Development need not take place immediately, but at some subsequent time, provided the prints be meanwhile stored in a calcium tube and in every way rigorously protected from damp.
The proportions which I have given for the oxalate of potash bath represent the standard developer as given by the makers of the paper for the hot-bath papers, and they recommend that this be diluted to about half strength for cold-bath papers. Personally, I use it at full strength for the cold process, and see no reason for diluting it.
It may be said that such a course is calculated to give strong, vigorous prints, for generally speaking, the stronger the bath, the stronger the contrasts of the print. The difference, however, produced by altering the strength of the bath is not very great.
There are two alternatives to the oxalate of potash developer, both possessing certain, if not very strongly marked characteristics. The first of these is known as the "D" salts. These are sold in tins by the Platinotype Company, and consist of a loose admixture of certain salts, and hence it is essential that the entire contents of a half-pound tin be dissolved at once and kept thus as a stock solution.
The proportions to be used are as follows: Dissolve 1/2 lb. of D salts in 50 ozs. water, and then take equal portions of this solution and water, in other words, dilute it to half-strength.
The "D" salts are said to give colder colours and more half-tone, but the colour derived from development on the first-named oxalate bath may be made colder by adding to 20 parts of developer 1 part of a saturated solution of oxalic acid, in like manner slightly warmer colour may be obtained if the oxalate bath be made alkaline by the addition of carbonate of potash, but only just enough should be added to turn a red litmus test paper blue.
If prints developed on D salts should appear mealy or granulated, the bath should be strengthened or used at the full strength of the stock solution (salts 1/2 lb. to water 50 oz.).
Another developer, the effect of which is to minimize half-tone and increase the vigour of the contrasts, and so give very brilliant and even hard blacks and whites, is as follows:
Oxalate of potash 16 ozs.
Phosphate of potash 4 ozs.
Sulphate of potash 1/2 oz.
Water 120 ozs.
This should be made with hot water, and to get the full advantage of its contrast-giving powers, used quite cold. Development will then probably take one or two minutes, but can be arrested sooner when the desired effect is attained.
It may now be as well to enumerate and describe the various kinds of platinotype paper obtainable, and whilst the general treatment of them all is the same as described in the foregoing, some special recommendations may be made in each case.
The papers for the Cold-Bath process are two called respectively AA and CC. AA is a smooth surface paper and is the kind usually employed for portraiture and general small work. CC is a heavier, stronger paper with a surface similar to stout cartridge or drawing paper. For pictorial work and for landscapes, also for large portraits or heads this paper is eminently suitable.
Next we have the papers for Hot-Bath process, to be presently described. These are firstly A and C, both precisely the same in character as the AA and CC just referred to, but intended to be developed in a bath at high temperature. These four kinds of paper all yield a picture of the normal platinotype black colour, the black tending to cooler or warmer tints according to slight modifications of treatment, but it is also possible to produce a platinotype print of a rich sepia brown by using the papers S and RS--these both in substance and character corresponding with AA or A and CC or C respectively. Thus we have a thin smooth and a thick rough paper for each Cold bath, Hot bath, and for Sepia printing.
DEVELOPMENT OF HOT-BATH AND SEPIA PAPERS.
With the Hot-Bath papers perhaps the precautions against damp should be rather more stringent than for Cold-Bath papers, certainly they may not be relaxed, and in the sepia papers, S and RS, there seems to be even greater susceptibility still, but for this, printing and development are performed precisely as already described, but the temperature of the oxalate bath should not be less than 150 to 170, whilst in some cases it may be convenient to raise it still higher. The oxalate solution should, moreover, always be at full strength, namely, 1/2 lb. in 25 ozs. of water or thereabouts, a much more diluted bath will result in granular prints.
As a general rule the colour of A and C prints is a rather browner black than their cold-bath equivalents--AA and CC--with also rather softer contrasts.
Development takes place in shorter time than with cold-bath papers, and is indeed so instantaneous that any control is next to impossible. On this account, rather more dexterity will be required in development, that is to say, between the time that one end of the print touches the developer and the rest of the print is brought into contact with it, the shortest possible time should elapse.
