The Barnet Book of Photography Part 3

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 quant.i.ties 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 gla.s.s or into a dish, the contents of which are afterwards transferred to a measuring gla.s.s and mixed with a small quant.i.ty 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.

[Ill.u.s.tration]

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.

[Ill.u.s.tration: 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 gla.s.s 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 gla.s.s. 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 circ.u.mstances, 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 quant.i.ty of alcohol and placed in a second quant.i.ty, 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 gla.s.s 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 l.u.s.trous 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 quant.i.ties 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 quant.i.ty of a solution of pota.s.sium 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 pa.s.s 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.

[Ill.u.s.tration: 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 pa.s.ses through the film and is reflected from the back surface of the gla.s.s 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 pa.s.sed 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 gla.s.s, 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 quant.i.ty of pota.s.sium ferricyanide (red prussiate of potash).

FERRICYANIDE SOLUTION.

Pota.s.sium 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 quant.i.ty 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.

The ferricyanide reducer can be applied locally for reducing high-lights, halated windows, etc., and this is often very valuable, especially in the case of under-exposed negatives. A small quant.i.ty of hypo and ferricyanide solution is mixed in a measuring gla.s.s or some other suitable vessel. The plate is immersed in plain hypo solution in a white dish for a short time and is then raised by one corner or one edge until the part to be reduced is above the solution. The mixture of hypo and ferricyanide is carefully applied with a camel's hair brush to the parts that are too opaque, and after a few moments the plate is allowed to slip back into the hypo solution and the dish is rocked. If the reduction is not sufficient, the same proceeding is gone through as often as necessary. The reducer should not be allowed to act too long before putting the plate back into the hypo, otherwise the reduction may spread further than is desired. Further, the reducer must not be too strong (_i.e._, contain too much ferricyanide), otherwise it will produce brownish stains and the action may be too energetic.

The other reducer is known as Belitzski's reducer, and is made up as follows:--