Acid, a mixture of Nitric and Sulphuric Acids in certain proportions be
used, the effect is peculiar. The fibres contract slightly, but undergo
no other visible alteration. Hence we are at first disposed to think the
mixed Acids ineffectual. This idea however is not correct, since on making
the experiment the properties of the cotton are found to be changed. Its
weight has increased by more than one-half; it has become soluble in
various liquids, such as Acetic Ether, Ether and Alcohol, etc., and, what
is more remarkable, it no longer burns in the air quietly, but explodes on
the application of flame with greater or less violence.
This change of properties clearly shows, that although the fibrous
structure of the material is unaffected, it is no longer the same
substance, and consequently chemists have assigned it a different name,
viz. Pyroxyline.
To produce the peculiar change by which Cotton is converted into
Pyroxyline, both Nitric and Sulphuric Acids are, as a rule, required; but
of the two the former is the most important. On analyzing Pyroxyline,
Nitric Acid, or a body analogous to it, is detected in considerable
quantity, but not Sulphuric Acid. The latter Acid, in fact, serves but
a temporary purpose, viz. to prevent the Nitric Acid from dissolving
the pyroxyline, which it would be liable to do if employed alone. The
Sulphuric Acid prevents the solution by removing water from the Nitric
Acid, and so producing a higher degree of concentration; Pyroxyline,
although soluble in a dilute, is not so in the strong Acid, and hence it
is preserved.
The property possessed by Oil of Vitriol of removing water from other
bodies, is one with which it is well to be acquainted. A simple experiment
will serve to illustrate it. Let a small vessel of any kind be filled to
about two-thirds with Oil of Vitriol, and set aside for a few days; at the
end of that time, and especially if the atmosphere be damp, it will have
absorbed sufficient moisture to cause it to flow over the edge.
Now even the strongest reagents employed in chemistry contain, almost
invariably, water in greater or lesser quantity. Pure Anhydrous Nitric
Acid is a white, solid substance; Hydrochloric Acid is a gas: and the
liquids sold under those names are merely solutions. The effect then of
mixing strong Oil of Vitriol with aqueous Nitric Acid is to remove water
in proportion to the amount used, and to produce a liquid containing
Nitric Acid in a high state of concentration, and Sulphuric Acid more or
less diluted. This liquid is the Nitro-Sulphuric Acid employed in the
preparation of Pyroxyline.
Various forms of Pyroxyline.--Very soon after the first announcement of
the discovery of Pyroxyline, most animated discussions arose amongst
chemists with regard to its solubility and general properties. Some
spoke of a "solution of Gun-Cotton in Ether;" whilst others denied its
solubility in that menstruum; a third class, by following the process
described, obtained a substance which was not explosive, and therefore
could scarcely be termed Gun-Cotton.
On further investigations some of these anomalies were cleared up, and
it was found that there were varieties of Pyroxyline, depending mainly
upon the degree of strength of the Nitro-Sulphuric Acid employed in the
preparation. Still the subject was obscure until the publication of
researches by Mr. E. A. Hadow. These investigations, conducted in the
Laboratory of King's College, London, were published in the Journal of the
Chemical Society. Constant reference will be made to them in the following
remarks.
We notice--first, the chemical constitution of Pyroxyline; secondly, its
varieties; and thirdly, the means adopted to procure a Nitro-Sulphuric
Acid of the proper strength.
a. _Constitution of Pyroxyline._--Pyroxyline has been sometimes spoken
of as a Salt of Nitric Acid, a Nitrate of Lignine. This view however is
erroneous, since it can be shown that the substance present is not Nitric
Acid, although analogous to it. It is the Peroxide of Nitrogen, which is
intermediate in composition between Nitrous Acid (NO{3}) and Nitric Acid
(NO{5}). Peroxide of Nitrogen (NO{4}) is a gaseous body of a dark red
colour; it possesses no acid properties, and is incapable of forming a
class of salts. In order to understand in what state this body is combined
with cotton fibre to form Pyroxyline, it will be necessary to digress for
a short time.
Law of Substitution.--By the careful study of the action of Chlorine,
and of Nitric Acid, upon various organic substances, a remarkable series
of compounds has been discovered, containing a portion of Chlorine or
of Peroxide of Nitrogen in the place of Hydrogen. The peculiarity of
these substances is, that they strongly resemble the originals in their
physical, and often in their chemical properties. It might have been
supposed that agents of such active chemical affinities as Chlorine and
Oxide of Nitrogen would, by their mere presence in a body, produce a
marked effect; yet it is not so in the case before us. The primitive type
or constitution of the substance modified remains the same, even the
crystalline form being often unaffected. It seems as if the body by which
the Hydrogen had been displaced had stepped in quietly and taken up its
position in the framework of the whole without disturbance. Many compounds
of this kind are known; they are termed by chemists "substitution
compounds." The law invariably observed is, that the substitution takes
place in equal atoms: a single atom of Chlorine, for instance, displaces
one of Hydrogen; two of Chlorine displace two of Hydrogen, and so on,
until, in some cases, the whole of the latter element is separated.
In illustration of these remarks, take the following instances:--Acetic
Acid contains Carbon, Hydrogen, and Oxygen; by the action of Chlorine
the Hydrogen may be removed in the form of Hydrochloric Acid, and an
equal number of atoms of Chlorine be substituted. In this way a new
compound is formed, termed _Chloracetic Acid_, resembling in many
important particulars the Acetic Acid itself. Notice particularly that
the peculiar properties characteristic of Chlorine are completely masked
in the substitution body, and no indication of its presence is obtained
by the usual tests! A soluble _Chloride_ gives with Nitrate of Silver a
white precipitate of Chloride of Silver, unaffected by Acids, but the
Chloracetic Acid does not; hence it is plain that the Chlorine exists in a
peculiar and Ultimate state of combination different from what is usual.
The substance we have been previously considering, viz. Pyroxyline,
affords another illustration of the Law of Substitution. Omitting, for
the sake of simplicity, the number of atoms concerned in the change,
the action of concentrated Nitric Acid upon ligneous fibre may be thus
explained:--
{ Carbon
Cotton _or_ { Hydrogen + Nitric Acid
{ Hydrogen
{ Oxygen
equals
{ Carbon
Pyroxyline _or_ { Hydrogen + Water
{ Peroxide Nitrogen
{ Oxygen
Or in symbols:--
CH{11}O + NO{5} = C (H{n-1}NO{4}) O + HO
By a reference to the formula, it is seen that the fifth atom of Oxygen
contained in the Nitric Acid takes one of Hydrogen, and forms an atom of
Water; the NO{4} then steps in, to fill the gap which the atom of Hydrogen
has left. All this is done with so little disturbance that even the
fibrous structure of the cotton remains as before.
b. _Chemical Composition of the varieties of Pyroxyline._--Mr. Hadow has
succeeded in establishing _four_ different substitution compounds, which,
as no distinctive nomenclature has been at present proposed, may be termed
compounds A, B, C, and D.
_Compound A_ is the most explosive Gun-Cotton, and contains the largest
amount of Peroxide of Nitrogen. It dissolves _only in Acetic Ether_, and
is left on evaporation as a white powder. It is produced by the strongest
Nitro-Sulphuric Acid which can be made.
_Compounds B and C_, either separate or in a state of mixture, form the
soluble material employed by the Photographer. They both dissolve in
Acetic Ether, and also in a mixture of Ether and Alcohol. The latter,
viz. C, also dissolves in glacial Acetic Acid. They are produced by a
Nitro-Sulphuric Acid slightly weaker than that used for A, and contain a
smaller amount of Peroxide of Nitrogen.
_Compound D_ resembles what has been termed _Xyloidine_, that is, the
substance produced by acting with Nitric Acid upon Starch. It contains
less Peroxide of Nitrogen than the others, and dissolves in Ether
and Alcohol, and also in Acetic Acid. The ethereal solution leaves,
on evaporation, an opaque film, which is highly combustible, but not
explosive.
By bearing in mind the properties of these compounds, many of the
anomalies complained of in the manufacture of Gun-Cotton disappear. If the
Nitro-Sulphuric Acid employed is too strong, the product will be insoluble
in Ether; whilst if it is too weak, the fibres are gelatinized by the Acid
and partly dissolved.
c. _Means adopted to procure a Nitro-Sulphuric Acid of the requisite
strength for preparing Pyroxyline._--This is a point of more difficulty
than would at first appear. It is easy to determine an exact formula for
the mixture, but not so easy to hit upon the proper proportions of the
acids required to produce that formula; and a very slight departure from
them altogether modifies the result. The main difficulty lies in _the
uncertain strength of commercial Nitric Acid_. Oil of Vitriol is more to
be depended upon, and has a tolerably uniform Sp. Gr. of 1·836;[13] but
Nitric Acid is constantly liable to variation; hence it becomes necessary
to make a preliminary determination of its real strength, which is done
either by taking the specific gravity and referring to tables, or, better
still, by a direct analysis. As each atom of Sulphuric Acid removes only a
given quantity of water, it follows that the weaker the Nitric Acid, the
larger the amount of Sulphuric which will be required to bring it up to
the proper degree of concentration.
[13] The later experience of the writer induces him to believe, that the
specific gravity of Oil of Vitriol cannot always be taken as an indication
of its real strength; which is best ascertained by analysis.
To avoid the trouble necessarily attendant upon these preliminary
operations, many prefer to use, in place of Nitric Acid itself, one of
the salts formed by the combination of Nitric Acid with an alkaline
base. The composition of these salts, provided they are pure and nicely
crystallized, can be depended on.
Nitrate of Potash, or _Saltpetre_, contains a single atom of Nitric Acid
united with one of Potash. It is an _anhydrous_ salt, that is, it has
no water of crystallization. When strong Sulphuric Acid is poured upon
Nitrate of Potash in a state of fine powder, in virtue of its superior
chemical affinities it appropriates to itself the Alkali and liberates the
Nitric Acid. If care be taken to add a sufficient excess of the Sulphuric
Acid, a solution is obtained containing Sulphate of Potash dissolved in
Sulphuric Acid, and free Nitric Acid. The presence of the Sulphate of
Potash (or, more strictly speaking, of the _Bi_-Sulphate) does not in any
way interfere with the result, and the effect is the same as if the mixed
acids themselves had been used.
The reaction may be thus represented:--
Nitrate of Potash _plus_ Sulphuric Acid in excess
= Bisulphate Potash _plus_ Nitro-Sulphuric Acid.
CHEMISTRY OF THE SOLUTION OF PYROXYLINE IN ETHER AND ALCOHOL, OR
"COLLODION."
The substitution compounds B and C, already alluded to as forming the
Soluble Cotton of Photographers, are both abundantly soluble in Acetic
Ether. This liquid however is not adapted for the purpose required,
inasmuch as on evaporation it leaves the Pyroxyline in the form of a white
powder, and not as a transparent layer.
The rectified Ether of commerce has been found to answer better than any
other liquid as a solvent for Pyroxyline.
If the sp. gravity be about ·750, it contains invariably a small
proportion of _Alcohol_, which appears to be necessary; the solution not
taking place with absolutely pure Ether. The Pyroxyline, if properly
prepared, begins almost immediately to gelatinize by the action of the
Ether, and is soon completely dissolved. In this state it forms a slimy
solution, which, when poured out on a glass plate, dries up into a horny
transparent layer.
In preparing Collodion for Photographic purposes, we find that its
physical properties are liable to considerable variation. Sometimes
it appears very thin and fluid, flowing on the glass almost like
water, whilst at others it is thick and glutinous. The causes of these
differences will now engage our attention. They may be divided into two
classes: first, those relating to the Pyroxyline; second, to the solvents
employed.
a. _Variation of Properties in different Samples of soluble
Pyroxyline._--The substitution compounds A, B, C, and D differ, as already
shown, in the percentage amount of Peroxide of Nitrogen present, and the
former are more explosive and insoluble than the latter. But it often
happens in preparing Pyroxyline, that two portions of Nitro-Sulphuric
Acid taken from the same bottle yield products which vary in properties,
although they are necessarily the same in composition.
Taking _extremes_ in illustration, we notice two principal modifications
of soluble Pyroxyline.
The first, when treated with the mixture of Ether and Alcohol, sinks down
to a gummy or gelatinous mass, which gradually dissolves on agitation The
solution is very fluid in proportion to the number of grains used, and
when poured out spreads into a beautifully smooth and glassy surface,
which is quite structureless, even when highly magnified. The film adheres
tightly to the glass, and when the finger is drawn across it, separates in
short fragments, and broken pieces.
The second variety produces a Collodion which is thick and glutinous,
flowing over the glass in a slimy manner, and soon setting into numerous
small waves and cellular spaces. The film lies loose upon the glass, is
apt to contract on drying, and may be pushed off by the finger in the form
of a connected skin.
This subject is not thoroughly understood, but it is known that the
_temperature_ of the Nitro-Sulphuric Acid at the time of immersing the
Cotton influences the result. The soluble variety is produced by _hot_
acids; the second, or glutinous, by the same acids employed cold, or
only slightly warm. The best temperature appears to be from 130° to 155°
Fahrenheit; if it rises much beyond that point, the acids act upon and
dissolve the Cotton.
b. _The physical properties of Collodion affected by the proportions and
purity of the Solvents._--Pyroxyline of the varieties termed B and C
dissolves freely in a mixture of Ether and Alcohol; but the characters
of the resulting solution vary with the relative proportions of the two
solvents.