There must be no hesitation, the whole surface must be brought down gradually but swiftly, and accompanied by a sliding movement in order to squeeze out or wipe out any air bubbles which might cling to the surface of the paper. If this be not done evenly and continuously, it is more than likely that there will be marks of unequal development on the surface.
[Ill.u.s.tration: Fig. 5.]
It is no uncommon thing for the tyro to let the print hover over the bath before giving it its plunge in the hot solution, but in so doing it should be remembered that he is submitting it to the direct action of the steam which the bath is giving off, and so exposing it to damp.
Whilst with prints of 1/2-plate size and under it may be sufficient to hold the print by one corner and wipe it across the surface of the solution, pressing it down with the fingers of the other hand, with larger sizes it will be well to cultivate a little trick in manipulation, and the accompanying figure may perhaps be suggestive (Fig. 5), in which it will be seen the left hand is bringing one end of the print into contact with the bath, whilst the right hand holds the opposite end above and well back, and the left hand will next be moved in the direction of the arrow, drawing the print with it along the surface of the bath, the right hand following but simultaneously lowering the whole of the print--thus the solution attacks the print smoothly and continuously, whilst the air is pressed out in the opposite direction. Instantly the entire print is floating on the bath it should be moved about a little, as a further means of disengaging any air bubbles.
As far as possible, prevent the developer from flowing over the back of the print, but this will be a far less evil than not bringing the whole printed surface immediately and at one stroke on to the developer. The print is next pa.s.sed direct and without intermediate washing into the hydrochloric acid bath, as already described.
The sepia papers, S and RS, are both hot-bath papers, and no special instructions need to be given as regards development, except that to get the full benefit of the sepia tint and secure a fine rich bright colour, the Special Sepia Solution prepared and supplied by the Platinotype Company should be employed in the developer.
Of this, one or two drams should be added to each ounce of oxalate bath, either before heating it in the dish or afterwards and just before floating the prints. In the latter case stir the whole so as to get it equally mixed, and wipe the surface to remove any sc.u.m.
A good subst.i.tute for the bath as above prepared for sepia prints may be made by adding one part of saturated solution of oxalic acid to each ten parts of oxalate of potash solution.
The Sepia papers are rather more sensitive to light than the Black papers, and hence all operations should be conducted in very subdued daylight, a precaution even extending to the first acid bath.
The bath containing the special solution should be used for sepia prints only, and when done with kept in a separate bottle for future use, but the bottle must be kept from the light, and the sediment which will fall should be left undisturbed at the bottom of the bottle or filtered out, and the dish used for sepia development should be well washed before using it for black prints.
Opinions seem to differ as to the wisdom of keeping old developing baths, but as far as my own experience goes I use the oxalate solution for black prints again and again, taking no heed of its discoloured condition.
After developing, the bath is poured into the stock bottle, and so long as undissolved crystals remain at the bottom of the bottle hot water may be added from time to time to make up the loss occasioned by spilling and waste, thus the stock solution is always a combination of old and freshly-dissolved oxalate, and I have had one large jar of solution thus in very frequent use for over twelve months, a greenish-black encrustation gradually acc.u.mulating at the bottom without detriment.
CONCERNING THE HYDROCHLORIC CLEARING OR FIXING BATH.
Little needs to be said as to the Hydrochloric Acid bath into which the prints are pa.s.sed immediately after development. The purpose of the acid bath is to dissolve out the sensitive salts which have been unaffected by light and which are still light-sensitive, the removal of these making the paper white and clean. Thus the acid bath is both fixing and clearing in its action.
Into the first acid bath the prints will carry a good deal of the oxalate solution in which they have been developed, and it therefore soon becomes very much discoloured, wherefore after a lapse of about five minutes the print should be removed to a second acid bath of the same strength as the first (pure hydrochloric acid 1 part, water 70 parts) and after five or ten minutes into a third.
After the prints (many may be done at the same time) have been in the third acid for five minutes, the bath should be examined, and if it is quite colourless, that is if the prints have not discoloured it at all, we may rest satisfied that clearing and fixation are complete, but if not, yet another acid bath should be given.
Whilst five or ten minutes in each acid bath is long enough, probably no harm to the print itself, yet no good, will follow a longer immersion. There may, however, be a danger of softening or rotting the paper, a danger which is increased should the bath be made stronger in acid.
