Sulphuric acid Small quantity
Water to make up to 10 oz. or 100 parts
About three-quarters of the total quantity of water is mixed with a
small quantity (not more than 50 minims per 10 ozs., or one part per
100) of sulphuric acid, and the ferrous sulphate (proto-sulphate of
iron) which must be in clear pale green crystals without any
yellowish incrustation, is dissolved in it with the aid of a gentle
heat. After the solution has cooled, it is made up to the specified
volume with water. This solution alters when exposed to air, and
should, therefore, be kept in small (2 oz.) bottles, filled up to
the neck and tightly corked.
OXALATE SOLUTION.
Potassium oxalate 10 oz. or 25 parts
Potassium bromide 40 grains or 0.23 part
Water to make up to 40 oz. or 100 parts
For use take four parts of oxalate solution and one part of ferrous
sulphate solution, pouring the latter into the former and _not vice
versa_. In order to obtain slower action with a rather softer image
and a slightly browner deposit, the developer may be diluted with an
equal volume of water. Slower action, with slightly increased
printing contrasts, and clearer shadows, results from an increase in
the proportion of bromide.
FIXING.
After development is finished, the dark-coloured reduced silver that
forms the image remains mixed with a considerable quantity of
semi-opaque, yellowish unaltered silver bromide, which would not
only interfere with the printing, but would also gradually darken
when exposed to light. The negative must therefore be "fixed" by
dissolving out the unaltered silver bromide, and this is
accomplished by immersing the plate in a fairly strong solution of
sodium thiosulphate (formerly called sodium hyposulphite) commonly
known as "hypo." The usual strength of the fixing is as follows:
FIXING BATH.
Hypo (sodium thiosulphate) 10 oz. or 25 parts
Water to make up to 40 oz. or 100 parts
A solution of double this strength is, however, not unfrequently
used, and acts more rapidly, especially in cold weather.
The developed plate, after being well rinsed with water, is placed
in the fixing bath and allowed to remain in it with frequent rocking
until the silver bromide has all been dissolved out of the film.
This is ascertained by lifting the plate out of the dish and looking
at the back by reflected light, the plate being held in front of
something dark. It is not difficult to see whether the silver
bromide has all disappeared or not, but in order to ensure complete
fixing the plate must not be taken out of the bath as soon as this
has happened, but should be left in for a few minutes longer, the
dish being rocked so that the dissolved silver salt may diffuse out
of the film into the fixing bath.
When removed from the fixing bath the plate should be allowed to
drain into the bath for a few moments and should then be washed for
five or ten minutes in running water under the tap. It is best to
put the plate in a dish standing on the sink and have a piece of
flexible indiarubber tubing reaching from the tap to within a couple
of inches or so of the top of the dish, so that the water may not
splash too much. After washing in this way, the plate is placed in
a grooved zinc rack, which is immersed in a tank (preferably of
zinc), containing sufficient water to completely cover the plates,
and here it remains until the whole batch of plates in hand has been
developed and they can all receive their final washing together. The
plates stand upright in the rack, and the entrance and exit of the
water must be so arranged that the water enters at the bottom and
overflows at the top, or, what is perhaps better, enters at the top
and is drawn off from the bottom, the waste pipe opening at the
bottom of the tank and being bent and carried upwards until its
mouth is at the level at which the water is to stand in the tank.
When running water is not available the plates may be washed in
dishes. After being well rinsed to remove the adhering hypo
solution, the plate is covered with water (about 3-1/2 oz. for a
half plate or 5 oz. for a whole plate) and allowed to remain with
frequent rocking for five or six minutes. The water is then well
drained off, a second quantity added and allowed to remain for the
same time as before, with frequent rocking, when it is poured off in
its turn. Treatment in this way with six successive quantities of
water will remove all the hypo, provided that the film has not been
treated with alum.
Another plan, rather less troublesome, but also less expeditious, is
to place the rack containing the plates in a tank not much more than
big enough to hold it, taking care that there is not less than two
inches between the lower edges of the plates and the bottom of the
tank. After standing for some time the rack and the plates are
slowly and carefully lifted out and allowed to drain, the tank
emptied and filled with fresh water, and the rack and plates then
replaced. Eight or ten successive quantities of water applied in
this way should remove all the hypo, but if there is any doubt on
this point the plates, after they are supposed to be washed and have
been removed from the tank, should be allowed to drain into a
measuring glass or into a dish, the contents of which are afterwards
transferred to a measuring glass and mixed with a small quantity of
a solution of silver nitrate. If the plates are really completely
washed nothing will happen, or at most a white precipitate will be
produced which _will remain white_ if not exposed to daylight. If,
on the other hand, the plates still retain hypo, the silver nitrate
will produce a precipitate which will gradually become orange and
eventually dark brown. Should this happen, the washing must be
continued.
[Illustration]
DRYING.--If the negatives are allowed to dry in the rack in which
they were washed, the process is slow, and sometimes if the washing
has not been complete, the middle portions of the negatives, which
dry last, are less opaque than the rest. An excellent method of
drying negatives rapidly and in such a way that no dust can fall on
the film, is to drive nails (preferably of copper) into a wall or a
board fixed against the wall, at distances apart depending on the
size of the plates. Each plate then rests, with the film
downwards, between a pair of nails, the lower corner of the plate
resting against the wall, as shown on previous page.
[Illustration: WINTER TIME ON THE ALPS. CAPT. W. DE W. ABNEY, C.B.,
F.R.S., ETC.]
ALUM BATH.--It is frequently recommended that all plates should be
immersed in a strong solution of alum, for the purpose of preventing
"frilling" by hardening the film. Its use is, however, attended with
the great disadvantage that liquids diffuse into and out of a film
so treated with much greater difficulty than in the case of an
ordinary film, and consequently if the film is alumed between
development and fixing, the fixing is not only much slower, but the
washing after fixing requires a very much longer time. If,
therefore, the alum bath is used at all, it should not be applied
until after the film has been well washed after fixing. The
following solution may be used:--
ALUM BATH.
Alum 1 oz. or 5 parts
Water 20 oz. or 100 parts
If a strong solution of alum is applied to the plate for a long
time, the film may become so thoroughly hardened that it partially
loses its adhesiveness, and there is a possibility that it will
begin to peel from the glass after the negatives have been stored
for some time in a dry place. The solution given above is quite
strong enough and plates need not be immersed in it for more than
five minutes, after which they must, of course, be again well
washed. The hardening of the film, if not carried too far, no doubt
makes it less liable to be injured by abrasion and the like.
FRILLING.--It sometimes happens that during the various operations
of development, fixing and washing, the film begins to leave the
plate and rise in puckers along the edges. This is known as
"frilling," and in bad cases it may spread until a large part of the
film has detached itself from the glass. It is due to excessive or
irregular absorption of water by the gelatine, and at one time was
commonly met with, but it rarely occurs with the dry plates of the
present day. It is most likely to arise if there is any considerable
difference of temperature between the various liquids and the
wash-water, or during very hot weather when all the liquids are much
warmer than usual.
When frilling does occur, the plate must be treated carefully, so as
to avoid tearing the film, but unless it is very bad and shows a
tendency to spread, all the operations, including washing after
fixing, should be completed before any special measures are taken to
remedy the defect. On the other hand, if the frilling spreads
rapidly, the plate should be carefully rinsed two or three times
with water and placed for five minutes in the alum bath, with
occasional gentle rocking, after which it is again well washed to
remove the alum, and the various operations are completed. There is
one exception to the procedure just indicated; if the frilling
becomes bad while the plate is being fixed or during washing after
fixing, the alum must not be applied until the fixing and the
washing after fixing are completed. If something must be done in
these circumstances, the plate, after draining, but without any
previous washing, may be placed for about ten minutes in a saturated
solution of common salt. It can afterwards be put back into the
fixing bath, also without any intermediate washing, and the
remainder of the process carried through.
Although the methods just described will check the frilling, they
will not remove its effects. For this purpose the plate after its
final washing is allowed to drain thoroughly and is then immersed in
methylated alcohol, preferably of the old kind, though the new kind
can be made to do. The alcohol abstracts water from the film, which
consequently shrinks to its original size and can be pressed back
with the fingers into its proper position on the plate. Should the
film be opalescent it should be removed from the first quantity of
alcohol and placed in a second quantity, after which it should be
set up to dry. The plates should not remain too long in the alcohol
or the gelatine will contract too much.
DEFECTS IN NEGATIVES.
A perfect negative presupposes a perfect plate, correct exposure,
and correct development stopped at exactly the right time. It is
almost unnecessary to say that all these conditions are rarely
satisfied, and consequently most negatives fall more or less short
of perfection. The defects may be broadly grouped under two heads,
namely, those due to imperfections existing in the film before
exposure, and those due to defects or errors in the way in which the
plate has been treated. It will be more convenient to deal with the
latter, and larger, group first, but there is really no hard and
fast division between them.
THE NEGATIVE IS THIN, or in other words, whilst showing good
gradation, and sufficient relative contrast between the different
parts, is as a whole lacking in opacity or printing strength, and
gives prints that are deficient in vigour and contrasts. The plate
has been removed from the developer too soon, and the remedy is to
intensify the image (see p. 51). Sometimes the want of opacity is
due to the fact that the developer was too cold.
THE NEGATIVE IS TOO DENSE OR OPAQUE and consequently although
showing good contrasts and gradations, takes a long time to print,
especially on dull days. The developer has been too energetic, or
development has been continued too long; the remedy is to reduce the
image (see p. 50).
