ACCELERATOR (Pr., AccaOrateur ; Ger., Beschleuniger) A substance added to developing solutions to shorten the duration of development and bring out the image more quickly. Usually it is an alkali which hastens the development owing to its power of absorbing the bromine set free from the silver salt during development, thus forming an alkaline bromide which acts as a restrainer, and as this increases with continued or repeated use of a developer, due allowance should be made.
Common accelerators are sodium carbonate, washing soda, ammonia, potassium carbonate, sodium hydrate (caustic soda), and potassium hydrate (caustic potash). " Hypo " (sodium hypo sulphite) has been recommended when develop ing with a mixture of ferrous-oxalate, but not infrequently it causes a partial reversal of the image ; merely adding a few drops of a weak solution of "hypo " to the normal developer has a wonderful accelerating effect in some cases. Attempts have been made to introduce substi tutes for alkaline accelerators in the form of acetone with sodium sulphite, tribasic sodium phosphate, and other chemicals, but only the two named have met with any success. Some
" one solution " developers—such as rodinal, azol, etc.—include an accelerator ; but in " two solution " developers, the developer proper is generally included in bottle " A " or " No. r," and the accelerator in bottle " B " or " No. 2." It was long thought that an increase of the accelerator in cases of under-exposure brought out more detail, but photographers are now growing out of the idea. It is never advisable to add much alkali, because this invariably tends to produce fog. Accelerators cannot be used as the fancy dictates, some being more suitable for certain developers than others. Ammonia and sodium carbonate, for example, are found to give their best results in conjunction with pyro. Some of the newer developers—amidol, for example—do not require an alkali accelerator, and they will work with sodium sulphite, which is a preservative rather than an accelerator. In regard to the comparative strengths of the numerous alkalis used for accelerating develop ment, a table will be found under the heading " Alkalis, Chemical Equivalence of."