Pyrogallic Acid (C6H3(OH)3) may now be obtained in concentrated crystalline form, as well as in the snow-white needle flakes, an ounce of which required so enormous a bottle to contain them. It is the oldest of developers, and is still the one used by nine-tenths of the photographic world.

It will admit of infinite modification to adapt it for all purposes, and being very energetic is regarded as the most suitable for open-dish development. The only fault to be found is the stains it frequently leaves, both on negative and hands. We find, however, that these stains are much more easy to remove than the similar discolorations caused by so-called stainless developers. The pyro-ammonia is an old favourite, but is not suitable for very rapid plates, and sometimes produces green fog. A good formula is:

A. Pyro........................ 90 gr.

Sulphite of Soda........................... 1 oz.

Nitric Acid.......................... 3 drops.

Water........................... 15 oz.

B. Ammonia (880)...................... 1/8 oz.

Ammonium Bromide .......................... 1/8 "

Water.................... 20 ,,

For development take equal parts of A and B, adding a further proportion of B if found necessary.

Pyro-soda has almost entirely supplanted pyro-ammonia during the last few years. It is cleaner, less liable to fog, and green fog is practically unknown in connection with it. The following, which we have used for some years, is a good all-round formula, which, for some purposes, will allow of dilution:

A. Pyro...................... 1/4 oz.

Sodium Sulphite ........................ 2 ,,

Nitric Acid........................... 5 m.

Water................................. 20 oz.

B. Sodium Carbonate (crystals)...... 2 oz.

Potassium Bromide....... 60 gr.

Water.......... 20 oz.

Mix in equal parts at time of using. For instantaneous exposures or for very soft negatives omit the bromide.

A variety of the above may be used with a caustic alkali, substituting for B:

Caustic Potash........100 gr.

(or Caustic Soda........70 gr.)

Water.........20 oz.

For use take A, 1 oz.; B, 1 oz.; Water 1 oz.

Pyro-Acetone

Messrs. Lumiere, a few years ago, introduced the employment of an aldehyde in place of the alkali. Acetone is the most suitable and most easy to obtain; with pyro and paramidophenol it appears to promote rapid action and at the same time the risk of injury to the film is less than when the strong alkalies are the accelerators.

A. Pyro......... 1/4 oz.

Sodium Sulphite........4 "

Water..........20 „

B. Acetone..........20 m.

Water (added at time of using)..... I oz.

Take 1 part of A to 2 parts of B. The simplest way is to first pour the developer into the measure, then add the acetone in the proportion of 40 minims to each ounce of A, and fill up the measure to three times the original volume. Metabisulphites or acids must not be used with acetone developers.

Variations In Pyro Formula

It is always a good practice in comparing various developing agents to calculate the quantities dissolved in each ounce of fluid developer. The formulae for pyro vary most considerably, some giving as much as 8 gr. of pyro to the ounce, others as little as 1 1/4 gr.; the amount of sulphite varies from 40 gr. to 5 gr., and the alkali in proportion. The formulae we have given average about 2 3/4 gr. of pyro; it is therefore not so economical a developer as some of more recent introduction, especially as fresh developer must be employed for each plate, whereas paramidophenol or amidol will develop several plates in succession, with but one to two grains to the ounce. Density of image is, however, less quickly obtained, and the character of negative not so readily controlled.

Pyro Stains

The deep colour which pyro-developer assumes during action is rather an advantage than otherwise, preventing fog from the lamp during prolonged development. Wash the plates well before fixing, and the acid alum bath will generally remove the last traces of yellow stain, especially if nitric acid is employed. Dip the fingers at the same time in the acid bath. Lemon peel or weak solution of potassium permanganate, followed by a dip into the hypo bath and then a wash with soap and water, are said to be efficacious for the fingers. The stains are easily removed when fresh.

Messrs. Lumiere have urged that if the alkaline salts of the tribasic acids, as for instance the phosphates of soda and potash, were employed as accelerators, the danger of stains occurring would be greatly diminished.

Paramidophenol (C6H4NH2OH)

This developer is very widely employed under the name of Rodinal, a proprietary article said to consist of a solution of paramidophenol hydrochlorate with sulphite of soda, to which sufficient of a caustic alkali has been added to redissolve the precipitate at first formed. The concentrated solution is cheap, keeps well, and produces a developing agent of considerable range, but slow in acquiring density. The image appears more quickly than with pyro, and details are distinguishable almost at once, but development is not complete for a considerable time. For ordinary use take:

Rodinal........ 1 part.

Water........30 parts.

For under-exposures or greater detail add more water; to increase contrast strengthen developer. Bromide acts as a mild restrainer, and may be freely added. The hydrochlorate is generally the salt of paramidophenol preferred and consists of needle crystals which easily turn a dark colour, but this does not materially affect development. A good stock solution is:

Paramidophenol........60 gr.

Sodium Carbonate . ....... 2 oz.

Sodium Sulphite........2 oz.

Water..........20 oz.

Dilute with from 3 to 5 times the bulk of water before using.

Metol (C6H3OHCH3NHCH3) is a very powerful and energetic developer, producing density very rapidly, and bringing up detail in the shadows almost simultaneously with the high lights. Softer results are obtained for short exposures than with most other. Unfortunately it produces a very painful affection of the skin. Rubber gloves should be used, and the fingers washed immediately after development. The usual formula is: