The Barnet Book of Photography Part 24

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 modic.u.m 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.

In procuring your carbon tissue order at the same time a packet of Single Transfer Paper, which is paper with a thin coating of hard gelatine. Now to proceed. Place a piece of single transfer paper into a dish of cold water, and in three or four minutes the coated side will feel slimy, then place in the same dish a piece of the printed tissue face upwards. This will probably curl up at first and afterwards flatten out again. When this has happened or in a few minutes after immersion bring the piece of single transfer paper and the print together, film to film, so that they may be in contact, and square one with the other. Now holding them by one edge, withdraw them together by sliding them out of the dish on to a sheet of thick gla.s.s, a large cutting gla.s.s serves well, or stout sheet of zinc.

This should be supported in readiness at the rim of the dish.

Having the transfer paper and print now on the gla.s.s or zinc, hold them firmly and with a rubber squeegee press them closely into contact, squeezing as much water out as possible.

A better way perhaps is, if the dish is large enough, to place the gla.s.s or zinc under the two papers whilst in the water and so raise them out.

The squeegeeing must be done thoroughly, firmly, and all over--several strokes being given in each direction.

Next lift the papers, now in firm contact and sticking together, and place them between blotting paper on which is a heavy weight. The next print may now be proceeded with and so on.

The print should be between blotting paper and under pressure for about twenty minutes, after which it is removed to a dish of hot water--almost as hot as the hands can comfortably bear, say 100 to 120 F.

After lying in this for a few moments the dark pigment will be seen to be oozing out from between the two papers. When this has begun to come pretty freely take one corner of the print and pull it away from the transfer paper. It should come quite easily, and on being peeled off entirely it is thrown away. We have now the transfer paper bearing the printed film reversed, that is, the side which was previously at the bottom and next the original paper support, is now uppermost and can therefore be got at.

If we splash it or lave it with the hand, using the hot water, we shall soon see what happens. The smudgy ma.s.s of pigment begins to wash away and the picture gradually appears.

This const.i.tutes development and we continue working it with hot water until the whole is clear and bright, being careful not to touch the film with fingers or anything but water, for being in a very delicate and soft condition it would be certain to sustain injury.

The hotter the water the greater its washing-off action, and hence in cases of over-exposure very hot water may go far to recover the print. When the desired result is secured, transfer the print to a dish of cold water, this instantly tends to slightly harden the film by cooling it, and after two or three minutes it is pa.s.sed into a dish of alum and water, which further hardens it and also "clears"

the print of any bichromate salts which may still remain. In the alum bath the print should remain until any sign of yellow stain has disappeared, when after a final rinse of a few minutes in cold water to remove the alum, the print may be hung up to dry.

It will be seen that there is no prolonged washing as with those processes in which hypo is employed, and the print is absolutely permanent.

It must be remembered, however, that in the finished picture we are looking at the back of the printed film as it received the light impressions from the negative, and hence the image is reversed, that is, the left is on the right and the right on the left. For landscape and views this reversed position will probably be of no importance, but if it is desired to have things right way round--in portraits it will be essential--we must either work from reversed negatives, or we must again transfer the film which will then const.i.tute a _double_ transfer. We shall now understand why previously we called the paper to which the film was transferred _single_ transfer.

Inasmuch as it will be seen that the print is not on paper, but consists of a transferable film of pigmented gelatine, it will be understood that the paper employed is merely a support to that film, hence it is customary to speak of the paper as the support, whilst moreover it maybe, and as often as not is ivory, gla.s.s, textile fabrics, wood, or other substances.

If now we wish to again transfer the film so as to correct the lateral reversal, we subst.i.tute for the single transfer paper a "_temporary_ support."

The temporary support which is to receive the film merely whilst it is being developed, and with the intention of its being subsequently transferred again to a _final_ support, may be paper or many other things.

Moreover, remembering that the film is mainly gelatine, it should be clear that whatever the nature of the surface of the temporary support, the soft glutinous film will take that surface just as we may make the impression of a seal in sealing-wax.

