The following toning baths have each their several advocates, and each worker must discover by experiment the one that gives him the particular brown colour he prefers:--

18. Potassium chloroplatinite 1 grain.

Water 1/2 ounce.

Add dilute nitric acid (one part strong acid, twenty parts water) drop by drop until the mixture just turns a bit of blue litmus paper a red tinge. Now take a glass rod and make of it a mop by tying a small bunch of clean cotton wool over one end, using for the purpose a bit of white cotton. Having thoroughly washed the print for _at least_ ten minutes in running water, lay it face up on a sheet of glass, and apply the above toning solution with the cotton wool mop.

Having got a tint or colour nearly what you want, but allowing for a loss of red in fixing, wash off the toning solution and immerse the print in:--

19. Washing soda 1 ounce.

Water 10 "

for three or four minutes, and then fix in the usual way in a ten per cent. hypo bath.

Here are some platinum toning baths well recommended:--

20. Lactic acid 2 drams.

Water 12 ounces.

Pot. chloroplatinite 2 grains.

21. Citric acid 20 grains.

Water 10 ounces.

Table salt 20 grains.

Pot. chloroplatinite 2 grains.

22. Phosphoric acid 3 drams.

Water 10 ounces.

Pot. chloroplatinite 2 grains.

The chief points to bear in mind in platinum toning are: (1) that the print must have practically all the free silver washed away before toning. To this end it is a very good plan to dip each print for a couple of minutes or so in a bath of table salt one ounce, water ten ounces, and again rinse under the tap for a minute or two.

(2) That the toning bath is acid, therefore one must either neutralize this acidity by passing through an alkaline bath, such as No. 19, or what perhaps is rather more convenient, though not quite so desirable--_i.e._, using a fixing bath made distinctly alkaline.

The following proportions are recommended:--

23. Hypo 1 ounce.

Water 10 ounces.

Soda sulphite 1/2 ounce.

Washing soda 1/2 "

_Toning with Gold and Platinum._--A large number of experimenters have tried to find out how to produce platinotype-like effects with P.O.P. papers. Perhaps none of them have been completely successful.

The following procedure, however, seems to give the nearest approach to that ideal.

The best results are obtained with a slightly matt-surfaced paper.

This should be printed a shade or two deeper than the print is intended to appear finally. The print is well washed and then _partly_ toned in a gold bath:--

24. Soda acetate 30 grains.

Borax 25 "

Water 10 ounces.

Gold chloride 1 grain.

It is then washed for a minute or so, and the toning continued in the following bath.

25. Phosphoric acid 1 dram.

Water 5 ounces.

Pot. chloroplatinite 2 grains.

Wash for five minutes and fix in bath 23.

_Intensifying and Reducing P.O.P._--When the negative is obtainable and printable it is _very_ much better, and altogether more satisfactory to make a fresh print than to attempt to intensify or reduce an unsatisfactory one.

Nevertheless, it sometimes happens that this course is not possible, and the best has to be made from an unsatisfactory print.

If the print is only very lightly printed, and comes straight from the printing frame, it is best to strengthen it by development (see formula 17 _et seq._). If the print has been toned and fixed, etc., the following may be tried:--

26. Make a _saturated solution_ of mercury bichloride in cold water, let it settle, and use only the quite clear supernatent liquid.

Immerse the print in this for 15 minutes, turning it from time to time, and see that no air bells are clinging to either side. Wash the print in running water for 15 minutes at least, and longer if convenient. Then immerse it in a bath consisting of strong ammonia one part, water ten or twelve parts. Again wash for five minutes under the tap.

_Reducing P.O.P._--

27. Hypo 120 grains (120) Uranium nitrate 4 "

Water 2 ounces.

The advocates of this solution claim for it that it can be used either _before_ or _after_ toning with equal facility and advantage.

Prints must be well washed both before and after its use in any case.

Another method, which is somewhat risky except in expert hands, is as follows:--

28. Dissolve metal iodine in alcohol to a rich dark port wine colour. Dilute a small quantity with cold water until the whole is a pale sherry colour. Now prepare a one in ten solution of potassium cyanide (_N.B.: a powerful poison_) and add this a _little_ at a time until the pale yellow colour of the iodine solution is just discharged.

The print may be immersed in this until sufficiently reduced, or it may be applied locally with cotton wool mop (as described above under platinum toning formula 18). The print must of course be quickly washed just before the desired degree of reduction has been produced. This solution acts somewhat quickly when once the action begins, and therefore it is well to deal with prints one at a time.

DEFECTS, ETC.

_Red-orange_ patches are usually due to touching the gelatine surface with dirty fingers, etc. These places, being somewhat greasy, repel the various fluids and cause uneven action of the developing, toning, etc.

_Brown Stains_ are also often produced in the same way. They may _sometimes_ be removed by the application of a saturated solution of alum. If this fails one may try "chloride of lime" ("bleaching powder") one part in twenty parts of hot water. Allow to stand until cold and apply with cotton wool mop.

_Yellow Stains_ may sometimes be removed by a dilute solution of potassium cyanide (poison) of strength one part cyanide in fifty parts water. (Yellow stains usually indicate hypo splashes.)

_General Fog from Age._--This sometimes may be considerably reduced by giving the prints the bath of: Soda sulphite (one in fifteen) _before_ toning, but well washing after this bath and before toning.

_Very Slow Toning_ generally points to the fact that the toning bath is too cold, or that it has been spoilt by a small quantity of hypo or developer, or that it does not contain sufficient gold.

_Uneven Toning, i.e._, blue edges, generally points to a bath too strong in gold, or that there are too many prints in the bath at once, so that the edges are getting more of the metal than the central parts, or it may arise from prints sticking together or to the bottom of the dish.

_Blue-Grey Tones_ indicate too long a time in the toning bath, or a bath too strong in gold.

_Red-Yellow Tones_ arise from just the opposite state of affairs.

_Pinking_ of the high-lights points to the bath being too weak or becoming worked out.