THE IMAGE IS "FLAT," or shows comparatively little contrast between
the highest lights and the deepest shadows. This may, of course, be
due to the absence of contrasts in the subject photographed; it is
commonly due to over-exposure; it may be caused by using a developer
containing too little reducer, or restrainer, or both, and too much
alkali; sometimes it arises from a defect in the quality of the
emulsion, or from the fact that the plate has been coated with an
abnormally thin film of emulsion.
THE IMAGE IS "HARD," or shows excessive contrasts between lights and
shadows, and is defective in the range of its half-tones. This is
probably due to under-exposure, but may have been aggravated by the
use of a developer containing too much bromide or too little alkali.
Local reduction (see p. 50) may partially remedy the defect.
FOG.--A more or less marked grey deposit of reduced silver extends
over the whole surface of the image. It may be due to over-exposure,
in which case the edges of the plate that have been protected by the
rebate of the dark slide usually remain clear. It may also be caused
by using a developer containing too much alkali, or too little
restrainer, or both, or by the plate having been exposed to actinic
light outside the camera, including the light from the dark-room
lamp if the glass or coloured fabric used as the screening material
is not efficient. In any of these cases the defect would be
observable up to the extreme edges of the film.
The character of the dark-room light should be tested by exposing
one half of a plate to it at a distance of say nine or twelve inches
for five or ten minutes, the other half of the plate being
protected by some opaque substance. The best plan is to put the
plate into a dark slide and draw out the shutter half-way. After
exposure the plate is treated with a developer in the usual manner,
and it can then be seen whether or no the light has exerted any
action on the plate.
Slight general fog may as a rule be neglected, but if the amount of
fog is at all considerable the plate should be treated with a
reducer, and afterwards the image can, if necessary, be intensified.
GREEN FOG.--The surface of the film shows a peculiar brilliant green
or yellowish-green lustrous appearance, generally in patches, when
examined by reflected light, but is more or less distinctly pink
when the plate is looked through. This effect is rarely observed
except when pyro-ammonia has been used as the developer, and it most
frequently occurs with old plates, especially if development has
been long continued or has been forced by the addition of
comparatively large quantities of ammonia.
If the green fog is only slight it does not affect the prints made
from the negative, but in bad cases the prints have a patchy
appearance and are less deeply printed at those points where the
green fog is worst. Two methods are available for the removal of
green fog.
In one of these the plate, after being fixed and washed, is placed
in a hypo solution of half the strength of the ordinary fixing bath,
and to this hypo solution is added a very small quantity of a
solution of potassium ferricyanide, and the mixture is allowed to
act on the plate for some time, the dish being rocked occasionally.
The green fog will gradually disappear and some more of the
ferricyanide may be added, if necessary, to secure this end, but it
is important to keep the proportion of ferricyanide as low as
possible, otherwise the image itself will be reduced. For this
reason, if it is seen or suspected that the green fog is likely to
be bad, development should be carried a little farther than usual in
order to allow for the slight reduction that accompanies the removal
of the green fog.
The other plan is to immerse the plate in a dilute solution of
ferric chloride (perchloride of iron) until the green fog has been
completely bleached, then wash, first in a dilute solution of oxalic
acid and afterwards in water, and finally treat with a developer,
preferably ferrous oxalate. The green fog is converted into a very
fine grey deposit which is almost invisible and has no appreciable
effect on the printing qualities of the negative.
BLACK SPOTS may be due to particles of dirt that have been allowed
to lodge on the film during one or other of the operations, or
during drying. They may also be due to particles in the emulsion,
and in the latter case are generally round and sharply defined.
BLACK MARKS of the nature of irregular streaks, looking, so to
speak, like black scratches, are generally due to mechanical
abrasion of the film. Pressure produces a developable image similar
to that produced by the action of light.
BLACK BANDS, indistinct or nebulous at the edges, are sometimes
caused during the coating of the plate with the emulsion, in which
case they, as a rule, extend all the way along or across the plate.
More commonly they are due to defects in the hinges of the dark
slides, which may produce the bands either by allowing light to pass
through, or by giving off exhalations that affect the plates if they
are allowed to remain in the dark slide for a long time. If the
bands are due to the hinges, they will, of course, correspond with
them in position, and if the hinge is double, in the distance
between them.
TRANSPARENT BANDS, or bands showing less opacity than the rest of
the image, are sometimes caused by exhalations from the material
forming the hinges of the dark slides.
TRANSPARENT SPOTS if small ("pinholes"), are generally due to the
presence of particles of dust on the surface of the plate when it
was exposed. Prevention lies, of course, in carefully dusting the
plate and the dark slide with a soft, clean, dry camel's hair brush,
before putting the former into the latter. If the spots are larger
and circular, they are due either to the formation of air bubbles on
the surface of the plate during development, or to the presence in
the film of insensitive particles.
UNEVEN OPACITY OR DENSITY, varying gradually from one end or side of
the plate to the opposite end or side, is due to uneven coating of
the plate. If there is a distinctly defined patch, less opaque than
the rest, the plate was not properly covered by the developing
solution.
STAINS.--A uniform stain, of a yellowish or brown colour, is
produced when the pyro developer contains too small a proportion of
sulphite or is allowed to act for a very long time. Such a stain is
rarely observed with the other developers mentioned above. The pyro
stain can be more or less completely removed by immersing the plate
for some time, with repeated rocking, in the alum solution given
above, 1 drachm of sulphuric acid being added to every 10 ounces.
The plate must afterwards be well washed in soft water. Similar
stains in patches may be caused by using dirty dishes or a developer
that has become turbid by being frequently used.
DEEP YELLOW-ORANGE OR BROWN STAINS, appearing gradually in patches
or all over the negative, some time after it has been fixed, and
washed, and dried, are due either to imperfect fixing or to
incomplete washing after fixing. There is no practicable remedy.
[Illustration: DOCK By Carine Cadby.]
HALATION.--When the subject photographed includes some part much
more brightly lighted than the rest, such as a window in an interior
subject, the details of the bright part are not only lost, but the
image of it seems to spread in all directions, obliterating the
details of the surrounding portions. The effect is especially
noticeable when the subject includes dark parts which necessitate a
somewhat long exposure. A window at the end of a long dimly lighted
interior, or dark trees against a bright sky are cases in point. The
effect is really due to the fact that the sensitive film is not
perfectly opaque, and some of the incident light passes through the
film and is reflected from the back surface of the glass on to the
under side of the film, producing a blurred image superposed, as it
were, on the normal image formed at the surface of the film by the
action of the direct light. The effect is known as "halation." It is
prevented by having a perfectly opaque film, which is a condition
difficult to realize in practice, and which, moreover, introduces
certain other disadvantages. It is also prevented by coating the
back of the plate with some substance that will absorb the rays that
have passed through the film, and so prevent their being reflected
back against the under side of the film. The substance used must
either be opaque or must have a deep orange, brown, or red colour,
and it must have the same refractive index as the glass, otherwise
the reflection will not be prevented. For practical convenience it
must also be easily applied and easily removed. Many substances have
been recommended but nothing is so good as caramel, prepared by the
action of heat on sugar. In order to get the mixture to dry
completely after it has been applied, a somewhat troublesome process
of purification is necessary, but caramel specially prepared for the
purpose can now be obtained from dealers in photographic materials.
The caramel (which is a solid substance) is dissolved in just enough
water to make a thick syrup, which is carefully applied to the back
of the plates in a thin layer by means of a flat brush.
If the caramel does not dry properly the solution may be thoroughly
mixed with about one quarter (or more) of its weight of very finely
powdered burnt sienna or burnt umber, "ground in water."
After being coated, the plates require some time to dry, and must,
of course, be carefully protected from light. If the dark-room is
thoroughly dark, the plates may be put up to dry in the same manner
as negatives (see page 40), but if the dark-room is not suitable,
some sort of drying box must be used.
After exposure and before development the backing is removed with a
damp sponge; if caramel only is used in a form completely soluble in
water, it need not be removed unless a developer is being used that
is to be applied to several plates in succession.
REDUCTION.
When a negative is too opaque or dense it must be reduced by
dissolving away part of the silver that forms the image. The same
process is also applied for the removal of general fog, sometimes
with a view to subsequent intensification.
The simplest solution to use for this purpose is known as the Howard
Farmer reducer and is a solution of hypo mixed with a small quantity
of potassium ferricyanide (red prussiate of potash).
FERRICYANIDE SOLUTION.
Potassium ferricyanide 1 oz. or 10 parts
Water to make up 10 oz. or 100 parts
This solution must be protected from light if it is to be kept for
any length of time.
The negative which, if it has been previously dried, must be soaked
in water for some time until it is thoroughly and uniformly wetted,
is placed in some fresh hypo solution (the ordinary fixing-bath
solution diluted with an equal volume of water) to which a small
quantity of the ferricyanide solution has been added, and the dish
is rocked repeatedly to ensure uniform action. The rapidity of the
reducing action depends on the proportion of ferricyanide solution
added, and it is very important not to add too much, otherwise the
process gets out of control and reduction goes too far. The image
should be carefully watched and the plate removed from the solution
and rapidly washed before the apparent reduction is quite as great
as it is intended to be. It is much better to stop too soon than too
late, because if it is found that a little further reduction is
necessary, the plate can be again immersed in the hypo and
ferricyanide.