When the Ether is in large excess, the film is inclined to be strong and
tough, so that it can often be raised by one corner and lifted completely
off the plate without tearing. It is also very contractile, so that a
portion of the Collodion poured on the hand draws together and puckers the
skin as it dries. If spread upon a glass plate in the usual way, the same
property of contractility causes it to retract and separate from the sides
of the glass.
These properties, produced by Ether in large proportion, disappear
entirely on the addition of more Alcohol. The transparent layer is now
soft and easily torn, possessing but little coherency. It adheres to the
surface of the glass more firmly, and exhibits no tendency to contract and
separate from the sides.
From these remarks it will be gathered that an excess of Ether, and a
low temperature in preparing the Pyroxyline, both favour the production
of a contractile Collodion; whilst on the other hand an abundance of
Alcohol, and a hot Nitro-Sulphuric Acid, tend to produce a short and
non-contractile Collodion.
The physical properties of Collodion are affected by another cause, viz.
by the _strength_ and purity of the solvents, or, in other words, their
freedom from dilution with water. If a few drops of water be purposely
added to a sample of Collodion, the effect is seen to be to precipitate
the Pyroxyline in flakes to the bottom of the bottle. There are many
substances known in chemistry which are soluble in spirituous liquids, but
behave in the same manner as Pyroxyline in this respect.
The manner in which water gains entrance into the Photographic Collodion
is usually by the employment of Alcohol or Spirit of Wine which has not
been highly rectified. In that case the Collodion is thicker, and flows
less readily than if the Alcohol were stronger. Sometimes the texture of
the film left upon evaporation is injured; it is no longer homogeneous and
transparent, but semi-opaque, reticulated, or honeycombed, and so rotten
that a stream of water projected upon the plate washes it away.
These effects are to be attributed not to the Alcohol, but to the water
introduced with it; and the remedy will be to procure a stronger spirit,
or, if that cannot be done, to increase the amount of Ether. Collodion
prepared with a large proportion of Ether, and water, but a small quantity
of Alcohol, is often very fluid and structureless at first, adhering to
the glass with some tenacity and having a short texture; but it tends to
become rotten when used to coat many plates successively, the water on
account of its lesser volatility accumulating in injurious quantity in the
last portions.
THE COLORATION OF IODIZED COLLODION EXPLAINED.
Collodion iodized with the Iodides of Potassium, Ammonium, or Zinc,
soon assumes a yellow tint, which in the course of a few days or weeks,
according to the temperature of the atmosphere, deepens to a full brown.
This gradual coloration, due to a development of Iodine, is caused partly
by the Ether and partly by the Pyroxyline.
Ether may, with proper precautions, be preserved for a long time in a
pure state, but on exposure to the joint action of air and light it
undergoes a slow process of oxidation, attended with formation of Acetic
Acid and a peculiar principle resembling in properties ozone, or Oxygen
in an allotropic and active condition. Iodide of Potassium or Ammonium is
decomposed by Ether in this state. Acetate of the Alkali, and Hydriodic
Acid (HI), being first produced. The ozonized substance then removes
Hydrogen from the latter compound, and liberates Iodine, which dissolves
and tinges the liquid yellow.
A simple solution of an Alkaline Iodide in Alcohol and Ether does not,
however, become so quickly coloured as Iodized Collodion; and hence it is
evident that the presence of the Pyroxyline produces an effect. It may
be shown that Alkaline Iodides slowly decompose Pyroxyline, and that a
portion of Peroxide of Nitrogen is set free: this body, containing loosely
combined oxygen, tends powerfully to eliminate Iodine, as may be seen by
adding a few drops of the yellow commercial Nitrous acid to a solution of
Iodide of Potassium.
The _stability_ of the particular Iodide used in Iodizing Collodion,
influences mainly the rate of coloration, though elevation of temperature
and exposure to light are not without effect. Iodide of Ammonium is the
least stable, and Iodide of Cadmium the most so; Iodide of Potassium being
intermediate. Collodion iodized with _pure_ Iodide of Cadmium usually
remains nearly colourless to the last drop, if kept in a cool and dark
place.
As the presence of free Iodine in Collodion affects its photographic
properties, it may sometimes be necessary to remove it. This is done by
inserting a strip of Silver-foil; which decolorizes the liquid, by forming
Iodide of Silver, soluble in the excess of Alkaline Iodide (p. 42).
Metallic Cadmium, and metallic Zinc, have the same effect.
When Methylated Spirits are employed in the manufacture of Collodion,
the Iodine first liberated is afterwards either partially or entirely
reabsorbed, the liquid acquiring at the same time an acid reaction to
test-paper.
SECTION II.
_The Chemistry of the Nitrate Bath._
The solution of Nitrate of Silver in which the plate coated with iodized
Collodion is dipped, to form the layer of Iodide of Silver, is known
technically as _the Nitrate Bath_. The chemistry of Nitrate of Silver
has been explained at page 13, but there are some points relating to the
properties of its aqueous solution which require a further notice.
_Solubility of Iodide of Silver in the Nitrate Bath._--Aqueous solution of
Nitrate of Silver may be mentioned in the list of solvents of Iodide of
Silver. The proportion dissolved is in all cases small, but it increases
with the _strength_ of the solution. If no attention were paid to this
point, and the precaution of previously saturating the Nitrate Bath with
Iodide of Silver neglected, the film would be dissolved when left too long
in the liquid.
This solvent power of Nitrate of Silver on the Iodide is well shown by
taking the excited Collodion plate out of the Bath, and allowing it to dry
spontaneously. The layer of Nitrate on the surface, becoming concentrated
by evaporation, eats away the film, so as to produce a transparent,
spotted appearance.
In the solution of Iodide of Silver by Nitrate of Silver a _double
salt_ is formed, which corresponds in properties to the double Iodide
of Potassium and Silver in being _decomposed_ by the addition of water.
Consequently, in order to saturate a Bath with Iodide of Silver it is
only necessary to dissolve the total weight of Nitrate of Silver in a
small bulk of water, and to add to it a few grains of an Iodide; perfect
solution takes place, and on subsequent dilution with the full amount of
water, the excess of Iodide of Silver is precipitated in the form of a
milky deposit.
_Acid condition of Nitrate of Silver._--A solution of _pure Nitrate of
Silver_ is neutral to blue litmus-paper, but that prepared from the
commercial Nitrate has usually an acid reaction; the crystals having been
imperfectly drained from the acid mother-liquor in which they were formed.
Hence, in making a new Bath it is often advisable not only to saturate it
with Iodide of Silver, but to neutralize the free Nitric acid it contains.
There is also a peculiar condition of Nitrate of Silver crystallized from
a solution of the metal in Nitric Acid, which renders it quite unfit for
photographic purposes (see p. 101). It is thought to depend upon the
presence of an oxide of Nitrogen, possibly of Nitrous Acid, and the remedy
is to dry the crystals very strongly, or, better still, to fuse them at a
moderate heat: mere neutralization with Carbonate of Soda does not suffice.
In melting Nitrate of Silver great care should be taken not to raise the
heat so high as to decompose the salt, or a basic Nitrite will be formed,
which affects the properties of the solution (p. 13): fused Nitrate of
Silver ought, when cold, to be quite white, and to dissolve perfectly in
water without leaving any residue. The only objection to the employment
of Nitrate of Silver in this form is the facility with which it may be
adulterated with Nitrates of Potash and Soda, the presence of which would
lessen the available strength of the Bath.
The Nitrate Bath, although perfectly neutral when first prepared, may
become _acid_ by continued use, if Collodion containing much _free
Iodine_ be constantly employed. In that case a portion of Nitric Acid is
liberated, thus:--
Nitrate of Silver + Iodine
= Iodide of Silver + Nitric Acid + Oxygen.
When Collodion is iodized entirely with alkaline Iodides, it liberates
Iodine by keeping; and hence the occasional use of Ammonia may be required
to remove acidity from the Bath. But since the introduction of the Iodide
of Cadmium, which preserves the Collodion nearly or quite colourless, the
necessity for neutralizing Nitric Acid in the Bath has ceased.
_Alkaline condition of the Bath._--By "alkalinity" of the Bath is meant
a condition in which the blue tint is rapidly restored to reddened
litmus-paper. This indicates that an Oxide of some kind is present in
solution, which, by combining with the acid in the reddened paper,
neutralizes it and removes the red colour.
If a small portion of caustic Potash or Ammonia be added to a strong
solution of Nitrate of Silver, it produces a brown precipitate, which is
Oxide of Silver.
Ammonia + Nitrate of Silver
= Oxide of Silver + Nitrate Ammonia.
The solution however, from which the precipitate has separated, is not
left in a neutral state, but possesses a faint alkaline reaction. Oxide
of Silver and Carbonate of Silver are also _abundantly_ soluble in water
containing Nitrate of Ammonia; which salt is continually accumulating in
the Bath when compounds of Ammonium are used for iodizing.
An alkaline Bath is perhaps of all conditions the one most fatal to
success in photography. It leads to that universal darkening of the film
on applying the developer to which the name of "fogging" has been given.
Hence care must be used in adding to the Bath substances which tend to
make it alkaline.
Collodion containing free Ammonia, often sold in the shops, gradually does
so. The use of Potash, Carbonate of Soda, Chalk, or Marble, to remove free
Nitric Acid from the Bath, has the same effect; and hence, when they are
employed, a trace of Acetic acid must afterwards be added.
The mode of testing a bath for alkalinity is as follows:-- a strip of
porous blue litmus-paper is taken and held to the mouth of a bottle of
glacial Acetic acid until it becomes reddened; it is then placed in the
liquid to be examined and left for ten minutes or a quarter of an hour. If
Oxide of Silver be present in solution, the original blue colour of the
paper will slowly but gradually be restored.
_Occasional formation of Acetate of Silver in the Nitrate Bath._--In
preparing a new Bath, if the crystals of Nitrate of Silver are acid, it is
usual to add an alkali in small quantity. This removes the Nitric Acid,
but leaves the solution faintly alkaline. Acetic Acid is then dropped in,
which, by combining with the Oxide of Silver, forms Acetate of Silver.
Acetate of Silver is not formed by the simple addition of Acetic Acid to
the Bath, because its production under such circumstances would imply the
liberation of Nitric Acid; but if an alkali be present to neutralize the
Nitric Acid, then the double decomposition takes place, thus--
Acetate of Ammonia + Nitrate of Silver
= Acetate of Silver + Nitrate of Ammonia.
Acetate of Silver is a white flaky salt, sparingly soluble in water. It
dissolves in the Bath only in small proportion, but yet sufficiently to
affect the Photographic properties of the film (see p. 111 and 117). The
observance of the following simple rules will obviate its production in
injurious quantity:--_First_, when it is required to remove free Nitric
Acid from a bath _not containing Acetic Acid_ a solution of Potash or
Carbonate of Soda may be dropped in _freely_; but the liquid must be
filtered before adding any Acetic Acid, otherwise the brown deposit of
Oxide of Silver will be taken up by the Acetic Acid, and the Bath will
be charged with Acetate of Silver. _Secondly_, in dealing with a Bath
containing both Nitric and Acetic Acids, employ an alkali _much diluted_
(Liquor Ammoniæ with 10 parts of Water), and add a single drop at a
time, coating and trying a plate between each addition; the Nitric Acid
will neutralize itself before the Acetic, and with care there will be no
formation of Acetate of Silver in quantity.
_Substances which decompose the Nitrate Bath._--Most of the common
metals, having superior affinity for Oxygen, separate the Silver from a
solution of the Nitrate; hence the Bath must be kept in glass, porcelain,
or gutta-percha, and contact with Iron, Copper, Mercury, etc., must
be avoided, or the liquid will be discoloured, and a black deposit of
metallic Silver precipitated.
All developing agents, such as Gallic and Pyrogallic Acids, the Protosalts
of Iron, etc., blacken the Nitrate Bath, and render it useless by reducing
metallic Silver.
Chlorides, Iodides, and Bromides produce a deposit in the Bath; but the
solution, although weakened, may again be used after passing through a
filter.
Hyposulphites, Cyanides, and all fixing agents decompose Nitrate of Silver.
Organic matters, generally, reduce Nitrate of Silver, either with or
without the aid of light. Grape Sugar, Albumen, Serum of Milk containing
caseine, etc., blacken the Bath, even in the dark. Alcohol and Ether
act more slowly, and produce no injurious effect unless the liquid is
constantly exposed to light.
These facts indicate that the Nitrate Bath containing volatile organic
matters must be preserved in a dark place; also that it should be kept
exclusively for sensitizing the Collodion plates, and not used in floating
papers intended for the printing process.
_Changes in the Nitrate Bath by use._--The solution of Nitrate of Silver
employed in exciting the Collodion film gradually decreases in strength,
but not so quickly as the Bath used in sensitizing papers for printing. If
the amount of Nitrate be allowed to fall as low as twenty grains to the
ounce of water, the decomposition will be imperfect, and the film will be
pale and blue, even with a highly iodized Collodion.
A gradual accumulation of Ether and Alcohol also takes place in the Bath
after long use, in consequence of which the developing solutions flow
less readily upon the Collodionized plates, and oily stains are apt to be
produced.
Diminished sensitiveness of the Iodide film is sometimes traced to
impurities in the Bath, when it is very old, and has been much used. These
are probably of an organic nature and may often be partially removed
by agitation with kaolin, or animal charcoal. The latter however is
objectionable, being usually contaminated with _Carbonate of Lime_, which
makes the Bath alkaline; or (in the case of _purified_ animal charcoal)
with traces of Hydrochloric Acid, which liberate Nitric Acid in the Bath.