If a number of prints are being made, or if numerous dishes for acid const.i.tute a difficulty or inconvenience, we may modify procedure as follows:--
Make up the first acid bath to about half the prescribed strength, say hydrochloric acid one part to water 120 to 140 parts. Into this each print may be flung as soon as developed, until the entire batch is thus far finished. In this weak acid bath the prints will take no harm if left for several hours, when an acid bath (one to seventy) of full strength having been prepared, the first weak solution may be poured off and the fresh poured on. In this the prints should be separately turned over, so that each receives thorough treatment, when the second bath may be thrown away and a third subst.i.tuted. One dish thus serves for the whole series of acid baths.
If adopting this course, it will be safer not to mix sepia and ordinary black prints in the same _first_ acid bath, after which, however, they may be treated altogether.
Sufficient washing to rid the paper of acid is all that is required to complete operations; but acid does not cling to the print as does hypo, moreover, we have not an absorbent gelatine surface to deal with, so that if prints were dealt with individually and washed by hand, probably a few minutes sluicing under a tap would suffice, but in a properly constructed print-washer, or even a large dish, twenty minutes to half-an-hour should be ample. If any doubt is felt, the last washing water may be tested with blue litmus paper.
MODIFICATIONS IN DEVELOPMENT.
To impart a warmer and richer tone to prints on CC (cold-bath) paper, the following slight modification may be resorted to, but it must be regarded merely as an exception for definite purposes, being in violation of the instructions and rules already laid down. It consists of developing CC paper as though it were hot-bath paper, using a bath of about 170 F and submitting it to the influence of damp to a slight degree. This latter very heterodox course may be effected by leaving the paper laid out all night in a room where there has been no fire to dry the air, or by using paper which has been kept for a week or so in its tube without calcium chloride and without sealing the lid, or yet again, the print may be held over the steam of the developer for a few minutes before developing it.
It must be remembered that in doing this we are taking liberties with the process, and if poor, "muddy" prints result, we can only blame ourselves, but as a rule this will not be the case, the effect being rather to impart a slight creamy tone to the whites without otherwise degrading their brilliance, whilst the use of a hot bath gives the whole a distinctly brown-black image, which combined with the cream tint of the high-lights has a very luminous and warm effect.
Another method of development which must also be taken as an exceptional one, only to be used in special cases to attain special ends, is local development with a brush, using glycerine as a medium.
As may have been seen from the foregoing descriptions, the development of a platinotype print, even with a cold bath, is so rapid that there is not a possibility of developing one portion more than another, or if such could be done, still it would be done with the certainty of leaving a mark where development had been stopped.
These difficulties, however, may be overcome by the use of glycerine, the effect of which is to r.e.t.a.r.d development to almost any degree, and by its soft, viscid character to soften and blend the line of demarcation where greater or less development ceased.
The method of applying it is as follows: On removing the print from the frame it should be fastened to a board with pins, print side upwards. Next pour on to the surface a small pool of _pure_ glycerine, and with the finger tip, a brush or soft pad, spread it _evenly_ and thinly over the print. It must not be allowed to remain on the surface in irregular patches of unequal depth, but after spreading it had better be wiped with a fresh pad of cotton wool, so as to remove any superfluous glycerine. Now have four small vessels at hand, and into No. 1 place an ounce or two of the ordinary oxalate developing solution, in No. 2 put equal parts of oxalate solution and glycerine, in No. 3 one part oxalate solution and two parts glycerine, and in No. 4 pure glycerine.
With a broad, soft hair brush apply the contents of No. 3 to the less printed portions of the image and wait results. These portions will presently begin to gain in depth and to slowly develop up, now spread the No. 3 mixture to the rest of the print and apply the contents of No. 2 to the portions first treated with No. 3. The most obstinate parts may be touched with No. 1, plain oxalate solution, whilst any spots which have come up too quickly may be promptly arrested from further progress by the application of pure glycerine.
Here we have a method of developing up any one part, and restraining or entirely stopping any other.
I do not think any good will be done by a more detailed description of its working, even if there be anything more to tell. It is essentially a method of development in which the individual worker will invent modifications and dodges for himself, and when all is said for it, it must be admitted only as a means of improving a subject when ordinary procedure fails.