The normal carbon print is shiny, due to the gelatine, and so, if as a temporary support we were to use ground gla.s.s or matt "opal," the carbon print film would receive the fine granulated surface and give a matted print as a result. This merely by the way as suggesting an additional advantage offered by the double transfer process as a set-off against the slight extra trouble.

If double transfer is determined upon, and it is not intended to experiment with ground gla.s.s, etc., then when purchasing the carbon tissue, some _temporary_ support (sheets of paper coated with gelatine and sh.e.l.lac) should be procured, also some pieces of _final_ support.

Whatever the temporary support, it must receive an application of waxing solution. This also may be bought, or can be made of:--

Yellow resin 36 grains.

Yellow wax 12 "

Ether 2 ounces.

Melt the wax, add the resin, stir together and then add the ether.

Pour a little of this mixture on to the temporary support and spread with a tuft of cotton wool, and rub over to make it even.

The final support for double transfer may be purchased, and is made ready for use by soaking for ten minutes in alum.

The temporary support, after being waxed and the waxing solution having become dry, is to take the place of the single transfer paper in every respect, and the film developed as already described. When it has reached the final washing, after the alum clearing bath, it is brought into contact with the final support (which has been for ten minutes in alum bath as just described) and is removed to the gla.s.s or zinc plate and squeegeed.

It is now hung up to dry, and when quite dry the blade of a knife should be inserted at one corner and the temporary support gently pulled off.

Such is the carbon process, neither difficult nor lengthy, and with this brief outline to form an introduction, the reader who is a tyro will the better appreciate the fuller description which follows.

Whilst the article that follows is more comprehensive than the beginner may require at first, he is nevertheless advised to read it carefully through, and some points which may not seem clear at first will explain themselves after a very little experience.

_The Carbon Process._

[Ill.u.s.tration]

Before proceeding to practical details of working, it may be as well to realize what a piece of carbon tissue is, and what takes place in the process of exposing such tissue to light. Mr. J. W. Swan, who is to be regarded as the inventor of carbon process as we now know it, was justified in giving the name "tissue" to the film of pigmented bichromatized gelatine, as at first it was a tissue unsupported by paper backing and containing pigment practically, if not entirely, carbon. The terms "carbon" and "tissue" have been generally accepted as describing a pigmented paper containing permanent colour, therefore little if any misunderstanding is caused by such general description. The carbon process, like other kindred methods, is based upon the well-known hardening action of light upon a bichromate salt in combination with organic matter. When paper is coated with a mixture of gelatine pigment and a bichromate salt, dried under favourable conditions and exposed to light under a negative it naturally follows that a positive image is produced. The negative acting as a screen, prevents any undue hardening of such portions of the picture as are intended to form the high-lights, only slightly interfering with what are to be the middle tints, and practically permitting full play in the shadows. The latent image is imprinted on and into the film of tissue compound with the most delicate portions on the surface, and means must therefore be adopted to protect the surface during the washing away of all parts of the film not intended or desired to form any part of the finished picture.[7] In Swan's process this object was secured by cementing the surface of the printed tissue to its temporary support with rubber solution, but after J. R. Johnson discovered that the printed tissue would adhere without any cement to any surface impervious to air and water simply by atmospheric pressure, the same end was gained by soaking the undeveloped print in water until about _half saturated_, then bringing it into contact _under water_ with either its temporary or permanent support, slightly squeegeeing or sponging to remove as much water as possible without injury to the print; as to _air_, _there ought not to be any present_ if care is taken to exclude it before lifting from the water bath. The half-soaked tissue after mounting absorbs every particle of water from between the surfaces, and thus secures optical contact.

[7] It is generally a.s.serted by non-practical carbon printers that all portions of the film behind that which finally forms the print, are unacted upon by light. That is to say, unchanged and quite as soluble as if not printed at all. The upholders of such a theory should try the following experiment:--Take a piece of tissue, cut it through the centre, expose one piece, then mount both under precisely similar conditions and wash in the same warm water bath.

