acid (IO{5}) be also present, it decomposes the
Hydriodic Acid first formed, oxidizing the Hydrogen into Water (HO), and
setting free the Iodine. The immediate production of a yellow colour
on adding a weak acid to aqueous solution of Iodide of Potassium, is
therefore a proof of the presence of an Iodate. As Iodate of Potash
renders Collodion insensitive, this point should be attended to.
Iodide of Potassium may be rendered very pure by recrystallizing from
Spirit, or by dissolving in strong Alcohol of sp. gr. ·823, in which
Sulphate, Carbonate, and Iodate of Potash are insoluble. The proportion of
Iodide of Potassium contained in saturated Alcoholic solutions varies with
the strength of the spirit (_vide_ Part III., article Iodide of Potassium).
Solution of Chloride of Barium is commonly used to detect impurities in
Iodide of Potassium; it forms a white precipitate if Carbonate, Iodate,
or Sulphate be present. In the two former cases the precipitate dissolves
on the addition of pure dilute Nitric Acid, but in the latter it is
insoluble. The commercial Iodide however is rarely so pure as to remain
quite clear on the addition of Chloride of Barium.
b. _The Iodide of Ammonium._--This salt may be prepared by adding
Carbonate of Ammonia to Iodide of Iron, but more easily by the following
process:--A strong solution of Hydrosulphate of Ammonia is first made, by
passing Sulphuretted Hydrogen gas into Liquor Ammoniæ. To this liquid,
Iodine is added until the whole of the Sulphuret of Ammonium has been
converted into Iodide. When this point is reached, the solution at once
colours brown from solution of free Iodine. On the first addition of
the Iodine, an escape of Sulphuretted Hydrogen gas and a dense deposit
of Sulphur take place. After the decomposition of the Hydrosulphate of
Ammonia is complete, a portion of Hydriodic Acid--formed by the mutual
reaction of Sulphuretted Hydrogen and Iodine--attacks any Carbonate
of Ammonia which may be present, and causes an effervescence. The
effervescence being over, the liquid is still acid to test-paper, from
excess of Hydriodic Acid; it is to be cautiously neutralized with Ammonia,
and evaporated by the heat of a water-bath to the crystallizing point.
The crystals should be thoroughly dried over a dish of Sulphuric Acid, and
then sealed in tubes; by this means it will be preserved colourless.
Iodide of Ammonium is very soluble in Alcohol, but it is not advisable
to keep it in solution, from the rapidity with which it decomposes and
becomes brown.
The most common impurity of commercial Iodide of Ammonium is Sulphate of
Ammonia; it is detected by its sparing solubility in Alcohol. Carbonate
of Ammonia is also frequently present to a large extent, in which case
an alkaline Collodion and eventually an alkaline Nitrate Bath will be
produced.
e. _Iodide of Cadmium._--This salt is formed by heating filings of
metallic Cadmium with Iodine, or by mixing the two together with addition
of water.
Iodide of Cadmium is very soluble both in Alcohol and Water; the solution
yielding on evaporation large six-sided tables of a pearly lustre, which
are permanent in the air. The commercial Iodide is sometimes contaminated
with Iodide of Zinc; the crystals being imperfectly formed and slowly
liberating Iodine when dissolved in Ether and Alcohol. Pure Iodide of
Cadmium remains nearly or quite colourless in Collodion, if the fluid be
kept in a cool and dark place.
CHAPTER II.
FORMULÆ FOR SOLUTIONS REQUIRED IN THE COLLODION PROCESS.
Section I.--Solutions for direct Positives.
Section II.--Solutions for Negative Photographs.
SECTION I.
_Formulæ for Solutions for direct Positives._
The solutions are taken in the following order;--The Collodion.--The
Nitrate Bath.--Developing fluids.--Fixing liquids.--Whitening solution.
THE COLLODION.
_Formula No. 1._
Purified Ether, sp. gr. ·720 5 fluid drachms.
Purified Alcohol, sp. gr. ·825 3 " "
Pyroxyline 3 to 5 grains.
Pure Iodide of Cadmium or Ammonium 4 grains.
_Formula No. 2._
Rectified Ether, sp. gr. ·750 6 fluid drachms.
Spirits of Wine, sp. gr. ·836 2 " "
Pyroxyline 2 to 4 grains.
Iodide of Potassium or Ammonium 3 to 4 "
If the operator wishes to prepare a stock of the plain Collodion, and to
iodize as required, the last formula will stand thus:--
Rectified Ether, ·750 3 fluid ounces.
Alcohol of ·836 2 fluid drachms.
Pyroxyline 8 to 14 grains.
Dissolve the Pyroxyline, and let the fluid stand for forty-eight hours to
subside, then draw off clear, with a siphon.
To each fluid ounce of this plain Collodion add about two fluid drachms of
the following iodizing mixture:--
Alcohol, sp. gr. ·836 1 fluid ounce.
Iodide of Potassium 16 grains.
Of the two formulæ above given, the first is considered the best, but the
second may be substituted for it when highly rectified spirits cannot be
obtained. Iodide of Ammonium chemically pure is perhaps superior to any
other Iodide for preparing a portrait Collodion, but Iodide of Cadmium,
with addition of free Iodine, possesses better keeping properties, and
gives very good results. A mixture of the two Iodides may also be used
advantageously, or Iodide of _Potassium_ may be combined with Iodide of
Cadmium: this preparation has been much recommended, but the Collodion
will be liable to produce a spotted film unless the salts are quite pure.
The exact quantity of Pyroxyline will vary with the temperature at which
the preparation was made. The Collodion should flow smoothly on the glass
and remain free from crapy lines on setting. When Iodide of Cadmium is
used, the tendency to glutinosity will be a little greater than usual,
which must be obviated by the directions given at page 83.
The film, after dipping in the Bath, should appear opalescent and not too
yellow and creamy. Pale-blue films yield very good Positives, but with
more liability to failure than thicker films (p. 109).
If the Positives are not perfectly clear and transparent in the
shadows, dissolve 5 grains of Iodine in an ounce of Spirits of Wine
(not methylated), and add a few drops until the Collodion assumes a
golden-yellow colour.
In hot weather advantage will be gained by somewhat increasing the
quantity of Alcohol in Collodion; the evaporation of the solvents
being retarded, and the film rendered less liable to become dry before
development. _Anhydrous_ Alcohol of Sp. Gr. ·796, may be mixed with pure
Ether of ·715, even to the extent of equal parts; but this is the extreme
limit, and with the strongest spirit ordinarily obtainable, the Collodion
will often become somewhat glutinous if the proportions (by measure) of 5
parts of Ether to 3 of Alcohol be exceeded.
Collodion prepared by Formula No. 1, and iodized with Iodide of Cadmium,
may be kept for weeks or months without much loss of sensitiveness; but
when Alkaline Iodides are employed as in the second Formula, Iodine is
liberated, and the fluid becomes at last brown and insensitive.
THE NITRATE BATH.
Nitrate of Silver 30 grains.
Nitric Acid 1/20 minim, or Acetic
Acid (glacial) 1/6 minim.
Alcohol 15 minims.
Distilled water 1 fluid ounce.
Nitrate of Silver which has been melted, in order to expel Oxides of
Nitrogen, is always the most certain in its action: but the heat must not
be raised too high or the salt will be contaminated with _Nitrite_ of
Silver.
In the Vocabulary (see Part III.) directions are given for the preparation
and purification of Nitrate of Silver; also for the testing of distilled
water, and the best substitutes when it cannot be obtained.
The Bath must be saturated with Iodide of Silver, and Nitric Acid
neutralized if it be present. Nitrate of Silver however which has
undergone fusion is free from Nitric Acid.
Weigh out the total quantity of crystals of Nitrate required for the Bath,
and dissolve in about two parts of water. Then take a quarter of a grain
of Iodide of Potassium to each 100 grains of Nitrate, dissolve in half
a drachm of water, and add to the strong solution; a yellow deposit of
Iodide of Silver first forms, but on stirring is completely re-dissolved.
When the liquid is clear, test for free Nitric Acid by dropping in a
piece of blue litmus-paper. If at the expiration of two minutes the paper
appears _reddened_, Nitric Acid is present, to neutralize which, add
solution of Potash or Carbonate of Soda (not Ammonia) until a distinct
turbidity, remaining after agitation, is produced (an excess does no
harm). Then dilute down the concentrated solution with the remaining
portion of the water, stirring all the time, and filter out the milky
deposit. If the liquid does not at first run clear, it will probably do so
on passing it again through the same filter.
Lastly, add the Acetic Acid (previously tested for impurities, see Part
III.) and the Alcohol to the filtered liquid.
As the bulk of the Bath becomes lessened by use, fill it up with a
solution containing 40 grains of Nitrate to the ounce, which will be found
sufficient to maintain the strength nearly at the original point.
The common practice of occasionally dropping Ammonia or Potash into the
solution, to remove Nitric Acid liberated by free Iodine in the Collodion,
is not recommended (see p. 89).
When the Bath becomes old, and yields Positives which are highly intense
or stained, and slightly foggy, with a deficiency of half-tone, it will be
advisable to precipitate it with a Chloride and prepare a new one.
THE DEVELOPING FLUIDS.
Either of the three following formulæ may be used, according to the taste
of the operator:--
FORMULA No. 1.
Sulphate of Iron, recrystallized 12 to 20 grains.
Acetic Acid (glacial) 20 minims.
Alcohol 10 minims.
Water 1 fluid ounce.
FORMULA No. 2.
Pyrogallic Acid 2 grains.
Nitric Acid 1 drop.
Water 1 fluid ounce.
FORMULA No. 3.
Solution of Protonitrate of Iron 1 fluid ounce.
Alcohol 20 minims.
In all these formulæ, if distilled water is not at hand, read the
directions in the Vocabulary, Part III., Article "Water," for the best
substitute.
_Remarks upon these Formulæ._--_Formula No. 1_ is the most simple, since
the solution can be used _as a Bath_, the same portion being employed
many times successively. If it acts too rapidly, lessen the proportion of
Sulphate of Iron. An addition of Nitric Acid, half a minim to the ounce,
makes the image whiter and more metallic; but if too much is used, the
development proceeds irregularly, and spangles of Silver are formed.
The Alcohol and Acetic Acid render the development uniform by causing
the solution of Protosulphate to combine more readily with the film.
The latter also has an effect in whitening the image and increasing its
brightness.
Solution of Sulphate of Iron becomes red on keeping, from a gradual
formation of _per_salt. When it is too weak, add more of the
Protosulphate. The muddy deposit which settles to the bottom of the Bath
is metallic Silver, reduced from the soluble Nitrate upon the plates.
Some operators add pure Nitrate of Potash to this developing solution, to
form a _small portion_ of Protonitrate of Iron. It is said to improve the
colour slightly. The proportions are 10 grains of Nitrate of Potash to
about 14 or 15 grains of Protosulphate of Iron.
_Formula No. 2._--In this formula, if the colour of the image is not
sufficiently white, try the effect of increasing the amount of Nitric Acid
slightly. On the other hand, if the development is imperfect in parts,
and patches of a green colour are seen, use _three grains_ of Pyrogallic
Acid to the ounce, with less Nitric Acid. A few drops of Nitrate of Silver
solution added to the Pyrogallic, immediately before use, will augment the
energy of development when blue and green spots occur.
_Formula No. 3_, or Protonitrate of Iron, does not require any addition
of Acid; but it will be advisable, in some cases, to add to it a few
drops of Nitrate of Silver immediately before developing. It gives a
bright metallic image, resembling that obtained by adding Nitric Acid to
Protosulphate of Iron.
The following process is commonly followed for preparing Protonitrate of
Iron:--
Take of Nitrate of Baryta 300 grains;--powder and dissolve by the aid of
heat in three ounces of water. Then throw in by degrees, with constant
stirring, crystallized Sulphate of Iron, _powdered_, 320 grains. Continue
to stir for about five or ten minutes. Allow to cool, and filter from the
white deposit, which is the insoluble Sulphate of Baryta.
In place of Nitrate of Baryta, the Nitrate of Lead may be used (Sulphate
of Lead being an insoluble salt), but the quantity required will be
different. The atomic weights of Nitrate of Baryta and Nitrate of Lead are
as 131 to 166; consequently 300 grains of the former are equivalent to 380
grains of the latter.
THE FIXING SOLUTION.
Cyanide of Potassium 2 to 12 grains.
Common Water 1 fluid ounce.
Cyanide of Potassium is usually preferred to Hyposulphite of Soda for
fixing direct Positives; it is less liable to injure the purity of the
white colour. The percentage of _Carbonate of Potash_ in commercial
Cyanide of Potassium is so variable that no exact directions can be given
for the formula. It is best however to use it rather dilute--of such a
strength that the plate is cleared gradually in from half a minute to a
minute.
The solution of Cyanide of Potassium decomposes slowly on keeping, but
it will usually retain its solvent power for several weeks. In order to
escape inconvenience from the pungent odour evolved by this salt, many
employ a vertical Bath to hold the solution; but in that case the plates
must be carefully washed before fixing, as the Iron salts hasten the
decomposition of the Cyanide.
THE WHITENING SOLUTION.
Bichloride of Mercury 30 grains.
Distilled Water 1 fluid ounce.
By a gentle application of heat the corrosive sublimate dissolves and
forms a solution as nearly as possible saturated at common temperatures.
The addition of a portion of Muriatic Acid enables the water to take up a
larger quantity of Bichloride; but this concentrated solution, at the same
time that it whitens more quickly than the other, is apt to act unequally
upon different parts of the image.