The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Tone Blocks, or Half-tone Blocks
Toning
Write with this on a dark portion of the print, when the letters will soon become visible by the conversion of the image into iodide of silver, which will be dissolved by the usual fixing bath.
Tone Blocks, or Half-tone Blocks. See Fish Glue Process.
Toning. If a silver print is placed direct into the fixing bath an unpleasant brick-red colour is the result. The operation of toning consists in changing this colour by the partial sub-stitution of gold or some other metal for the silver in the print. The chemistry involved in toning is comparatively simple. All toning baths contain gold in such a state that it is readily deposited upon any reducing substance which may be placed in it. There are practically three conditions of toning bath to consider; first, the acid, secondly, the neutral, and, thirdly, the alkaline toning bath. At the present time the acid bath is but little used; it strongly attacks the image, gives bluish tones, and causes very slow toning. When a weak acid, such as acetic, boric, or phosphoric, is added to a neutral bath, the tones are reddened and the image weakened. When in practice a neutral salt, such as acetate, borate, or phosphate of soda, is added to acid solution of chloride of gold the hydrochloric acid immediately combines with the base and sets free the weaker acid. The second class of toning baths are those which give the most beautiful purple tones, and as they are comparatively stable they maybe used over and over again, and strengthened by the addition of more neutral solution of gold. The third class, or alkaline baths, have a tendency to give rather more bluish shades than the neutral. The addition of carbonate of soda, or any alkaline carbonate, to solution of chloride of gold neutralises the free acid, chloride of sodium being formed ; and if an excess of alkali be present, stable compounds of gold are formed, which are not reduced by silver. If no alkali be present in excess, the toning power lasts longer, only toning takes place somewhat more slowly. With freshly mixed alkaline baths it is stated by Lainer and Davanne that one atom of gold only .replaces three atoms of silver.
AuCl3+3Ag=3AgCl + Au. With a small excess of alkali the chloride of gold is decomposed
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