various similar ways (_i.e._, where economy is necessary as,
unfortunately, it sometimes is) the cost of practising our pet
recreation is very materially reduced.
_W. Ethelbert Henry, C.E._
_The Gum-Bichromate Process._
[Illustration]
Pictorial photography is answerable for the revival of this, one of
the almost forgotten methods of printing. Results unacceptable to
bygone requirements have been reintroduced with advantage, where
suggestive individuality and artistic effect have been desired.
The gum process has an unlimited range of possibilities, it would be
impossible to describe them all. The minutest details, or the
broadest diffusion together with the power of working from the
highest to the lowest keys of _chiaroscuro_ are values that can only
be realized when the infatuation consequent on successfully working
the process is experienced.
This method of printing, as with the so-called "carbon process," is
dependent upon the characteristic behaviour of the chromic salts
when in combination with organic substances, such as gelatine, gums
of various kinds, starch, etc.
When any of these mixtures are submitted to the action of actinic
light, they become more or less insoluble.
This property was partially discovered as far back as 1798, by
Vauquelin. Professor Sucrow, Mungo Ponton, Beauregard and others
advanced its application to photography up to about 1840, but it was
not until some ten years later that its great value as a
photographic agent was definitely established.
Hunt, Fox Talbot and Poitevin, each worked indefatigably to bring
the application of the chromic process to a successful issue; but to
Poitevin must be accredited the honour of being the original
inventor of the chromated pigment or carbon process. This brings us
up to about 1855.
None of these investigators appear to have been remarkably
successful, beyond having established definite, but valuable facts
of the changes produced.
This want of success may possibly be accounted for by the general
employment of gelatine and direct printing. It was not until Pouncey
and others, about 1859, employed gum as the colloid medium, that any
great advance was made.
About this time an important commission of inquiry decided that to
Pouncey, Gamier and Salmon, and Beauregard the honour of producing
permanent prints must be equally credited, and accordingly divided
the Duc de Luyue's prize between them, giving to Poitevin the credit
of the priority of invention.
Pouncey appears to have followed up the process with some
considerable success, as some of his existing examples are
excellent; it is much to be regretted that we have not more detailed
particulars of his methods of working; but he evidently was before
his time and met with but little encouragement.
To Alfred Maskell and M. Demachy must be accredited the revival of
this long neglected process, and during the last three years much
advancement has been made towards perfecting it.
Serious workers, both at home and abroad, are industriously
exhausting the possibilities of the process, and crude as some of
the earlier examples of this revival have been, improvements and
simplicity of working are giving us productions of every
description, of such excellent quality that it may soon be expected
to satisfy even the caustic criticism that has so persistently
opposed its re-introduction.
Dexterity in the various stages of practical manipulation is
necessary before skilful efficiency can be secured, and in order to
arrive at this, due consideration must be given to the selection of
the paper the colour most suitable to the subject and the effect
desired.
Almost any kind of paper will be found workable, if it be of fairly
good quality. Those that are thickly coated with soluble sizing
media are unsuitable, for although they may give clear whites they
sometimes produce harsh prints, the half-tones are also liable to be
lost in development unless very deeply printed. Several of the
continental kinds are well adapted to the process and work in an
excellent manner, giving soft and even results; of course, it will
be understood that for definition and fine detail the finer grained
descriptions are the best, but where diffusion is desired those of a
coarser texture may be advantageously used, they give a granulation
that tends materially to secure the peculiarities of gradation
characteristic of this process.
A few of the continental papers that will be found to work with ease
to the beginner, are as follows:--
Michallet paper is rather coarse, but takes the gum coating easily,
it has a series of lines running in both directions, which are
rather objectionable for some subjects; but it is an excellent paper
for first experiments.
Ingrés, is also a paper of similar character, and can be worked with
equal facility. Lallane is another paper of the same class, but much
finer.
Allongé paper is entirely free from the markings peculiar to those
previously mentioned. This paper is best worked on the reverse side,
which can be distinguished by examining the name marked in one
corner.
Among the English papers the ordinary cartridge, Whatman's drawing
papers and many others are adaptable, but it must be borne in mind
that those with a toothed or grained surface are preferable.
There are two methods of working, and results of equal excellence
have been produced by either. Some of the most proficient workers of
the process adopt the easier one of coating the paper, without
previous preparation, with a mixture of gum, bichromate of potass
and pigment. Others adopt the precaution of first saturating the
paper with a strong solution of bichromate, and when dry coating it
with a mixture containing only gum and pigment.
Experience is in favour of the previous saturation of the paper,
this is recommended especially for beginners, as there are several
kinds of paper that will not work efficiently by the first method;
but when skill and practical knowledge of the special behaviour of
the materials employed is acquired, either method can be adopted.
We may presume that the advantage of the previous saturation of the
paper with the chromic salt is, that should there be any inequality
in its structural character, or should it be unequally sized, the
bichromate appears to act as a kind of resist to the penetration of
the pigment, thereby securing an increased range of tone and a
corresponding purity of the whites.
The process may be divided into the following operations:--
Saturation or sensitizing of the paper.
Preparation of the gum mucilage.
Mixing and preparing the pigments.
Coating the paper.
Printing and exposure.
Development.
For working by the previously chromated paper method, the
sensitizing solution is made up of one part of bichromate of
potassium dissolved in ten parts of water. This strength will not
keep at all temperatures. Should the salt crystallize out, it is
necessary to warm a portion of the solution and re-dissolve the
crystals. The solution may be used repeatedly, but it will be
necessary to filter it occasionally.
Before saturation it is convenient to cut the paper into the most
useful sizes--quarter sheets are handy. Having decided which is to
be the working side, mark the back distinctly. Into a dish of
sufficient depth pour in the one in ten bichromate solution to a
depth of about one inch, and immerse your paper sheet by sheet,
until you have in it all you intend to sensitize. As each sheet is
placed in the solution, remove air bells and turn it over and repeat
this precaution. The time necessary for immersion is of no
importance so that the saturation is absolute, about five minutes
being generally sufficient for the thickest of papers. By removing
the bottom sheet to the top and passing through the whole in this
manner, turning over each sheet and removing all air bells, even
saturation is secured. Each sheet is carefully and slowly removed
from the solution and dried in the dark. The paper is now very
sensitive to actinic light, which must during all future operations
be carefully guarded against.
After the paper is dry, it will--if kept so--be in good condition
for a long time.
To prepare the gum mucilage, take two ounces of Soudan or Turkey gum
and dissolve it in five fluid ounces of cold water, strain out the
floating impurities through fine muslin, and allow others, and finer
to subside. This mucilage will keep in good condition in a
well-corked bottle, for a considerable time. M. Demachy employs gum
mucilage of twice this density.
Pigments in powder are more suitable than in any other form, if in
cakes or paste. The medium in which they are prepared, does not work
kindly with the gum, and it is also difficult to accurately measure
quantities. No advantage is gained by using expensive colours, they
can all be purchased at a good colourman's, and at a small cost.
Lamp or any carbon, black, red ochre, yellow ochre, burnt sienna,
and raw sienna, all work well; there is some uncertainty with the
umbers and sepias. It will be found that much time will be saved if
a combination of these dry colours is made up in bulk, as for
instance--one hundred and seventy-five grains of vegetable black and
one hundred of burnt sienna, give a rich soft brown colour. These
must be finely and intimately mixed with each other, which is
conveniently accomplished by grinding with a small pestle and
mortar; after which the mixture may be kept in a wide-mouthed
bottle. Another advantage in thus keeping combinations of dry
colours in bulk, is the absolute certainty of repeating the actual
tint when required.
Various combinations of similar mixtures can be made. Of course it
will be understood that any or all of the above-named colours may be
used singly.
The grinding of the pigment with the mucilage is easily done on a
stone slab with a palette knife. Take half a fluid ounce of the two
in five gum mucilage, to which add the same quantity of water and
thoroughly mix. Weigh out fifteen grains of the mixed pigment and
place in a heap on the slab, add a few drops of the diluted
mucilage, grind and regrind the mixture until it is completely
smooth, then remove it to a cup, and clean the stone with another
portion of the reduced mucilage, finally adding the whole of the
ounce, intimately mix, and it is then ready for coating the paper.
For extra fine work on smooth paper, and in fact for all classes of
work, the fine grinding of the colours adds materially to extend the
range of gradation, and although the trituration may be carried out
fairly well with a palette knife, when the finest possible grades
are desired, recourse must be had to the muller and stone. Mullers
are obtainable of any artist's colourman, they are made in glass,
and a convenient size is about one inch in diameter.
The most convenient brush for applying the mixture of combined gum
and pigment to the paper, is of the description known as bear's
hair, these are usually set in tin; a flat one about two inches wide
is a useful size.
In order to coat the paper evenly, pin it down to a drawing board by
each corner with a double layer of blotting paper an inch or two
larger than the paper to be coated. The blotting paper will absorb
the excess of colour at the margins and enable you to secure an even
coating up to the extreme edge.
Take a fairly full brush of the mixture after thoroughly
incorporating the colour and spread it evenly over the paper,
crossing and recrossing it with the brush. Allow the mixture to lie
upon the paper for a second or two so that the paper may expand; now
release each of the corners and pin the paper down again. Upon the
next operation depends the evenness of the coating.