Even the kaolin may as a preliminary precaution be washed with dilute
Acetic Acid to remove Carbonate of Lime if any should be present.
SECTION III.
_The Conditions which affect the Formation and Development of the Latent
Image in the Collodion process._
It will be necessary to preface the observations contained in this Section
by defining two terms which are frequently confounded with each other, but
are in reality of distinct meaning. These terms are "Sensitiveness" and
"Intensity."
By Sensitiveness is meant a facility of receiving impression from very
feeble rays of light, or of receiving it quickly from brighter rays.
Intensity, on the other hand, relates to the appearance of the finished
Photograph, independently of the time taken to produce it,--_to the degree
of opacity of the image_, and the extent to which it obstructs transmitted
light.
It will be seen as we proceed that the conditions necessary to obtain
extreme sensitiveness of the Iodide film are different from, and often
opposed to, those which give the maximum intensity of image.
CAUSES WHICH INFLUENCE THE SENSITIVENESS OF IODIDE OF SILVER ON COLLODION.
Some of the most important are as follows:--
a. _The presence of free Nitrate of Silver._--When the sensitive film is
removed from the Nitrate Bath, the Iodide of Silver is left in contact
with excess of Nitrate of Silver. The presence of this compound is not
_essential_ to the action of the light, since, if it be removed by washing
in distilled water, the image may still be impressed. In such a case
however the effect is produced slowly, and a longer exposure in the Camera
is required.
The sensitiveness of the Iodide film does not increase uniformly with the
amount of the excess of Nitrate of Silver, as measured by the strength of
the Bath. It is found that no advantage in this respect can be gained by
using a proportion of Nitrate of Silver greater than 30 or 35 grains to
the ounce of water, although solutions of three times this strength have
been sometimes employed.
It has been asserted that a chemically pure Iodide of Silver, which is
unaffected in colour by the direct action of light, is also incapable of
receiving the invisible image in the Camera; and that the sensitiveness of
a washed Collodion film is due to a minute quantity of Nitrate of Silver
still remaining. Iodide of Silver in the state in which it is thrown down
on diluting with water a strong solution of the salt known as the double
Iodide of Potassium and Silver,--and which must, from the mode of its
preparation, be free from Nitrate of Silver,--is quite insensitive; but
this form of Iodide differs from the other in colour, and not only so, but
is likely to contain an excess of Iodide of Potassium. The application
of a solution of Nitrate of Silver to this compound at once renders it
sensitive to light.
b. _Free acids in the Nitrate Bath._--Strong oxidizing agents, such as
Nitric Acid, greatly diminish the sensibility of the film, and hence the
importance of removing the free acid often met with in commercial samples
of the Nitrate of Silver. The effect of even a single drop of strong
Nitric Acid in an eight-ounce Nitrate Bath will be appreciable; and when
the proportion is increased to one drop per ounce, it will be difficult to
obtain a rapid impression.
Acetic Acid has far less effect upon the sensitiveness than Nitric
Acid, and being found useful during the development of the image is
commonly employed; but when great rapidity is desired, it should be added
cautiously, and in a proportion very much less than that in the solution
known as the Aceto-Nitrate of Silver, which contains about one drop of the
glacial acid to each grain of Nitrate of Silver.
c. _Addition of certain organic matters._--It has long been remarked that
the use of bodies like Albumen, Gelatine, Caseine, etc., which combine
with Oxides of Silver, retard the action of light upon Iodide of Silver;
and the recent observations of the Author enable him to confirm this
statement. It is probable that one cause, amongst others, of the great
sensibility of the Collodion film is due to the fact that Pyroxyline is
a substance peculiarly indifferent to the Salts of Silver, exhibiting
no tendency to reduce them to the metallic state; and it is proved by
experiment that the addition of Grape Sugar, or of the resinous body,
Glycyrrhizine, which resembles Albumen in causing a white precipitate in
strong solution of Nitrate of Silver, renders necessary a longer exposure
in the Camera. Alkaline Citrates have a still more marked effect, as also
have Tartrates, Oxalates, etc.
d. _Impurities in the soluble Iodides._--Commercial Iodide of Potassium
often contains _Iodate_ of Potash, which is found to have a retarding
effect upon the action of light; also Carbonate of Potash, which, in
Collodion, produces Iodoform,[14] and in the paper processes, where
"Aceto-Nitrate" is used for sensitizing, forms Acetate of Silver. Iodoform
has a marked influence in diminishing the sensitiveness of Iodide of
Silver; Acetate of Silver may perhaps increase it a little by securing the
absence of free Nitric Acid (p. 117). Iodide of Potassium prepared by the
process in which Sulphuretted Hydrogen and Alcohol are used, and having
a smell of Garlic, contains probably Xanthate of Potash, and is nearly
useless for Photography.
[14] See the Vocabulary, Part III., Art. Iodoform.
Commercial Iodide of Cadmium is a purer salt than the Iodide of
Potassium, and may be advantageously substituted for it; but it possesses
the property of coagulating Albumen, and hence cannot be employed in
conjunction with that substance.
e. _Presence of free Iodine._--Both in the waxed paper and the Collodion
processes, the solutions often contain a small quantity of free Iodine.
This Iodine, in contact with the Nitrate of Silver of the Bath, produces
a mixed Iodide and _Iodate_ of Silver, and liberates Nitric Acid. It thus
retards the sensitiveness of the film in proportion to the quantity of
Iodine present. Collodion of a full yellow colour is perceptibly less
sensitive than the same rendered colourless; and when enough Iodine has
been liberated to give a red or brown tint, double the original exposure
will probably be required.
If brown Collodion be much used, the Nitrate Bath may by degrees become
sufficiently contaminated with free Nitric Acid to interfere with the
sensitiveness of the film; but if colourless or lemon-yellow tinted
Collodion be employed, this evil need not be anticipated.
Certain substances may be added to coloured Collodion which possess the
property of counteracting the retarding influence of the free Iodine,
such, for instance, as the Oils of Cloves, Cinnamon, etc.; they probably
act in virtue of their affinity for Oxygen, by preventing the formation
of Iodate of Silver. In colourless Collodion they produce little or no
effect, neither do they remove the insensitiveness of the film when
dependent upon a too acid condition of the Nitrate Bath.
f. _Addition of Bromide or Chloride to Collodion._--In the Daguerreotype
a very exalted state of sensibility is obtained by exposing the silvered
plate first to the vapour of Iodine, and afterwards to that of Bromine or
Chlorine; but this rule does not apply to the Collodion process, which
differs essentially in principle. Soluble Bromides added to Collodion
lessen its sensibility to an appreciable extent, as also do Chlorides.
This rule however may perhaps be liable to an exception when artificial
light is used, which contains a greater proportion of the rays of small
refrangibility, known to act more powerfully upon the Bromide than upon
the Iodide of Silver (p. 66).
g. _Density of the sensitive film._--When the proportion of soluble Iodide
in the iodizing solution is too great, the film is very dense, and the
Iodide of Silver is apt to burst out upon the surface, and fall away in
loose flakes into the Bath. This condition, which is highly unfavourable
to sensitiveness, is very common in Collodion, and constitutes what is
termed "over-iodizing." The Iodide, in fact, is formed in such a case too
much upon the surface, and consequently, when the fixing agent is applied,
the image not being retained by the film, is washed off and lost.
On the other hand, the sensibility of the film is not lessened by reducing
the amount of Iodide in Collodion to a minimum, if all the solutions are
neutral; but the pale blue films formed by a dilute Collodion, and which
almost rival the Daguerreotype itself in delicacy, are nearly useless in
practice; for if free Iodine or other bodies of a retarding nature are
present in any quantity, either in the Collodion or in the Bath, they
almost destroy the action of a weak light, producing a far more injurious
effect than if the film were more yellow and opaque.
h. _Impurities in Ether and Alcohol._--Pure Ether should be neutral to
test-paper, but the commercial samples of this article have usually either
an acid or an alkaline reaction. The frequent occurrence of a peculiar
oxidizing principle in Ether has also been pointed out (p. 85). Each of
these three conditions is injurious to sensitiveness; the first and last
by liberating Iodine when alkaline Iodides are used; and the second,
by producing Iodoform under the same circumstances. In this case the
Collodion remains colourless, but gives inferior results.
The Author has also observed that Ether which has been redistilled from
the residues of Collodion may contain a volatile principle (probably a
compound Ether?) which produces a retarding effect upon the action of
light.
Commercial Spirit of Wine is not always uniform in composition, as
sufficiently evidenced by the test of smell. It may contain "fusel oil" or
other volatile substances, which become milky on dilution with water, and
are believed to injure the quality of the spirit for Photographic use.
i. _Relative proportions of Ether and Alcohol in Collodion._--It was
shown at p. 84 that the addition of Alcohol to Collodion lessens the
contractility of the film, and renders it soft and gelatinous. This
condition is favourable to the formation of the invisible image in the
Camera, the play of affinities being promoted by the loose manner in which
the particles of Iodide are held together. It is therefore usual to add to
Collodion as much Alcohol as it will bear without becoming glutinous, or
leaving the glass; the exact quantity required varying with the strength
of the spirit or its freedom from dilution with water.
k. _Decomposition in the Collodion._--Collodion iodized with the metallic
Iodides generally, excepting the Iodide of Cadmium, becomes brown and
loses sensitiveness in the course of a few days or weeks. If the free
Iodine, the cause of the brown colour, be removed, the greater part,
but not the whole, of the sensitiveness is regained. The experiments of
the Author, and of others, have proved that a solution of Pyroxyline in
contact with an unstable iodide, slowly undergoes decomposition, the
result of which is that Iodine is set free, and an equivalent quantity of
the base remains in union with certain organic elements of the Collodion.
Decomposition also gradually ensues when iodized Collodion is placed in
contact with reducing agents, such as Proto-iodide of Iron, Gallic Acid,
Grape Sugar, Glycyrrhizine, etc., so that these combinations do not retain
a constant sensibility for any length of time. Even plain Collodion
uniodized cannot be preserved many months without a small but perceptible
amount of change.
l. _Decomposition in the Nitrate Bath._--A Collodion Nitrate Bath which
has been much used, often gives a less sensitive film than when newly
made. It is known also that many organic substances which reduce Nitrate
of Silver, if added to the Bath, produce a state which is favourable to
sensitiveness whilst the decomposition is taking place, but is eventually
unfavourable; hence the solution will be injured by adding either Gallic
or Pyrogallic Acid, and by organic matters generally if exposed to light.
_Recapitulation._--The conditions most favourable to extreme sensitiveness
of the Iodide of Silver on Collodion may be condensed as follows:--perfect
neutrality of the solutions employed; a soft, gelatinous state of the
film; absence of Chlorides and other salts which precipitate Nitrate of
Silver; an undecomposed Collodion, containing no organic matter of that
kind which is precipitated by basic Acetate of Lead, and combines with
oxides of Silver.
THE CONDITIONS WHICH AFFECT THE DEVELOPMENT OF THE LATENT IMAGE.
The general theory of the development of a latent image by means of a
reducing agent, having been simply explained in the third Chapter, may
now be more fully examined in its application to the Iodide of Silver on
Collodion.
a. _The presence of free Nitrate of Silver essential to the
development._--This subject has already been mentioned (p. 36). A
sensitive Collodion plate, carefully washed in distilled water, is still
capable of receiving the radiant impression in the Camera, but it does not
admit of development until it has been redipped in the Bath, or treated
with a reducing agent to which Nitrate of Silver has been added: and if
the proportion of free Nitrate of Silver on a Collodion film be too small,
the image will be feeble or altogether imperfect in parts, with patches of
green or blue, due to deficient reduction.
b. _Comparative strength of deducing Agents._--No increase of power in a
developer will suffice to bring out a perfect image on an under-exposed
plate, or upon a film containing too little Nitrate of Silver. But there
is considerable difference in the length of time which the various
developers require to act. Gallic Acid is the most feeble, and Pyrogallic
Acid the strongest, producing at least four times more effect than an
equal weight of the crystallized Protosulphate of Iron, and twenty times
more than the Protonitrate of Iron.
c. _The effect of free Acid upon the development._--Acids tend to retard
the reduction of the image as well as to diminish the sensibility of the
film to light. Nitric Acid especially does so, from its powerful oxidizing
and solvent properties. The effect of Nitric Acid is particularly seen
when the film of Iodide of Silver is very blue and transparent, and the
quantity of Nitrate of Silver retained upon its surface small. Under such
circumstances the proper development of the image may be suspended, and
spangles of metallic Silver separate. This indicates that the quantity of
the acid should be diminished, or the strength of the Nitrate Bath and of
the reducing agent be increased, as a counterpoise to the retarding action
of acid upon the development.
Acetic Acid also moderates the rapidity of development, but it has not
that tendency altogether to suspend it possessed by Nitric Acid. It is
therefore usefully employed, to enable the operator to cover the plate
evenly with liquid before the development commences, and to preserve the
white parts of the impression from any accidental deposit of metallic
Silver due to irregular action of the reducing agent.