Paying special attention to the backing papers, they will find the one unacted upon by light will have parted with its load of coloured material in much less time than the piece that formed the backing of the print.

The squeegee, handy tool as it is, ought to be used with great care, in no case with any degree of force, or serious injury will result, particularly to the finer kinds of work, such as double transfer prints of all kinds, either on paper, ivory or opal. The rubber edge of the squeegee should be free from notches, often caused by contact with the sharp edges of gla.s.s plates. The notches can be removed by rubbing on a sheet of gla.s.s paper placed on a plane surface.

TISSUE MAKING.

The tissue compound consists of a mixture of the following ingredients:--Gelatine, sugar, pigment and water. The proportions are of infinite variety according to season, the nature of the pigment used, and the purpose for which the tissue is intended. For convenience it is the rule for tissue makers to prepare what is termed stock jelly by dissolving, by the aid of a water bath, gelatine and sugar in water, in varying proportions--roughly speaking:--

Gelatine 2 parts.

Water 4 to 7 "

Sugar[8] 3/4 to 1-1/4 "

[8] For some purposes (instead of sugar), glycerine, sugar of milk, or treacle may be subst.i.tuted.

The pigments are made up into what are termed jelly colours, which are ground either by hand on a slab of gla.s.s, marble or granite, using a suitable muller for the purpose, or when large quant.i.ties are required a paint mill driven by steam or other power is employed. In hand grinding the colour is kept moist by syrup on greatly reduced stock jelly. After grinding by hand the pigment is lifted from the slab with a palette knife and stirred into melted stock jelly. When the mill is used, the pigment is mixed with the jelly before grinding. The proportion of pigment to jelly varies enormously according to the nature of the pigment, and may be anything between 2-1/2 per cent. and 25 per cent. Having prepared stock jelly and jelly colours, and allowed both to set, they are weighed out in proper proportions, the jelly being dissolved in a tin vessel placed in a water bath. The colour, generally speaking, is dissolved in a small proportion of the stock jelly placed in the mill and again ground into the bulk of the jelly. In some cases the pigment is dissolved in warm water and filtered through cotton wool, fine felt or flannel. After adding powdered recrystallized bichromate, the jelly compound is ready for coating or spreading on the paper. The coating may be done by hand or machine. Several forms of machine are in use, including the first form invented by Mr.

Swan. When only a small quant.i.ty is required, it is the general practice to coat by hand.

In hand coating, the tissue compound may be strained through fine muslin into a flat tin dish placed on a water bath; the surface cleared of air bubbles by dragging over it a strip of stiff paper.

The sheet of paper to be coated is held in an upright position at the further end of the dish with its bottom edge just touching the surface of the solution, gently lowered until the whole surface of the sheet is in contact with the solution. If the lowering is properly done there will not be any default in contact, but if allowed to rest on the solution a few moments, the presence of air bubbles, if any, will be detected by the presence of little lumps on the back of the paper, these may be removed by raising a corner and touching the spots with a finger tip. The sheet is then raised with a rather slow and steady motion, allowed to drip, then clipped to a line by its top corners and left to dry in a warm dry room from which white light has been excluded. When this method of coating is adopted it is best to have the sheets of paper an inch longer than the dish; the blank edge prevents contamination of the fingers and distortion of the sheet caused by contraction in drying. Another method of hand-coating is to roll the sheet into a tube shape, placing the roll on the surface of the jelly compound one and a half inches from the top of the free end, raising with rather slow and steady motion as before. When the second method is chosen an oblong and somewhat deep dish will be found better than the flat shape; the flat dish may be used if tilted to give greater depth of solution in a corner.

[Ill.u.s.tration: AT THE FOUNTAIN. J. W. WADE.]

In the manufacture of tissues the greatest care must be taken to avoid over or long-continued heating of the gelatine solution.