Take a four inch wide artist's badger's hair softener, hold it
vertically and lightly by two fingers and the thumb about an inch
and a half from the top of the handle, and pass it rapidly over the
whole surface of the paper as quickly and evenly as possible. The
motion producing the best effect is not the usual sweeping action,
but a series of sudden short jerks, difficult to describe but easily
acquired. Continue this softening down until the paper has an even
semi-transparent surface without uneven cloudy spots. Allow it to
dry spontaneously, but before it is stored for future use dry it
carefully by the fire, but avoid overheating.
Uncertainty of result is a defect often brought into argument against
this process; but absolute uniformity is not difficult if strictly
accurate quantities only are employed. With constant strength of
bichromate and gum, uniform weights and combination of pigment,
similarity of repeats are obtained: but these can only be secured when
each sheet of paper is coated identically with its fellow. To get this
evenness the badger hair softener must be washed out and dried after
coating each sheet. This is very quickly accomplished by an energetic
shaking and drying upon a smooth towel. If the paper has been coated
properly, it has an even semi-transparent surface slightly glassy.
Failures often occur from using an excess of pigment and allowing
the gum to become too thick in consequence of evaporation. Excess
of pigment gives dense heavy shadows and increases the difficulty of
printing; excess of gum gives clear high-lights, tending to hardness
and easy solubility endangering the half-tones.
The paper, if it has been correctly coated will work satisfactorily,
if on steeping a small piece of it downwards upon cold water, the
pigmented gum dissolves and drops from the surface leaving the paper
nearly clean. From ten to fifteen minutes should complete this test.
The method of working without previously chromatizing the paper is
as follows:--Take half a fluid ounce of four-in-ten gum mucilage and
add to it an equal quantity of saturated solution of bichromate of
potass; to this, with all care as to grinding and mixing, add the
pigment; coat the paper as before directed. This method will be
considerably slower in printing than that in which the paper had
been previously saturated with the bichromate; neither are the
whites as a rule quite so clear; but it will possess a peculiar
grain and softness not otherwise obtainable, which is much approved
by some workers of the process.
Exposure is so much dependent on circumstances that it is difficult
to give precise directions, being governed by the density of the
negative, the thickness of the coating and the intensity of the
light. Even and not too dense negatives are the more suitable, for
if the intermediate and high-lights are over dense the shadows are
considerably over printed before the lighter parts can be brought
out. Skill in development can do much to overcome these defects, but
they may be considerably modified by the judicious employment of
matt varnish, and by other methods of locally retarding printing.
The greatest assistance in obtaining uniformity in printing is the
employment of a reliable actinometer, Wynne's print meter is
probably the most useful for this purpose, with ordinary
gelatino-chloride paper as a register; from twelve to sixteen
numbers will be mostly sufficient for an ordinary negative, on not
too thickly coated paper. Another method of judging exposure is by
the appearance of the shadows; they may frequently be seen by
transmitted light, and when well out printing may be judged to be
correct, but this is a slovenly method and only approximately
correct at the best.
If the bichromate is used only in the pigmented gum, without
previous saturation of the paper, exposure must be much more
prolonged.
By no other process is it possible to obtain such diversity of
effect as by this; much will, however, depend on the skill which is
exercised in development. Should the printing exposure have been
fairly correct it is a simple procedure. The print is floated face
downward upon cold water contained in a deep dish; see that all
parts are equally acted upon by the water, and that no air bells
exist; if any, they may be easily removed by gently raising the
print and immersing it again once or twice. After it has been
soaking some five or ten minutes it may be examined; if all is going
well, and the exposure has been approximately correct, the pigmented
gum on the unexposed margins will have left the paper, and possibly
some of the high-lights and half-tones may be making an appearance,
if so, the treatment must be of a gentle character, and the print
may be safely left for some time longer in the same position face
downward; never allow it to lie either in or out of the water face
upwards for any long time, or unremovable stains will be developed.
Many prints will develop almost entirely without assistance, or
with only an occasional laving of water if allowed to lie in this
position for a long time. On the other hand some may, even when only
slightly over-printed, give no indication of development. When this
is the case remove the print from the water and place it face
upwards upon a thin, smooth board, fix it in position with one
drawing pin on the extreme margin, then gently lave cold water over
it; should some of the darker parts still resist this action, longer
soaking will be found advantageous. If there are still parts on
which the colour will not move, recourse must be had to the brush,
and for this purpose nothing is better than a large camel's hair
mop. Keeping the brush always full of water, touch where necessary
very softly; do not sweep it up or down, but just dab here and there
as may be required, constantly flowing over the surface a copious
supply of water.