On comparing the retarding effects of free acid upon the light's action,
and upon the development, we see that the former is the most marked,--that
a small quantity of Nitric Acid produces a more decided influence upon the
impression of the image in the Camera than upon the bringing out of that
image by means of a developer.
d. _Accelerating effect of certain organic matters._--Organic bodies,
like Albumen, Gelatine, Glycyrrhizine, etc., which combine chemically
with oxides of Silver, and were shown in the last Section to lessen the
sensitiveness of the Iodide film,--facilitate the development of the
image, producing often a dense deposit of a brown or black colour by
transmitted light.
In the same way, viz. by a retention of organic matter, may partly be
explained the fact, that the image developed by Pyrogallic Acid, although
proved by the application of tests to contain no more than an equal
quantity of Silver, possesses greater opacity by transmitted light, than
that resulting from the use of protosalts of Iron: and in the case of
the Collodion itself the same rule applied--if it be pure, it is liable
to give a less vigorous impression than when by long keeping a partial
decomposition has taken place, and products have been formed which
combine with reduced Oxide of Silver more easily than the unaltered
Pyroxyline.
e. _Molecular conditions affecting Intensity._--The physical structure
of the Collodion film is thought to exert an influence upon the mode in
which the reduced Silver is thrown down during the development. A short
and almost powdery state, such as Collodion iodized with the alkaline
iodides acquires by keeping, is considered favourable, and a glutinous,
coherent structure unfavourable, to density. This is certainly the case
when the film is allowed to dry before development, as in the process
with desiccated Collodion and, to some extent, in the Oxymel preservative
process.
The mode of conducting the development also affects the density; a rapid
action tending to produce an image of which the particles are finely
divided and offer a considerable resistance to the passage of light,
whilst a slow and prolonged development often leaves a metallic and almost
crystalline deposit, comparatively translucent and feeble.
The writer has observed, that with certain samples of Collodion the
image is much enfeebled by keeping the plate for a considerable time,--a
quarter of an hour or longer,--after sensitizing, but before development.
This effect is not the result of the Nitrate of Silver having partially
drained away, since a second dip in the Nitrate Bath immediately before
applying the Pyrogallic Acid, does not remedy it. An alteration of
molecular structure may therefore be the correct explanation, and if so, a
contractile Collodion would suffer more than one possessing less coherency.
The actinic power of the light at the time of taking the picture,
influences the appearance of the developed image; the most vigorous
impressions being produced by a strong light acting for a short time. On a
dull dark day, or in copying badly lighted interiors, the photograph will
often lack bloom and richness, and be blue and inky by transmitted light.
f. _Development of images upon Bromide and Chloride of Silver._--Of the
three principal Salts of Silver, the Iodide is the most sensitive to
light, but the Bromide and Chloride, under some conditions, are more
easily developed and give a darker image. In the Collodion process the
difference is principally seen when organic bodies, like Grape Sugar,
Glycyrrhizine, etc., are introduced in order to increase the intensity; a
far more decided effect being produced by adding both Glycyrrhizine and a
portion of Bromide or Chloride, than by using the Glycyrrhizine alone.[15]
[15] See the Author's Paper on the chemical composition of the
photographic image, in the eighth Chapter.
g. _The intensity of the image affected by the length of exposure._--This
point has been briefly alluded to in the third Chapter. If the exposure in
the Camera be prolonged beyond the proper time the development takes place
rapidly but without any intensity, the picture being pale and translucent.
The effects produced by over-action of the light are particularly seen
when the Nitrate Bath contains Nitrite of Silver, or Acetate of Silver;
the image being frequently in such a case dark by reflected light, and
red by transmitted light,--more nearly resembling in fact a photographic
print, developed on paper prepared with Chloride of Silver. When Collodion
plates are coated with honey without previously removing the free Nitrate
of Silver, a slow reducing action is set up, which may give rise to the
characteristic appearance above referred to, after development. Other
organic substances, such as biliary matter, etc., will act in the same way.
h. _Certain conditions of the Bath which affect development._--Attention
may be called to a peculiar state of the Nitrate Bath, in which the
Collodion image developes unusually slowly, and has a dull grey metallic
appearance, with an absence of intensity in the parts most acted on by
the light. This condition, which occurs only when using a newly mixed
solution, is thought by the Author to depend upon the presence of an
Oxide of Nitrogen retained by the Nitrate of Silver. It is removed
partially by neutralizing the Bath with an alkali, more perfectly so by
adding an excess of alkali followed by Acetic Acid; but most completely by
carefully _fusing_ the Nitrate of Silver before dissolving it.
Commercial Nitrate of Silver has sometimes a fragrant smell, similar to
that produced by pouring strong Nitric Acid upon Alcohol. When such is the
case, it contains organic matter, and produces a Bath which yields red and
misty pictures.
Nitrate of Silver which has been sufficiently strongly fused to decompose
the Salt, and produce a portion of the basic Nitrite of Silver exhibits
great peculiarity of development, the image coming out instantaneously and
with great force. This condition is exactly the reverse of that produced
by the presence of acids, in which the development is slow and gradual.
In summing up the different conditions of the Nitrate Bath which affect
the development of the image, as many as _four_ might be mentioned, each
of which gives a more rapid reduction than the one which precedes it.
These are--the acid Nitrate Bath, the neutral Bath, the Bath of strongly
fused Nitrate of Silver, and the Bath containing _Ammoniacal_ Nitrate of
Silver, which is quite unmanageable, and produces an instantaneous and
universal blackening of the film on the application of the developer.
Greater intensity of image is commonly obtained in a Nitrate Bath which
has been a long time in use, than in a newly mixed solution: this may be
due to minute quantities of organic matter dissolved out of the Collodion
film, which, having an affinity for Oxygen, partially reduce the Nitrate
of Silver; and also to the accumulation of Alcohol and Ether in an old
Bath producing a short and friable structure of the film.
i. _Effect of Temperature on Development._--Reduction of the oxides of
noble metals proceeds more rapidly in proportion as the temperature
rises. In cold weather it will be found that the development of the image
is slower than usual, and that greater strength of the reducing agent and
more free Nitrate of Silver is required to produce the effect.
On the other hand, if the heat of the atmosphere be excessive, the
tendency to rapid reduction will be greatly increased, the solutions
decomposing each other almost immediately on mixing. In this case the
remedy will be to use Acetic Acid _freely_ both in the Bath and in the
developer, at the same time lessening the quantity of Pyrogallic Acid, and
omitting the Nitrate of Silver which is sometimes added towards the end of
the development.
Also in the case of films which are to be kept for a long time in a
sensitive condition by means of honey, etc., the modifying influence of
temperature must be observed, and the quantity of free Nitrate of Silver
left upon the film be reduced to a minimum if the thermometer stands
higher than usual.
SECTION IV.
_On certain Irregularities in the Developing Process._
The characteristics of the proper development of a latent image are--that
the action of the reducing agent should cause a blackening of the Iodide
in the parts touched by light, but produce no effect upon those which have
remained in shadow.
In operating both on Collodion and paper however there is a liability to
failure in this respect; the film beginning, after the application of the
developer, to change in colour to a greater or less extent over the whole
surface.
There are two main causes which produce this state of things:--the first
being due to an irregularity in the action of the light; the second to a
faulty condition of the chemicals employed.
If from a defect in the construction of the instrument, or from other
causes which will be pointed out more particularly in the Second Part
of this work, diffused white light gains entrance into the Camera, it
produces indistinctness of the image by affecting the Iodide more or less
universally.
The luminous image of the Camera not being perfectly pure, mere
_over-exposure_ of the sensitive plate will usually have the same effect.
In such a case, when the developer is poured on, a faint image first
appears, and is followed by a general cloudiness.
The clearness of the developed Collodion picture is much influenced by the
condition of all the solutions employed, but particularly so by that of
the Nitrate Bath. If this liquid be in the state termed alkaline (p. 88),
it will be impossible to obtain a good picture; and even when neutral,
care and avoidance of all disturbing causes will be required to prevent
a deposition of Silver upon the shadows of the image: especially so when
Nitrite of Silver or Acetate of Silver are present, both of these salts
being more easily reduced than the Nitrate of Silver.
The use of _Acid_ is the principal resource in obviating cloudiness of the
image. Acids lessen the facility of reduction of the Salts of Silver by
developing agents (p. 98), and hence when they are present the metal is
deposited more slowly, and only on the parts where the action of the light
has so modified the particles of Iodide as to favour the decomposition:
whereas if acids be absent or present in insufficient quantity, the
equilibrium of the mixture of Nitrate of Silver and reducing agent which
constitutes the developer is so unstable, that any rough point or sharp
edge is likely to become a centre from which the chemical action, once
started, radiates to all parts of the plate.
Various acids have been employed, such as Acetic acid, Citric acid,
Tartaric acid, etc. Nitric acid is the most effectual of all, but is
seldom used, because, although the image can often be developed with great
clearness when the Bath contains a small quantity of Nitric acid, yet
such a condition is not favourable to _intensity_; on the other hand,
films which are prone to irregular reduction, such as those prepared in a
chemically neutral bath or a bath containing Acetate or Nitrite of Silver,
are likely to give the greatest vigour of impression. Hence, when this
quality is desired, the use of Nitric Acid will be adopted cautiously.
The state of the Collodion must be attended to as well as that of the
bath; it should be either acid or neutral, not alkaline. Colourless
Collodion may be used successfully as a rule, but sometimes a little
free Iodine is advantageously added. Care should be taken in introducing
organic substances, many of which dissolve out into the bath, and spoil it
for giving clear pictures. Glycyrrhizine, however, which is recommended
to produce intensity of Negatives, has no effect of that kind, and may be
employed with safety.
The condition of the developing agent is a point of importance in
producing clear and distinct pictures. The Acetic acid, which is advised
in the formulæ, cannot be omitted or even lessened in quantity without
danger. This is particularly the case in hot weather or under any other
condition which favours reduction, such as neutrality of the bath, etc.;
at all times, in fact, when the solutions of Pyrogallic acid and Nitrate
of Silver decompose each other with unusual rapidity.
In addition to the points now mentioned, viz. the state of the Bath,
of the Collodion, and of the developer, the reader should also study
the remarks made in the Third Section of Chapter III. on the effect
of _surface conditions_ in modifying the deposition of vapour and of
metallic Silver: he will then in all probability experience but little
difficulty in dealing with those numerous irregularities in the action
of the developing fluid, which often prove the greatest hindrance to the
successful practice of the Collodion process.
CHAPTER VII.
ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS.
The terms "Positive" and "Negative" occur so frequently in all works upon
the subject of Photography, that it will be impossible for the student to
make progress without thoroughly understanding their meaning.
A Positive may be defined to be a Photograph which gives a natural
representation of an object, as it appears to the eye.
A Negative Photograph, on the other hand, has the lights and shadows
reversed, so that the appearance of the object is changed or negatived.
In Photographs taken upon _Chloride of Silver_, either in the Camera or by
superposition, the effect must necessarily be Negative; the Chloride being
_darkened by luminous rays_, the lights are represented by shadows.
The following simple diagrams will make this obvious.
[Illustration: Fig. 1. Fig. 2. Fig. 3.]
Fig. 1 is an opaque image drawn upon a transparent ground; fig. 2
represents the effect produced by placing it in contact with a layer of
sensitive Chloride and exposing to light; and fig. 3 is the result of
copying this negative again on Chloride of Silver.
Fig. 3 therefore is a Positive copy of Fig. 1, obtained by means of a
Negative. By the first operation the tints are reversed; by the second,
being reversed again, they are made to correspond to the original. The
possession of a Negative therefore enables us to obtain Positive copies of
the object, indefinite in number and all precisely similar in appearance.
This capability of multiplying impressions is of the utmost importance,
and has rendered the production of good Negative Photographs of greater
consequence than any other branch of the Art.
The same Photograph may often be made to show either as a Positive or
as a Negative. For instance, supposing a piece of silver-leaf to be cut
into the shape of a cross and pasted on a square of glass, the appearance
presented by it would vary under different circumstances.
[Illustration: Fig. 1. Fig. 2.]
Fig. 1 represents it placed on a layer of black velvet; fig. 2 as held
up to the light. If we term it Positive in the first case, _i. e._
by reflected light, then it is Negative in the second, that is, by
transmitted light. The explanation is obvious.
Therefore to carry our original definition of Positives and Negatives
a little further, we may say, that the former are usually viewed by
reflected, and the latter by transmitted, light.
All Photographs however cannot be made to represent both Positives and
Negatives. In order to possess this capability, it is necessary that a
part of the image should be transparent, and the other opaque _but with a
bright surface_. These conditions are fulfilled when the Iodide of Silver
upon Collodion is employed, in conjunction with a developing agent.
Every Collodion picture is to a certain extent both Negative and Positive,
and hence the processes for obtaining both varieties of Photographs are
substantially the same. Although however the general characters of a
Positive and a Negative are similar, there are some points of difference.
A surface which appears perfectly opaque when looked down upon, becomes
somewhat translucent on being held up to the light; hence, to give the
same effect, the deposit of metal in a Negative must be proportionally
thicker than in a positive; otherwise the minor details of the image, will
be invisible, from not obstructing the light sufficiently.
With these preliminary remarks, we are prepared to investigate more
closely the _rationale_ of the processes for obtaining Collodion Positives
and Negatives. All that refers to paper Positives upon Chloride of Silver
will be treated in a subsequent Chapter.
SECTION I.
