The correct exposure (about one-third of that required with silver printing) is ascertained by inspection of the paper in a rather weak white light in the usual manner. A little experience will enable the exposure to be determined very accurately ; but, generally speaking, we may say that the printing should be till the detail in the high-lights is just faintly visible.

When examining the prints in the printing frames, care should be taken not to expose them unduly to light ; for the degradation of the whites of the paper due to slight action of light is not visible until after development.

Damp paper gives a less visible image than dry paper, hence it may easily be over-exposed.

Remove the prints to a calcium tube as soon as exposure is complete, unless they are to be at once developed.

DEVELOPMENT. Development should be conducted in a feeble white light, similar to that used when cutting up the paper, or by gaslight.

It may take place immediately after the print is exposed, or at the end of the day's printing.

The developer will keep indefinitely when made of full strength, but requires to be diluted by at least an equal quantity of water before use. When diluted the developer does not keep so well, and therefore it is advisable only to dilute the quantity actually required. The formula we recommend for rich black tones is :-A. D. Solution. Contents of Platinotype Co.'s tube of D salts, dissolved in 48 ounces

of water........................

1 part.

Water (distilled or well boiled) ......

1 „

N.B. - The D salts or developing salts, are the special salts recommended by the company for development, and are sold in 1/2-lb. tubes.

The whole contents of the tube must be dissolved at one time, as the salts are mixed ; for, if this is not done, too large a proportion of one of the ingredients may be used.

An Alternative Solution. - If the D salts cannot be obtained, the following developer may be used : -

B. Oxalate of potash solution, normal strength (i.e., 1 lb. oxalate dissolved in 54 oz.

1 part.

Water..................

2 parts.

A saturated solution of oxalic acid may be added in proportion of 1 part to 20 parts of mixed developer.

In cases where prints appear granular, or where intense blacks are wanted, use the following formula : -

C. D solution as above ..........

2 parts.

Water ................

1 part.

Full development is essential, and at 6o°F. thirty seconds should be allowed. Less is required at higher temperatures.

A gives slightly more half-tone than B.

B gives slightly warmer tones than A.

C is advisable when prints appear granular, or where intense blacks are wanted.

Develop by floating the print, exposed side downwards, on one of the above solutions A, B or C. In cases where workers find a difficulty in doing this, the print may be immersed face up and immediately turned over. With rough papers such as C.C. and R.S. immersion is almost compulsory if air bells are to be avoided. The object in turning face downwards is to prevent particles of platinum, which sometimes become disengaged from the surface of the print, settling on the surface and producing black spots and comet-like marks.

Development may take thirty seconds or more : a correctly exposed print cannot be over-developed.

Solutions must never be less than 60°F. and 70°F. is recommended.

REMARKS. 1st. - If the print is too light after development it shows that the exposure has been insufficient. 2nd. - The developers, in consequence of their dilution, will deteriorate after a certain number of prints have been developed on them.

If defective development be observed, replace the worn-out bath by a new solution from the stock bath. Keep the used solutions separate- from the stock. 3rd. - Warmer tones are given by the B bath if it be made slightly alkaline. If made acid this bath will give colder tones than if neutral.

Alkali should on no account be added in larger quantity than is just sufficient to blue red litmus paper. Ammonia is unsuitable ; potash, soda or the carbonates of these alkalis may be used.

4th. - As scum is apt to form on the surface of the solution, it is well to skim it with a stiff piece of paper to remove the scum, in order to avoid developing marks on the print.

Always rock the bath or stir the solution between each development ; this will break up any scum which may be left by the previous print.

5th. - The developers may be warmed. At a temperature of about 100°Fah. under-exposed prints may often be saved. It is usual for a warm developer to give rather warmer tones. It is not advisable to use a bath of a lower temperature than 60°Fah., or brown and muddy tones may result.