If there are still parts in the shadows, or even in the high-lights
that will not move, a jet of water from an enema syringe or from the
household service pipe is very useful.
As a last resource a prolonged steeping in water of varying degrees
of temperature, even up to the boiling point, may be resorted to,
but the application of increased temperature requires judicious
management.
When the print is sufficiently developed, if the creamy yellowness
of the chromate stain is not desired, the print must be cleared or
bleached, either in a solution of alum, sulphite of soda, or
hyposulphite of soda, strength being immaterial with a careful
after-washing. If the print is only just sufficient or only slightly
over-printed, care must be taken that the clearing bath is not acid,
neither must the washing be too prolonged, but if the print is
first dried and submitted to light, this precaution is unnecessary.
Always allow the prints to dry spontaneously. It will not do to use
any kind of pressure or blotting paper, for the surface of the
colour is very tender and delicate.
_Jas. Packham, F.R.P.S._
_An Introduction to Carbon Printing for Beginners._
[Illustration]
In the article that follows next will be found a complete exposition
of the carbon process, with its various adaptations from the
preparation of the paper and material forwards.
Whilst at the present time carbon printing is more largely used by
professional photographers, yet its simplicity, the absence of
chemical formulæ and complications combined with the beauty of the
results, makes it eminently suitable for amateur workers, and hence
it has been thought desirable that as an introduction to the
subsequent article, a brief and simple outline of the process should
be given for the benefit of those who have not hitherto made its
acquaintance.
In the first place then let it be understood that in carbon printing
instead of depending on light to make a visible alteration of the
sensitive salts as in silver printing, we expose the prepared paper
or "tissue," as it is called, under a negative and secure a positive
in insoluble gelatine, the gelatine having combined with it a
pigment, and hence we get an image in pigment, not in platinum, or
silver, or gold, but in a simple pigment which may be of any colour.
If bichromate of potash is mixed with an organic substance such as
gelatine, that gelatine becomes insoluble after exposure to light,
and if that gelatine carries with it a pigment, then on becoming
insoluble it holds the pigment with it. If now, paper or other
material be coated with bichromate, gelatine, and pigment, and
exposed to light under a negative in the usual way, the thin
portions of the negative will admit of the light acting on this
coating and making it insoluble, whilst the parts which are
protected from light, as for instance the sky or white objects, will
remain unchanged and soluble, and on being washed in water will
dissolve away, leaving white paper, whilst the light-affected
portions which have become insoluble remain in proportion as the
light has penetrated the various densities of the negative. This
then is how we obtain our print.
For fuller explanation of the paper or "tissue" and its manufacture
the reader is referred to the next article.
The beginner will certainly first obtain his tissue ready made, and
he can purchase it ready sensitized or otherwise. The former will be
best at the outset, but it must be borne in mind that it should not
be kept longer than can be helped before use, and never more than
ten to fourteen days at the utmost. Various shades of blacks,
browns, and reds are the usual colours, also grey, green, and blue.
The tissue is rather more sensitive to light than silver paper, and
should therefore be opened and handled in subdued light. It must be
kept as dry as possible. A rather vigorous negative is best for
carbon printing, one not too strong in contrasts. Before placing the
negative in the frame, we must give it what is termed a "safe edge."
This is done by making a narrow border, say of about a quarter of an
inch or less, round the negative, either on the glass or film side,
with opaque black varnish, or it may be done by gumming on narrow
strips of paper, such as lantern-slide binders. If binders and not
black varnish are used, they must be applied to the glass side.
The Carbon printing paper which will hereinafter be called the
"tissue" will be found to present an unpromising appearance, and as
the coating is the full colour of the pigment in which the print is
eventually to appear, it follows that the progress of printing will
not be visible, and a mechanical means of gauging the exposure must
be resorted to. An actinometer, similar to that described in the
article on Platinotype, will do, and another and simpler form is
described in the next article.
Printing will occupy about one-third of the time occupied by
gelatino-chloride of silver paper.
Development is conducted in daylight, but not too close to a window.
The absence of chemical solutions has been suggested as an
advantage, in this process the developer being merely hot water.
It is not necessary to have this laid on, a can of hot water close
at hand and a kettle on the fire or gas stove not far off are all
that are required.
We shall require four or five dishes, one at least of which should
be a good deal larger than the size of the prints we are to develop
and several inches deep--a good-sized pie-dish or a basin will do.
Development merely consists of washing away the unaffected and
therefore soluble coating, but it must be remembered that the less
affected portions representing the half-tones have received their
modicum of light on the surface, and therefore the soluble part of
the film is underneath the part that has like a surface skin become
insoluble. This necessitates the printed film or tissue being
transferred to another paper or "support," so that we may develop or
wash away from the back.