_On Collodion Positives._
Collodion Positives are sometimes termed _direct_, because obtained by a
single operation. The Chloride of Silver, _acted upon by light alone_,
is not adapted to yield direct Positives, the reduced surface being dark
and incapable of representing the lights of a picture. Hence a developing
agent is necessarily employed, and the Iodide of Silver substituted for
the Chloride, as being a more sensitive preparation. Collodion Positives
are closely allied in their nature to Daguerreotypes. The difference
between the two consists principally in the surface used to sustain the
sensitive layer, and the nature of the substance by which the invisible
image is developed.
In a Collodion Positive the lights are formed by a bright surface of
reduced Silver, and the shadows by a black background showing through the
transparent portions of the plate.
Two main points are to be attended to in the production of these
Photographs.
First, to obtain an image distinct in every part, _but of comparatively
small intensity_.--If the deposit of reduced metal be too thick, the
dark background is not seen to a sufficient extent, and the picture in
consequence is deficient in shadow.
Secondly, to _whiten_ the surface of the reduced metal as much as
possible, in order to produce a sufficient contrast of light and shade.
Iodide of Silver developed in the usual way presents a dull yellow
appearance which is sombre and unpleasing.
_The Collodion and Nitrate Bath for Positives._--Good Positives may be
obtained by diluting down a sample of Collodion with Ether and Alcohol
until it gives a pale bluish film in the Bath. The proportion of Iodide
of Silver being in that case small, the action of the high lights is less
violent, and the shadows are allowed more time to impress themselves. The
dilution lessens the amount of Pyroxyline in the Collodion at the same
time with the Iodide, which is an advantage, the slight and transparent
films always giving more sharpness and definition in the picture.
The employment of a very thin film for Positives is not however always a
successful process. The particles of the Iodide of Silver being closely
in contact with the glass, unusual care is required in cleaning the
plates in order to avoid stains; and the amount of free Nitrate of Silver
retained upon the surface of the film being small, circular patches of
imperfect development are liable to occur, unless the reducing agent be
scattered evenly and perfectly over the surface. Also if free Iodine or
organic substances which have a retarding effect on the action of light
are present to a considerable extent, the Collodion will not work well
with a small proportion of Iodide. The Author found in experimenting on
this subject that with perfectly pure Collodion and a _neutral_ Bath most
vigorous impressions were produced when the density of the film had been
so far reduced by dilution that scarcely anything could be seen upon the
glass; but with Collodion strongly tinted with Iodine, or with a Bath
containing Nitric Acid, it was necessary to stop the dilution at a certain
point or the film became absolutely insensitive to feeble radiations of
light, and the shadows could not be brought out by any amount of exposure.
In this case, by adding more Iodide a better effect was obtained.
A thicker Collodion may be used for positives if a little free Iodine be
added, for the purpose of diminishing intensity and keeping the shadows
clear during the development. This process is easier to practice than the
last, but does not always give the same perfect definition.
No organic substance of the class to which Glycyrrhizine and Sugars belong
should be added to Collodion which is to be used for Positives. By so
doing the image would be rendered intense, and the high lights liable to
solarization, _id est_, a dark appearance by reflected light.
_The Nitrate Bath._--If the materials are pure, the Nitrate Bath may
advantageously be diluted down at the same time with the Collodion, when
Positives are to be taken; but the employment of a very weak Nitrate
Bath (such as one of 20 grains to the ounce), although highly useful
in obviating excess of development, has some disadvantages; it becomes
necessary to exclude free Nitric Acid, and to avoid the employment of a
Collodion too highly tinted with Iodine. On the other hand, with a strong
Nitrate Bath, and a tolerably dense film of Iodide of Silver, a better
result is often secured by the use of Nitric Acid. The sensitiveness of
the plates is impaired, but at the same time the intensity is diminished,
and the picture shows well upon the surface of the glass.
A new Bath is better for taking Positives than one which has been a long
time in use. The latter often causes _haziness_ and irregular markings on
the film during the action of the developer. This is due partly to the
accumulation of Alcohol and Ether in the Bath, which causes the solution
of Sulphate of Iron to flow in an oily manner; and partly to a reduction
of the Nitrate of Silver by organic matter.
The presence of _Acetate of Silver_ is objectionable in a Positive Nitrate
Bath as producing solarization and intensity of image; hence those
precautions which obviate its formation must be adopted (p. 89).
If fused Nitrate of Silver be used for the Positive Nitrate Bath, it is
very important that the fusion should not be carried too far, or the
solution would contain a basic Nitrite of Silver, and yield an intense,
solarized, and misty image.
_The Developers for Collodion Positives._--Pyrogallic Acid when used with
Acetic Acid, as is usual for negative pictures, produces a surface which
is dull and yellow. This may be obviated by substituting Nitric Acid in
small _quantity_ for the Acetic. The surface produced by Pyrogallic Acid
with Nitric Acid is lustreless, but very white, if the solution be used
of the proper strength. On attempting to increase the amount of Nitric
Acid the deposit becomes metallic, and the half-tones of the picture are
injured; Pyrogallic Acid, although an active developer, does not allow
of the addition of mineral acid to the same extent as the Salts of Iron.
It requires also, when combined with Nitric Acid, a fair proportion of
Nitrate of Silver on the film, or the development will be imperfect in
parts of the plate.
_Sulphate of Iron._--The Protosalts of Iron were first employed in
Photography by Mr. Hunt. The Sulphate is a most energetic developer, and
often brings out a picture when others would fail. To produce by means of
it a dead white tint with absence of metallic lustre, it may be used in
conjunction with Acetic Acid, and in a somewhat concentrated condition, so
as to develope the picture quickly.
The addition of _Nitric Acid_ to Sulphate of Iron modifies the
development, making it more slow and gradual, and producing a bright
sparkling surface of reduced Silver. Too much of this acid however must
not be used, or the action will be irregular. The Nitrate Bath also must
be tolerably concentrated, in order to compensate for the retarding effect
of Nitric Acid upon the development. The blue and transparent films of
Iodide of Silver, formed in a very dilute Nitrate Bath, are not well
adapted for Positives to be developed in this way. They are injured by the
acid, and the development of the image becomes imperfect.
_Protonitrate of Iron._--This salt, first used by Dr. Diamond, is
remarkable as giving a surface of brilliant metallic lustre without any
addition of free acid. Theoretically, it may be considered as closely
corresponding to the Sulphate of Iron with Nitric Acid added. There are
however slight practical differences between them, which are perhaps in
favour of the Protonitrate.
The reducing powers of _Protoxide_ of Iron appear to be in inverse ratio
to the strength of the acid with which it is associated in its salts;
hence the _Nitrate_ is by far the most feeble developer of the Protosalts
of Iron.
The rules already given for the use of Sulphate of Iron acidified with
Nitric Acid, apply also to the Nitrate of Iron; the proportion of free
Nitrate of Silver must be large, and the film of Iodide of Silver not too
transparent.
In developing direct Positives either by Pyrogallic Acid or the Salts of
Iron, the colour of the image will be found liable to some variation;
the character of the light, whether bright or feeble, and the length of
exposure in the Camera, affecting the result.
_A Process for whitening the Positive Image by means of Bichloride of
Mercury._--In place of brightening the Positive image by modifying the
developer, it was proposed some time since by Mr. Archer to effect the
same object by the use of the salt known as _Corrosive Sublimate_, or
Bichloride of Mercury.
The image is first developed in the usual way, fixed, and washed. It is
then treated with the solution of Bichloride, the effect of which is to
produce almost immediately an interesting series of changes in colour. The
surface first _darkens considerably_, until it becomes of an ash-grey,
approaching to black; shortly it begins to get lighter, and assumes a
_pure white_ tint, or a white slightly inclining to blue. It is then
seen, on examination, that the whole substance of the deposit is entirely
converted into this white powder.
The _rationale_ of the reaction of Bichloride of Mercury appears to be,
that the Chlorine of the mercurial salt divides itself between the Mercury
and the Silver, a portion of it passing to the latter metal and converting
it into a Protochloride. The white powder is therefore probably a compound
salt, as is further evidenced by the effects produced on treating it with
various reagents.
SECTION II.
_On Collodion Negatives._
As in the case of a direct Positive we require an image which is _feeble_
though distinct, so, on the other hand, for a negative, it is necessary to
obtain one of considerable intensity. In the Chapter immediately following
the present, it will be shown that in using glass Negatives to produce
Positive copies upon Chloride of Silver paper, a good result cannot
be secured unless the Negative is sufficiently dark to obstruct light
strongly.
_The Collodion and Nitrate Bath for Negatives._--A Collodion containing
a very small portion of Iodide and yielding a blue transparent film in
the Bath is not well adapted for taking Negatives. Pale opalescent films
often give too little intensity in the high lights, and, unless the
Nitrate Bath be acid, do not admit of being exposed in the Camera for
the proper length of time without cloudiness and indistinctness of image
being produced under the action of the developer. The effect known as
"solarization of negatives," _i. e._ a red and translucent appearance
of the highest lights, is also more liable to occur when operating with
a very pale film. On the other hand, if the layer of Iodide be too
yellow and creamy, the half-tones of the image will often be imperfectly
developed, so that a middle point between these extremes is the best.
A pure and newly prepared Collodion, although highly sensitive to
light, does not always give, with one application of the developer, a
sufficiently vigorous image to serve as a negative matrix; and this
particularly in the most brightly illuminated parts, such as the sky in a
landscape photograph, or the white borders of an engraving. But on keeping
the Collodion for some weeks or months it becomes yellow, if iodized with
the alkaline iodides, and a decomposition takes place in it, as before
shown (p. 97), which lessens the rapidity of action, but adds to the
intensity of the negative.
Grape Sugar may be employed for the purpose of giving intensity to newly
mixed Collodion: also Glycyrrhizine, which is a resinous body extracted
from the root of Liquorice; but as both substances have an effect in
lessening the sensitiveness and keeping qualities of the fluid, they
should be used cautiously. In taking portraits in the open air, on bright
days, and with a Bath which has been mixed for a considerable time, it
will rarely be found that the intensity will be deficient; and especially
so if the developer be applied a second time to the film with a few drops
of solution of Nitrate of Silver added. In landscape Photography however,
or in copying engravings, where extreme sensitiveness is not an object,
the Glycyrrhizine may sometimes be added with advantage in order to obtain
perfect opacity of the blacks.
When the use of this substance is resorted to, the mode of iodizing the
Collodion appears to be of importance, the increase of intensity being
greater with the Iodide of Cadmium than with the Iodides of the Alkalies;
the latter probably exercising a decomposing action. An addition of a
Bromide or a Chloride to the Collodion in small quantity has also a marked
effect in adding to the intensity when Glycyrrhizine is used with alkaline
Iodides (p. 101).
Substances which produce intensity of the Collodion image have often,
if added in too large quantity, a tendency to lower the half-tone, and
prevent the darker parts of the picture from being sufficiently brought
out. The print from the Negative is then pale and white, or "chalky" as
it is termed, in the high lights. Collodion in this condition is often
preferred by the beginner, from the facility with which the Negatives
are obtained, but it does not give the finest results. An excess of
Glycyrrhizine in Collodion has also the effect of interfering with the
precipitation of the Iodide of Silver, producing a blue and smoky film
which is nearly useless for Negatives.
A judicious employment of free Iodine in Collodion which has been
previously intensified with Glycyrrhizine, has a remarkable effect in
improving the gradation of tone. The excessive opacity of the high lights
is diminished, and hence the operator is enabled by a longer exposure of
the sensitive plate to bring out the shadows and minor details of the
image with great distinctness. Collodion prepared in this manner is too
slow to be used for portraits, excepting in a strong light, but often
gives an image with great roundness and stereoscopic effect.
The Iodine and the liquorice sugar employed conjointly, tend also to
preserve the clearness of the plates under the influence of the developer,
and to give sharpness to the lines and dots of engravings, etc., which,
with a new and sensitive Collodion, are often imperfectly rendered. These
advantages will be appreciated by the operator who has failed from
working with a too feeble Collodion; but it must be borne in mind, that
all substances acting as intensifiers have a bad effect when the state of
the film is not such as to call for their employment.
The Proto-iodide of Iron has been recommended as an addition to Negative
Collodion. In the Nitrate Bath it forms, in addition to Iodide of Silver,
Protonitrate of Iron, an unstable substance and a developer. The use of
Iodide of Iron gives great sensibility, but it is difficult to preserve
it pure and unchanged. It also decomposes the Collodion in the course of
a few hours, becoming itself peroxidized, and producing an insensitive
condition of film. In addition to this, the negatives taken by the aid of
Iodide of Iron are commonly of an inferior kind, the reduction being too
marked in the high lights; so that its employment is of doubtful utility.
_The Nitrate Bath._--This should be prepared from Nitrate of Silver which
has been melted at a moderate heat (see pp. 13 and 101). If this point be
neglected, the best Collodion will sometimes fail in producing an intense
negative.
Acetic Acid must be added in minute quantity, to preserve the solution
from a too ready reduction by the Alcohol and Ether of the Collodion.
Also, unless the Nitrate of Silver be quite pure and free from organic
matter (p. 104), clear pictures will not be obtained without the use of
Acid.
Acetate of Silver has often been advised as an addition to the Negative
Nitrate Bath. It is produced by dropping into the solution an alkali, such
as Ammonia, followed by Acetic Acid in excess. The Negatives are rendered
blacker and more vigorous by this proceeding, but especially so when the
Bath is contaminated with Nitric Acid; which neutralizes itself at the
expense of the Acetate of Silver, thus:--
Acetate of Silver + Nitric Acid
= Nitrate of Silver + Acetic Acid.
As a rule, it will be better to avoid adding Acetate of Silver to the
Bath, since with, pure melted Nitrate of Silver no Nitric Acid can be
present, and perfect intensity is easily obtained. When the Bath is
saturated with Acetate of Silver, it is in a more reducible state, and
hence unless the glass plates are very perfectly cleaned, black lines
and markings, the results of irregular action, will be produced on the
application of the developer to the film (p. 104). Solarization, or
reddening by over-exposure, is also promoted by the presence of Acetate of
Silver.
_Developing solutions for Negatives._--The Protosalts of Iron are not
usually employed in developing Negative impressions. They are liable
to yield a violet-coloured image, which cannot easily be rendered more
intense by continuing the action.
Gallic Acid is too feeble for developing Collodion pictures. Pyrogallic
Acid is much superior, and may be used of any strength, according to
the effect desired. When the light is bad, the temperature low, and
the Negative developes slowly and appears blue and inky by transmitted
light, the proportion of the reducing agent should be increased. But with
an intense Collodion, on a clear summer's day, the finest gradation is
obtained with a weak solution, which does not begin to act until the plate
has been evenly covered. A strong developer might in such, a case produce
too much opacity in the highest lights, and would probably occasion stains
of irregular reduction.
_Modes of strengthening a finished impression which is too feeble to be
used as a Negative._--The ordinary plan of pushing the development cannot
be applied with advantage after the picture has been washed and dried. In
that case, if it is found to be too feeble to print well, its intensity
may be increased by one of the following methods.--
It must be premised however, that the same degree of excellence is not to
be expected in a Negative Photograph which has been improperly developed
in the first instance and more especially if the exposure to light was
too short. Any "instantaneous Positive" may be rendered sufficiently
intense for a Negative, but in that case the shadows are almost invariably
imperfect.
1. _Treatment of the image with Sulphuretted Hydrogen or Hydrosulphate of
Ammonia._--The object is to convert the metallic Silver into _Sulphuret
of Silver_, and if this could be done it would be of service. The mere
application of an Alkaline Sulphuret has however but little effect upon
the image, excepting to darken its surface and destroy the Positive
appearance by reflected light; the structure of the metallic deposit being
too dense to admit of the Sulphur reaching its interior.
Professor Donny ('Photographic Journal,' vol. i.) proposes to obviate this
by first converting the image into the white Salt of Mercury and Silver by
the application of Bichloride of Mercury, and afterwards treating it with
solution of Sulphuretted Hydrogen or Hydrosulphate of Ammonia. Negatives
produced in this way are of a brown-yellow colour by transmitted light,
and opaque to chemical rays to an extent which would not, _à priori_, have
been anticipated.
2. _MM. Barreswil and Davanne's process._--The image is converted into
Iodide of Silver by treating it with a saturated solution of Iodine in
water. It is then washed--to remove the excess of Iodine,--exposed to
the light, and a portion of the ordinary developing solution, mixed with
Nitrate of Silver, poured over it. The changes which ensue are precisely
the same as those already described; the whole object of the process being
to bring the metallic surface back again into the condition of Iodide of
Silver modified by light, that the developing action may be commenced
afresh, and more Silver deposited from the Nitrate in the usual way.
3. _The process with Bichloride of Mercury and Ammonia._--The image is
first converted into the usual white double Salt of Mercury and Silver
by the application of a solution of the Corrosive Sublimate. It is then
treated with Ammonia, the effect of which is to _blacken_ it intensely.
Probably the alkali acts by converting Chloride of Mercury into the black
Oxide of Mercury. In place of Ammonia, a dilute solution of Hyposulphite
of Soda or Cyanide of Potassium may be used, with very similar results.
CHAPTER VIII.
ON THE THEORY OF POSITIVE PRINTING.
The subject of Collodion Negatives having been explained in the previous
Chapter, we proceed to show how they may be made to yield an indefinite
number of copies with the lights and shadows correct as in nature.
Such copies are termed "Positives," or sometimes "Positive prints," to
distinguish them from direct Positives upon Collodion.
There are two distinct modes of obtaining photographic prints;--first by
development, or, as it is termed, _by the Negative process_, in which
a layer of Iodide or Chloride of Silver is employed, and the invisible
image developed by Gallic Acid; and second, by the direct action of light
upon a surface of Chloride of Silver, no developer being used. These
processes, involving chemical changes of great delicacy, require a careful
explanation.
The action of light upon Chloride of Silver was described in Chapter II.
It was shown that a gradual process of darkening took place, the compound
being reduced to the condition of a coloured _subsalt_; also, that the
rapidity and perfection of the change were increased by the presence of
excess of Nitrate of Silver, and of organic matters, such as Gelatine,
Albumen, etc.
We have now to suppose that a sensitive paper has been prepared in this
way, and that a Negative having been laid in contact with it, the
combination has been exposed to the agency of light for a sufficient
length of time. Upon removing the glass, a Positive representation of the
object will be found below, of great beauty and detail. Now if this image
were in its nature fixed and permanent, or if there were means of making
it so, without injury to the tint, the production of Paper Positives would
certainly be a simple department of the Photographic Art; for it will be
found that with almost any Negative, and with sensitive paper however
prepared, the picture will look tolerably well on its first removal from
the printing-frame. Immersion in the bath of Hyposulphite of Soda however,
which is essentially necessary in order to fix the picture, produces
an unfavourable effect upon the tint; decomposing the violet-coloured
Subchloride of Silver, and leaving behind a red substance which appears
to be united to the fibre of the paper, and, when tested, reacts in the
manner of a Suboxide of Silver.
Other chemical operations are therefore required to remove the
objectionable red colour of the print, and hence the consideration of the
subject is naturally divided into two parts; first, the means by which
the paper is rendered sensitive, and the image impressed upon it;--and
secondly, the subsequent fixing and _toning_, as it may be termed, of the
proof.
The present Chapter will also include, in two additional Sections, a
condensed account of the most important facts relating to the properties
and the mode of preservation of photographic prints.
SECTION I.
_The Preparation of the Sensitive Paper._
In this Section the general theory of the preparation of Positive paper,
in so far as it affects the tone and intensity of the print, will be
described; the reader being referred to the second division of the Work
for the formulæ required.
_The Preparation of the Sensitive Paper._--The conditions which are
required for producing a sharp and well defined print are--that an even
layer of Chloride of Silver should exist upon the very surface of the
paper, and that the particles of this Chloride should be in contact with
a sufficient excess of Nitrate of Silver. These points have been already
referred to at an early part of the Work (p. 19).
The material used for _sizing_ the paper is of importance. English papers
are usually sized with Gelatine, which is a photographic agent, and acts
chemically in forming the image. Foreign papers on the other hand being
sized with starch only, require an addition of Gelatine, Caseine, or
Albumen, to retain the Salt at the surface of the paper, and to assist
in producing the picture: if otherwise, the print will be flat and
"mealy," as it is termed. Albumen especially produces a beautifully smooth
surface, and is advantageously employed in printing small portraits and
stereoscopic subjects.
The uniform surface distribution of the Chloride of Silver is sometimes
interfered with by a faulty structure of the paper, causing it to absorb
liquids unevenly, and in consequence the pictures, when removed from the
printing frame, appear _spotted_. Another cause producing the same effect,
is the employment of too weak a solution of Nitrate of Silver, or the
removal of the sheet from the Nitrate bath before the Chloride of Ammonium
has been perfectly decomposed; it is thus rendered unequally sensitive at
different portions of the surface, and the prints have the characteristic
marbled appearance above referred to.
A sufficient excess of Nitrate of Silver being essential, it is important
to bear in mind, that the quantity of this salt eventually remaining in
the paper, is much influenced by the manner in which the solution is
applied. If it be laid on by _floating_, then the proportion of Nitrate to
that of Chloride of Sodium should be about as 3 to 1 (the atomic weights
are nearly as 5 to 2); but if the plan of brushing or spreading with a
glass rod be adopted, 7 to 1 or 8 to 1 will not be too much.
_The Darkening of the Sensitive Paper by Light._--The operator should be
familiar with the changes of colour which indicate the progress of the
reduction of the sensitive layer. Much in this respect depends upon the
kind of organic matter used, but there is always a regular sequence of
tints; in the case of a paper prepared simply with Chloride of Ammonium
and Nitrate of Silver, it is as follows: pale violet, violet-blue,
slate-blue, _bronze_ or copper-colour. When the _bronzed_ stage is
reached, there is no further change. On immersion in the fixing bath of
Hyposulphite, the violet tones due to Subchloride of Silver are destroyed,
and the print assumes a red or brown colour, which is deepest and most
intense in the parts where the light has acted longest.
Hence we see, that, to produce a good print, it is essential that the
Negative should possess considerable intensity in the dark parts. Pale
and feeble Negatives yield proofs which are wanting in vigour, and have
a flat and indistinct appearance. The combination cannot be exposed to
light for a sufficient length of time to bring about the requisite degree
of reduction of the Chloride of Silver; and hence the deepest shadows of
the resulting Positive are not sufficiently dark, and there is _a want of
contrast_ which is fatal to the effect.
A good Negative should be so opaque as to preserve the lights of the
printed image beneath clear, _until the darkest shades are about to pass
into the bronze or coppery condition_. If the amount of intensity be less
than this, the finest effect cannot be obtained.
CONDITIONS AFFECTING THE SENSITIVENESS OF THE PAPER AND THE INTENSITY OF
THE IMAGE.
Some of the principal of these are as follows:--
a. _The Strength of the Salting Bath._--The sensibility of the paper is
regulated up to a certain point by the amount of salt[16] used in the
preparation. The quantity of alkaline Chloride determines the amount of
Chloride of Silver; and with a proper excess of Nitrate of Silver, papers
are to a certain point more sensitive in proportion as they contain more
of the Chloride.
[16] The difference in the atomic weights of the various soluble Chlorides
used in salting must be borne in mind. Ten grains of Chloride of Ammonium
contain as much Chlorine as eleven of Chloride of Sodium, or twenty-two
grains of Chloride of Barium. (See the Vocabulary, Part III.)
Highly sensitized papers darken rapidly, and pass very completely into the
bronze stage. Those containing less Chloride darken more slowly, and do
not become bronzed with the same intensity of light. A Photographic print,
formed upon paper highly salted and sensitized, is usually vigorous, with
great contrast of light and shade; particularly so when the printing is
conducted in a strong light. Hence it will be an advantage, with a feeble
Negative, and in dull weather, to _double_ the ordinary quantity of Salt,
whereas in the case of an intense Negative, and with direct sunlight, the
deep shadows will be too much bronzed unless the quantity of Chloride and
Nitrate of Silver in the paper be kept low.
In proportion as Photographic papers are highly salted and sensitized,
they become more prone to change colour spontaneously in the dark.
b. _Proportion of Nitrate of Silver._--The compound on which a positive
print is formed is a Chloride, or an organic Salt of Silver, _with
an excess of Nitrate of Silver_. Nothing is gained by increasing the
proportion of Chloride of Sodium, unless at the same time an addition be
made to the quantity of free Nitrate in the sensitizing Bath.
A surface of Chloride of Silver with a bare excess of Nitrate, darkens on
exposure, but it does not reach the bronzed stage; the action appearing to
stop at a certain point. On placing the print in Hyposulphite of Soda, it
becomes very red and pale, and when tinted, looks cold and slaty, without
depth or intensity.
c. _The sensitiveness and intensity affected by substituting the Oxide
of Silver for the Nitrate._--Many operators employ a solution of Oxide
of Silver in Ammonia[17] or Nitrate of Ammonia, in preparing Chloride of
Silver paper. By doing so, a great increase of sensitiveness, and also of
intensity of image, is obtained. This will be understood if we remember
that the action of light in producing the print is of a reducing nature.
Hence the substitution of Oxide for Nitrate of Silver facilitates the
decomposition; just as _Ammonio-Nitrate_ of Silver is more readily reduced
by Gallic or Pyrogallic Acid than the simple Nitrate (see p. 31).
[17] The chemistry of Ammonio-Nitrate of Silver is explained in the
Vocabulary, Part III.
Ammonio-Nitrate paper has the disadvantage of soon _discolouring_ when
kept; but it is very serviceable in printing during the winter months.
The proportion of Chloride in the salting Bath may, if desired, be
considerably reduced; the intensity of action being greatly exalted by the
use of the Oxide of Silver.
d. _Employment of organic matters._--Those recommended in this work
are--Albumen, Gelatine, and Iceland Moss. Albumen adds much to the
sensibility of the paper, and gives very fine surface definition. A less
amount of Chloride is required than in the case of plain paper simply
salted, the glutinous character of Albuminous liquids causing more of
the fluid to be retained upon the surface of the paper, and the animal
matter assisting the reduction. By varying the proportion of salt, both
feeble and intense Negatives may be printed successfully upon albuminized
paper. No process gives better results, either as regards sensitiveness,
or in faithfully rendering all the finer details of the Negative, than the
process with Albumen.
Iceland Moss, when boiled in water, yields a mucilaginous liquid which is
conveniently employed as a vehicle for Chloride of Silver; it increases
the sensitiveness of the paper and gives additional power of bronzing,
by assisting to reduce the free Nitrate of Silver. Many other organic
matters, tending to absorb oxygen, would act in the same way.
_Gelatine_ is used in positive printing; it is analogous to Albumen in
composition, and, like it, forms a red compound with Suboxide of Silver.
