Stock solution as above I fluid drachm Ordinary water up to 2 fluid ounces Nitric acid 1 or 2 drops In winter it is preferable to use warm water to bring the temperature to about 8o°F. It must not, however, be over-heated; yellow half-tones will be the result.
It is very essential that the true chloroplatinite of potassium be used. (For method of manufacture see Potassium Chloroplatinite.) With a bath, as given above, the toning commences instantly the print touches the liquid. It is, therefore, preferable to tone the prints singly. The two ounces of solution will be sufficient to tone several 12 in. x io in. prints. Clark thus describes his method of working:—" For hold ing the toning bath I employ one of the plate glass bottomed dishes as generally used for the development of bromide papers. I usually turn it upside down, so as to have the sides as low as possible, and attach three ordinary levelling screws to the upper edge, which now becomes the lower one, and with these and the aid of an ordinary spirit lever I adjust the dish. By this means the toning solution is spread over a large area on a thin but uniform layer, two ounces being according to my experience, sufficient to cover a dish measuring 18in. x 15 in. I then take the wet proof and holding it by the opposite corners, one in each hand, place one corner on the sur face of the liquid with the left hand first, and then lowering the right hand allow the print to gradually float on to the liquid in this manner, pushing in front any bubbles that may have been formed on the surface. The print may be lifted off the toning bath as often as is thought desir able, and examined by transmitted light, and if any redness is to be seen in the deep shadows replace it on the bath. When all redness has disappeared the toning is complete. The print is then lifted slowly by one end off the liquid to allow as much as possible of the liquid on the sur face to run back again into the bath." The print is then plunged into a bath containing water made slightly alkaline by carbonate of potash or soda. The use of the bath is to destroy any acid in the print carried over from the toning bath which would decompose the hyposulphite fixing bath. If only a small quantity of
prints are toned the hyposulphite solution itself can be rendered alkaline, and the prints placed in it direct from the toning bath.
To get the required tone the process must be stopped at the right time. The bath given is, however, so rapid that but little control can be had over the toning. When this is desired the following bath is recomended :— Stock solution i fluid drachm.
Water up to 8 fluid ounces.
Nitric acid 2 to 3 drops.
With this bath the prints may be toned as with gold solution, that is to say, immersed in the bath and moved about until the required toned is obtained, when they are removed to the weak alka line water.
The fixing bath is the ordinary sodium hyposulphite one, with a strength of about i in 5 of water. A few drops of ammonia are added until it smells slightly of it. If the the parts have been toned to the full extent, that is to say, to a rich black or brown-black, they should not under go any change in color when immersed in the fixing bath, unless it be very strong, and they are allowed to remain in it a long time. If they have only been partially toned, however, a general change in color will be the result.
After fixing for about ten minutes, the prints are thoroughly well washed and dried.
Professor Burton recommends immersing the prints before toning in a solution of common salt instead of the preliminary washing, or to add the salt directly to the toning bath, and the prints placed in directly without previous treatment.
PLUMBAOO.—See Graphite. PLUMBIC ACID.—See Lead Acetate.
PLUMB INDICATOR.—In photographing buildings, etc., where we have a number of vertical lines, it is absolutely necessary that the sensitive surface be perfectly up right. For this purpose an arrangement termed a " plumb indicator" is sometimes used. It consists of a swinging pointer, attached to a plate (see fig. 354), which can be screwed to the swing back of the camera.
The device shows at once when the plate is vertical.