It is serviceable in keeping the print at the surface of the paper, but
does not alter the sensibility or the general appearance of the finished
picture so greatly as Albumen.
e. _Impurities in Nitrate of Silver._--Nitrate of Silver used for
Photographic printing should be free from even a trace of Protonitrate of
Mercury, since it is known that the precipitation of Chloride of Mercury
prevents the darkening of Chloride of Silver by light.
The peculiar condition of Nitrate of Silver spoken of at page 101, in
which it is thought to contain Oxides of Nitrogen, is likely to interfere
with Photographic printing. This is probably the explanation of a
faulty state of the Nitrate solution, in which it yields red and feeble
positives, and does not darken in colour in exciting albuminized paper.
The remedy will be, to fuse the Nitrate of Silver at a moderate heat
before dissolving it.
THE COLOUR OF THE IMAGE INFLUENCED BY THE PREPARATION OF THE SENSITIVE
PAPER.
This subject should be studied by those who desire to print with taste.
By introducing a few simple modifications into the mode of preparing the
sensitive paper, almost any variety of tint may be obtained.
The tendency of the "toning" process, to which the print is afterwards to
be submitted, is to darken the colour, and, if gold be used, to give a
shade of _blue_. Hence, if the Positive be printed of a red tone, it will
change in the gold Bath to a purple; whereas if left, after exposure to
light and fixing, of a dark brown or sepia tint, it passes by toning into
a pure black.
The Positive should look warm and bright on its removal from the printing
frame; but the tint which remains after immersion in Hyposulphite of Soda
is the proper colour of the simply fixed print.
The following points may be mentioned as affecting the colour and general
appearance of the picture.
a. _The proportions of Salt and Nitrate of Silver._--Highly salted and
sensitized papers give a _darker_ image than those which, containing a
small proportion of Chloride of Silver, are less sensitive to light.
Hence in printing upon paper weakly sensitized, in order to bring out the
finer details of a highly intense negative, we find the image unusually
red after fixing, and of a brown or mulberry colour when toned. The above
remarks apply also in some degree to the strength of the Nitrate Bath, and
especially so when no organic matter excepting Gelatine is employed,--in
such a case the image will be _darker_ after fixing, if the proportion of
free Nitrate of Silver be large.
b. _Effect of Oxide of Silver on the colour._--Prints formed upon
Ammonio-Nitrate papers highly salted are of a sepia colour after fixing,
and usually of a pure black or a purple-black when toned. With the
increased facility of reduction by light afforded by use of _Oxide_ of
Silver, there is also less redness in the print. But if the quantity of
salt used in preparing the paper be reduced to a minimum (one grain to
the ounce or less), for the sake of economy or to improve the half-tone,
then the usual red colour returns, and the Positive is brown or purple
after toning, in place of black. Thus by employing a solution of Oxide of
Silver, the operator is enabled, without the addition of organic matter,
to print Positives of a pleasing variety of tint, combined with a peculiar
softness and delicacy, which cannot easily be obtained with the simple
Nitrate of Silver.
c. _The colour affected by organic matter._--Albumen is coagulated by
Nitrate of Silver, and forms a permanent gloss upon the paper. The
sensitive albuminized paper darkens in the sun to a chocolate-brown
colour, which becomes very red on immersion in the Hyposulphite. The
finished prints are clear and transparent; usually of a brown tone, or
with a shade of purple when the gold Bath is newly made and active; pure
blacks are not easily obtained.
Iceland Moss affects the colour of the proof to a certain extent, but less
than Albumen; the finished prints are nearly black if the paper is highly
salted.
The Gelatinous sizing used for the English papers, and obtained by boiling
hides in water, and hardening the product by an admixture of Alum, has
a _reddening_ influence upon reduced Silver salts, analogous to that of
Albumen, or of Caseine, the characteristic animal principle of milk.
Positives printed upon English paper, commonly assume some shade of brown
more or less removed from black; the darker tones being more readily
obtained upon the foreign papers.
Citrates and Tartrates have a marked effect upon the colour of prints.
Paper prepared with Citrate, in addition to Chloride of Silver, darkens to
a fine purple colour which changes to brick-red in the fixing Bath. The
Positives, when toned, are usually of a violet-purple or of a bistre tint,
with a general aspect of warmth and transparency.
SECTION II.
_The Processes for Fixing and Toning the Proof._
This part of the operation is one to which great attention should be
paid, in order to secure bright and lasting colours: it involves more of
delicate chemical change than perhaps any other department of the Art.
The first point requiring explanation is the process of fixing; to which
(p. 41) brief reference has already been made. The methods adopted to
improve the tint of the finished picture will then be described.
CONDITIONS OF A PROPER FIXING OF THE PROOF.
This subject is not always understood by operators, and consequently they
have no certain guide as to how long the prints should remain in the
fixing Bath.
The time occupied in fixing will of course vary with the strength of
the solution employed; but there are simple rules which may be usefully
followed. In the act of dissolving the unaltered Chloride of Silver
in the proof, the fixing solution of Hyposulphite of Soda converts it
into Hyposulphite of Silver (p. 43), which is soluble in an _excess_ of
Hyposulphite of Soda. But if there be an insufficient excess,--that is, if
the Bath be too weak, or the print removed from it too speedily,--then the
Hyposulphite of Silver is not perfectly dissolved, and begins by degrees
to _decompose_, producing a brown deposit in the tissue of the paper.
This deposit, which has the appearance of yellow spots and patches, is
not usually seen upon the surface of the print, but becomes very evident
when it is held up to the light, or if it be split in half, which can be
readily done by gluing it between two flat surfaces of deal, and then
forcing them asunder.
_The reaction of Hyposulphite of Soda with Nitrate of Silver._--In order
to understand more fully how _decomposition_ of Hyposulphite of Silver may
affect the process of fixing, the peculiar properties of this salt should
be studied. With this view Nitrate of Silver and Hyposulphite of Soda may
be mixed in equivalent proportions, viz. about twenty-one grains of the
former salt to sixteen grains of the latter, first dissolving each in
separate vessels in half an ounce of distilled water. These solutions are
to be added to each other and well agitated; immediately a dense deposit
forms, which is Hyposulphite of Silver.
At this point a curious series of changes commences. The precipitate, at
first white and curdy, soon alters in colour: it becomes canary-yellow,
then of a rich orange-yellow, afterwards liver-colour, and finally black.
The _rationale_ of these changes is explained to a certain extent by
studying the composition of the Hyposulphite of Silver. The formula for
this substance is as follows:--
AgO S{2}O{2}.
But AgO S{2}O{2} plainly equals AgS, or Sulphuret of Silver, and SO{3}, or
Sulphuric Acid. The acid reaction assumed by the supernatant liquid is due
therefore to Sulphuric Acid, and the black substance formed is Sulphuret
of Silver. The yellow and orange-yellow compounds are earlier stages of
the decomposition, but their exact nature is uncertain.
The instability of Hyposulphite of Silver is principally seen when it is
in an isolated state: the presence of an excess of Hyposulphite of Soda
renders it more permanent, by forming a double salt, as already described.
In fixing Photographic prints, this brown deposit of Sulphuret of Silver
is very liable to form in the Bath and upon the picture; particularly
so when the _temperature_ is high. To obviate it, observe the following
directions:--It is especially in the reaction between _Nitrate of Silver_
and Hyposulphite of Soda that the blackening is seen; the Chloride and
other _insoluble_ Salts of Silver being dissolved, even to saturation,
without any decomposition of the Hyposulphite formed. Hence if the
print be washed in water to remove the soluble Nitrate, a very much
weaker fixing Bath than usual may be employed. But if the proofs are
taken at once from the printing frame and immersed in a dilute Bath of
Hyposulphite (one part of the salt to six or eight of water), _a shade of
brown_ may often be observed to pass over the surface of the print, and
a large deposit of Sulphuret of Silver soon forms as the result of the
decomposition. On the other hand, with a strong Hyposulphite Bath there is
little or no discoloration, and the black deposit is absent.
The print must also be left for a sufficient time in the fixing bath,
or some appearance of brown patches,[18] visible by transmitted light,
may occur. Each atom of Nitrate of Silver requires _three_ atoms of
Hyposulphite of Soda to form the _sweet and soluble double salt_, and
hence, if the action be not continued sufficiently long, another compound
will be formed almost tasteless and insoluble (p. 44). Even immersion in
a new Bath of Hyposulphite of Soda does not fix the print when once the
yellow stage of decomposition has been established. This yellow salt is
insoluble in Hyposulphite of Soda, and consequently remains in the paper.
[18] The writer has noticed that when sensitive paper is _kept for some
time_ before being used for printing, these yellow patches of imperfect
fixation are very liable to occur. The Nitrate of Silver appears gradually
to enter into combination with the organic matter of the size of the
paper, and cannot then be so easily extracted by the fixing bath.
In fixing prints by Ammonia the Author has found that the same rule may be
applied as in the case of Hyposulphite of Soda, viz. that if the process
be not properly performed, the white parts of the print will appear
_spotted_ when held up to the light, from a portion of insoluble Silver
Salt remaining in the paper. Prints imperfectly fixed by Ammonia are also
usually brown and discoloured upon the surface of the paper.
More exact directions as to the strength of the fixing bath and the time
occupied in the process, will be given in the Second Part of the Work;
at present it may be noticed only that _Albuminized_ paper, from the
horny nature of its surface-coating, requires a longer treatment with the
Hyposulphite than the plain paper.
THE SALTS OF GOLD AS TONING AGENTS FOR PHOTOGRAPHIC PRINTS.
The Salts of Gold have been successfully applied to the improvement of the
tones obtained by simply fixing the proof in Hyposulphite of Soda. The
following are the principal modes followed:--
_M. Le Grey's Process._--The print, having been exposed to light until
it becomes very much darker than it is intended to remain, is washed in
water to remove the excess of Nitrate of Silver. It is then immersed in a
dilute solution of Chloride of Gold, acidified by Hydrochloric Acid. The
effect is to reduce the intensity considerably, and at the same time to
change the dark shades to a violet or bluish tint. After a second washing
with water, the proof is placed in plain Hyposulphite of Soda, which fixes
it and alters the tone to a pure black or a blue-black, according to the
manner of preparing the paper and the time of exposure to light.
The _rationale_ of the process appears to be as follows:-- the Chlorine,
previously combined with Gold, passes to the reduced Silver Salt; it
bleaches the lightest shades, by converting them again into white
Protochloride of Silver, and gives to the others a violet tint more or
less intense according to the reduction. At the same time metallic Gold
is deposited, the effect of which is not visible at this stage, since the
same violet tint is perceived when a solution of _Chlorine_ is substituted
for Chloride of Gold.
The Hyposulphite of Soda subsequently employed, decomposes the violet
Subchloride of Silver, and leaves the surface of a black tint, due to the
Gold and the reduced Silver Salt.
M. Le Grey's process is objectionable on account of the excessive
over-printing required. This however is to a great extent obviated by a
modification of the process in which an _alkaline_ instead of an acid
solution of the Chloride is employed; one grain of Chloride of Gold is
dissolved in about six ounces of water, to which are added twenty to
thirty grains of the common Carbonate of Soda. The alkali moderates the
violence of the action, so that the print washed with water and immersed
in the Gold Bath, is less reduced in intensity, and does not acquire the
same _inky_ blueness. On subsequent fixing in the Hyposulphite, the tint
changes from violet to a dark chocolate-brown, which is permanent.
_The Tetrathionate and Hyposulphite of Gold employed in toning._--After
the discovery of Le Grey's mode, it was proposed, as an improvement,
to add Chloride of Gold to the fixing solution, so as to obviate the
necessity of using two Baths. The print, in that case, although darkened
considerably, is less reduced in intensity, and the same amount of
over-printing is not required. The chemical changes which ensue are
different from before: they may be described as follows:--
Chloride of Gold, added to Hyposulphite of Soda, is converted into
Hyposulphite of Gold, Tetrathionate of Gold, and (if the Chloride of Gold
be free from excess of acid) a red compound, containing more of the metal
than, either of the others, but the exact nature of which is uncertain.
Each of these three Gold Salts possesses the property of darkening the
print, but not to the same extent. The activity is less as the stability
of the salt is greater, and hence the red compound, which is so highly
unstable that it cannot be preserved many hours without decomposing and
precipitating metallic Gold, is far more active than the Hyposulphite of
Gold, which, when associated with an excess of Hyposulphite of Soda, is
comparatively permanent.
When rapidity of colouring is an object it will therefore be advisable to
add Chloride of Gold to the fixing Bath of Hyposulphite rather than an
equivalent quantity of Sel d'or; and by dropping a little Ammonia into the
Chloride of Gold so as to precipitate "fulminating gold"[19] (a compound
which dissolves in Hyposulphite of Soda with considerable formation of the
unstable red salt), the activity of the Bath will be promoted.
[19] Read the observations on the Explosive Properties of Fulminating Gold
in the Vocabulary, Part III.
The Author explains the action of these Salts of Gold upon the Positive
print as follows:--they are unstable, and contain an excess of Sulphur
loosely combined; hence, when placed in contact with the image, which
has an affinity for Sulphur, the existing compound is broken up, and
Sulphuret of Silver, Sulphuric Acid, and metallic Gold are the results.
That a minute proportion of Sulphuret of Silver is formed seems certain;
but the change must be superficial, as the stability of the print is very
little lessened when the process is properly performed.
_Sel Or employed as a toning agent._--This process, which was communicated
to the 'Photographic Journal' by Mr. Sutton of Jersey, has been found
serviceable.
The prints are first washed in water, to which is added a little Chloride
of Sodium, to decompose the free Nitrate of Silver. They are then immersed
in a dilute solution of "Sel d'or," or double Hyposulphite of Gold and
Soda, which quickly changes the tint from red to purple without destroying
any of the details or lighter shades. Lastly, the Hyposulphite of Soda is
employed to fix the print in the usual way.
This process differs theoretically from the last in some important
particulars. The toning solution is applied to the print _before fixing_,
which experience proves to have an important influence upon the result,
it having been found that when the print is previously acted upon by
Hyposulphite of Soda, the rapidity of deposition of the Gold is interfered
with;--thus, a dilute solution of Sel d'or colours a print rapidly, but if
to this same liquid a few crystals of Hyposulphite of Soda be added, the
picture becomes red and may be kept in the Bath for comparatively a long
time without acquiring the purple tones.
As Hyposulphite of Soda in excess lessens the action of the Sel d'or, so
on the other hand the addition of an acid increases it. The acid does
not precipitate _Sulphur_, as might be expected from a knowledge of the
reaction of Hyposulphite with acid bodies (p. 137), but it favours the
reduction of metallic Gold. Hence it is usual to add a little Hydrochloric
Acid to the toning solution of Sel d'or, to increase the rapidity and
perfection of the colouring process.
THE CONDITIONS WHICH AFFECT THE ACTION OF THE FIXING AND TONING BATH OF
GOLD AND HYPOSULPHITE OF SODA.
Although the process of toning Positives by Sel d'or is very certain in
its results and gives good tints, yet, as involving a somewhat greater
expenditure of time and trouble, it is not at present universally adopted.
The ordinary plan of fixing and toning in one bath has been proved to
yield permanent prints if the proper precautions are observed, but it is
quite necessary, in order to ensure success, that the conditions by which
its action is modified should be understood. The more important of these
are as follows:--
a. _The_ AGE _of the Bath._--When Chloride of Gold is added to
Hyposulphite of Soda, several unstable salts are produced, which decompose
by keeping. Hence the solution is very active during the first few days
after mixing; but at the expiration of some weeks or months, if not used,
it becomes almost inert, a reddish deposit of Gold first forming, and
eventually a mixture of black Sulphuret of Silver and Sulphur, the former
of which often adheres to the sides of the bottle in dense shining laminæ.
When the Bath is constantly kept in use there is a loss of Gold, which,
although it is less perceived than it otherwise would be, from the fact
that sulphuretting principles are formed (see next page) capable of
replacing the Gold as toning agents--yet makes the Bath work more slowly,
and hence over-printing is required.
b. _Presence of free Nitrate of Silver upon the surface of the
proof._--This produces an accelerating effect, as may be shown by soaking
the print in salt and water, to convert the Nitrate into Chloride of
Silver; the action then takes place more slowly.
The free Nitrate of Silver increases the instability of the Gold salts;
but if present in too great an excess, it is apt to cause a decomposition
of Hyposulphite of Silver, and consequent yellowness in the white parts
of the proof. It is therefore particularly recommended to wash the print
in water before immersing it in the fixing and toning Bath.
c. _Temperature of the solution._--In cold weather, the thermometer
standing at 32° to 40°, the Bath works more slowly than usual; whereas
in the height of summer, and especially in hot climates, it occasionally
becomes quite unmanageable. The best temperature for operating
successfully appears to be about 60° to 65° Fahrenheit; if higher than
this the solutions must be employed more dilute.
d. _Addition of Iodide of Silver._--Some operators associate Iodide with
Chloride in the preparation of sensitive paper for printing. Another
source of the same salts is the admixture of a portion of the fixing Bath
used for Negatives with the Positive toning solution. The presence of
Iodides in the fixing and toning Bath is injurious: when in large excess,
they dissolve the image, or produce yellow patches of Iodide of Silver on
the lights; in smaller quantity, the deposition of the Gold is hindered,
and the action proceeds more slowly. Bromides and Chlorides have not the
same effect.
e. _Mode of preparing the paper._--The rapidity of toning varies with
causes independent of the Bath: thus, plain paper prints are toned more
quickly than prints upon albuminized paper, and the use of English paper
sized with Gelatine retards the action. Foreign papers rendered sensitive
with Ammonio-Nitrate tone the most quickly.
_On certain states of the fixing and toning Bath which are injurious to
the proofs._--The object of using the Hyposulphite Bath is to fix the
proof and to tone it by means of Gold. But it is a fact familiar to the
photographic chemist, that Positives can also be toned by a sulphuretting
action, and that the colours so obtained are not very different from
those which follow the employment of Gold.[20] Now the Hyposulphite of
Soda is a substance which can be very readily made to yield up Sulphur
to any bodies which possess an affinity for that element, and as the
reduced Silver compound in the print has such an affinity, there is
always a tendency to absorption of Sulphur when the proofs are immersed
in the Bath. Consequently in many cases a sulphur toning-process is
set up, and as the picture is improved by it in appearance, losing its
brick-red colour and assuming a purple shade, it was at first adopted
by Photographers. Experience however has shown that colours brightened
in this way are less permanent than others, and are liable to fade
unless kept perfectly dry. Hence the process will be discarded by all
careful operators, and the object will be to avoid sulphuration as far
as possible. This can be done to a great extent, and, when the Bath is
properly managed, the prints will be toned almost entirely by Gold, and
will, with care, be permanent.
[20] For a more detailed account of the toning process by Sulphur, see the
Third Section of this Chapter, page 145. The instability of sulphuretted
prints is shown in the fourth Section.
Some of the conditions which facilitate a sulphuretting action upon the
proof are as follows:--
a. _The addition of an Acid to the Bath._--It was at one time common to
add a few drops of Acetic Acid to the fixing Bath of Hyposulphite of
Soda, immediately before immersing the proofs. The Bath then assumes an
opalescent appearance in the course of a few minutes, and, when this
milkiness is perceptible, the print begins to _tone_ rapidly and becomes
nearly black.
The chemical changes produced in a Hyposulphite Bath by addition of acid,
may be explained thus:--The acid first displaces the feeble Hyposulphurous
acid from its combination with Soda.
Acetic Acid + Hyposulphite Soda.
= Acetate Soda + Hyposulphurous Acid.
Then the Hyposulphurous Acid, _not being a stable substance when
isolated_, begins spontaneously to decompose, and splits up into
Sulphurous Acid--which remains dissolved in the liquid, communicating the
characteristic odour of burning Sulphur--and _Sulphur_, which separates in
a finely divided state and forms a milky deposit.[21]
[21] From the Vocabulary, Part III., it will be seen that commercial
Chloride of Gold usually contains _free Hydrochloric Acid_; hence
a considerable deposit of Sulphur takes place on adding it to the
Hyposulphite solution, and the liquid must not be used immediately.
Observe therefore that free acids of all kinds must be excluded from the
fixing Bath, or, if inadvertently added, the liquid must be set aside for
some hours until the Hyposulphurous Acid has decomposed, and, the Sulphur
having settled to the bottom, the Bath has regained its original neutral
condition.[22]
[22] The chemical reader will understand the decomposition of free
Hyposulphurous Acid by the following equation:--S{2}O{2} = SO{2} and S.
b. _Decomposition of the Bath by constant use._--It has long been known
that a solution of Hyposulphite of Soda undergoes a peculiar change in
properties when much used in fixing. When first prepared it leaves the
image of a red tone, the characteristic colour of the reduced Silver
Salt, but soon acquires the property of darkening this red colour by a
subsequent communication of Sulphur. Hence a simple fixing Bath becomes at
last an active toning bath, without any addition of Gold.
This change of properties will be found more fully explained in the
abstract of the Author's researches given in the next Section (p. 156). At
present we remark only that it is due principally to a reaction between
Nitrate of Silver and Hyposulphite of Soda, attended with decomposition
of Hyposulphite of Silver (p. 130); and hence, if the prints are washed
in water before immersion in the Bath, the solution will be less quickly
liable to change.
Many operators state that the toning Bath having at first been prepared
with Chloride of Gold, no further addition of this substance will be
required. This no doubt is correct, but in such case the proofs will at
last be toned by Sulphur more than by Gold, and will not possess the
same stability; the Bath will also, after long use, be found to acquire
a distinct _acid_ reaction to test-paper, the acidity being due to a
peculiar principle generated by decomposing Hyposulphite of Silver, and
which is shown to have an injurious action upon the print (p. 158). To
avoid this the solution should be kept _neutral to test-paper_ by means of
a drop of Ammonia, if required; and when it begins to be exhausted, and
does not tone (quickly) a print from which the free Nitrate of Silver has
been removed by washing, a fresh quantity of Chloride of Gold should be
added.
c. _Tetrathionate in the Hyposulphite Bath._--The Author has shown that
the Tetrathionates, which are analogous to the Hyposulphites, have an
active sulphuretting action upon Positive prints (see the papers in the
next Section). Very fine colours can be obtained in this way; but toning
by Sulphur having been proved to be wrong in principle, the formulæ given
in the first two editions of this Work have been omitted.[23]
[23] The preparation of a toning bath by Tetrathionate, without Gold, is
described in the next Section, but it is not recommended for practical use.
The bodies which produce Tetrathionate when added to a solution of
Hyposulphite of Soda, and hence are inadmissible in the toning process,
are as follows:--Free Iodine, Perchloride of Iron, Chloride of Copper,
Acids of all kinds (in the latter case the acid first produces Sulphurous
Acid, and the Sulphurous Acid, if present in any quantity, by reacting
upon Hyposulphite of Soda, forms Tetrathionate and Trithionate of Soda).
Chloride of Gold also produces a mixed Tetrathionate of Gold and Soda when
added to the fixing Bath (p. 133); but as the quantity of Chloride used
is small, the prints are far less sulphuretted than in the case of toning
Baths prepared by Tetrathionate without Gold.
SECTION III.
_The Author's Researches in Photographic Printing._
Having been long engaged in conducting experiments upon the composition
and properties of the reduced material forming the Photographic image, and
especially with a view of determining the exact conditions under which the
picture may be considered permanent, the Author has thought it advisable
to give the results of these researches in the form of an abstract of the
original papers read at the meetings of the Photographic Society.
A previous perusal of these papers will put the reader in possession of
the principal facts upon which are founded the precautions advised in the
next Section for the preservation of Photographic prints. In order to keep
the Work as nearly as possible within its original limits, and also for
the purpose of distinguishing the present Section from the others, as one
referring principally to scientific details, the type has been reduced to
the size of that used in the Appendix.
ON THE CHEMICAL COMPOSITION OF THE PHOTOGRAPHIC IMAGE.
The determination of the chemical nature of the Photographic image in
its various forms is a point of much importance, both as indicating the
conditions required for the preservation of works of art of that class,
and also as a guide to the experimenter in selecting bodies likely to have
an effect as chemical agents in Photography.
It has been stated by some who have given attention to the subject,
that the image is formed in all cases of pure metallic Silver, and that
any observable variations in its colour and properties, are due to a
difference in the molecular arrangement of the particles. But this
hypothesis, although involving much that is correct, yet does not contain
the whole truth, for it is evident that the chemical properties of the
Photographic image often bear no resemblance to those of a metal. One
Photograph may also differ essentially from another, so that we are led
to infer the existence of two varieties, the first of which is less of a
metallic nature than the second.
In investigating the subject, the principal point appeared to be to
examine the action of light upon Chloride of Silver, and afterwards
to associate the Chloride with organic matter in order to imitate the
conditions under which Photographs are obtained.
The following is an epitome of the conclusions arrived at:--
_Action of Light upon Chloride of Silver._--The process is accompanied
by a separation of Chlorine, but its product is not a mere mixture of
Chloride of Silver and Metallic Silver; if it were so, we cannot suppose
that the darkening would take place beneath the surface of Nitric Acid,
which it is found to do. A definite Subchloride of Silver seems to be
formed, the most important property of which is its decomposition by
fixing agents, such as Ammonia, and Hyposulphite of Soda, both of which
destroy the violet colour, dissolving out Protochloride of Silver, and
leaving a small quantity of a grey residue of metallic Silver.
Inasmuch therefore as all Photographic pictures require fixing, we may
conclude that if they could be produced upon pure and isolated Chloride
of Silver (which however is not the case), they would consist solely of
metallic Silver.
_Decomposition of organic Salts of Silver by Light._--Compounds of Oxide
of Silver with organic bodies, are as a rule darkened by exposure to
light, but the process does not always consist in a simple reduction to
the metallic state. This assertion is proved by the employment of the
following tests.
a. _Mercury._--Little or no amalgamation takes place on triturating the
darkened salt with this metal.
b. _Ammonia and fixing agents._--These usually produce only a limited
amount of action. Thus, the Albuminate of Protoxide of Silver is perfectly
soluble in Ammonia; but after having been reddened by exposure to light,
it is little or not at all affected.
c. _Potash._--Animal matters coagulated by Nitrate of Silver, and reduced
by the sun's rays, are dissolved by boiling Potash, the solution being
clear and of a blood-red colour. Metallic Silver, it is presumed, if
present, would remain insoluble.
d. _Boiling Water._--Gelatine treated with Nitrate of Silver and exposed
to light, loses its characteristic property of dissolving in hot water.
This experiment is conclusive.