Sequence viewing > Index - Alternative Photo Emulsions - Resource - © Lloyd Godman

Toning and after treatments:

After washing the image can be toned in gold baths similar to salted paper. Gold toning is thought to increasing the permanence of the image by attaching gold molecules to the less stable silver molecules. The colour the print takes on is dependent on the size of the gold particles deposited on the emulsion layer, and this is effected by the pH of the toner and the rate at which toning takes place. A bluer tone can be achieved with a more alkaline toner which acts quickly creating a course deposit. A redder tone can be achieved with a more acid toner which acts slowly and creates a finer deposit

Borax Toning bath for redder tones

Water 38°C ………………. 400ml Borax (Before use, read Health hazards)……………………… 3grms Gold Chloride (1% solution) (Before use, read Health hazards).6ml Dissolve the borax in hot water before adding the gold chloride. Store in a brown bottle, and before use, allow the chemical to sit for 1hr to cool to about 21°C. Mild alkalinity is achieved with the borax, which will keep, but further gold may need to be added when the chemical loses its potency. Each time gold is added allow to sit for an hour. Toning time varies from 6-12mins with the print becoming colder with a longer toning time. Like many photographic processes the colour of the image when dry can be quite different when wet.

Thiocyanate Toning bath for bluer tones

Water…………………… 400ml Ammonium thiocyanate Before use, read Health hazards) ….12.5 Grams Tartic acid………………. 1grm Sodium Chloride 2.5 grms Gold Chloride (1% solution) 10ml Water to make total volume 500ml This thiocynate bath will not keep well, only mix it before use. This acid toner has a silver solvent, ammonium thiocynate, which accelerates the action. Six mins in this toner will give blue grey tones.

Contrast Control: Potassium dichromate can be added to developing water to increase the contrast of the image. Potassium dichromate Before use, read Health hazards)…. 2grms Water ……………………..……28ml Begin with ten drops of a 10% Potassium dichromate solution in 560 ml of water. More can be added if a further increase in contrast is desired.

Detailed Chemical Reaction:

It was already known by Herschel's time that when ferric salts of organic acids are exposed to light the iron(III) ions are reduced to iron(II). (You might recognize this as the same reaction that is essential for the cyanotype process.) In this case, I've written the reaction using ferric ammonium oxalate (NH4)3Fe(C2O4)3 rather than ferric ammonium citrate as the photo active compound. In fact, the two reactions give the same important result: iron(III) or ferric ion is reduced to iron(II) or ferrous ion. Fe3+ + e- ---> Fe2+ While iron is reduced, some of the oxalate anion is oxidized to carbon dioxide: C2O42- ---> 2 CO2 + 2 e- We can combine these two half equations to obtain the balanced redox equation: 2 (NH4)3Fe(C2O4)3 + light ---> 6 NH4+ + 2 Fe2+ + 2 CO2 + 5 C2O42- When paper coated with ferric ammonium oxalate is exposed to light through a negative, the positive image appears. (ONLY VIEW THIS PICTURE IN A DARKROOM IF YOU WISH TO KEEP IT.)

However, this image is unstable; further exposure to light would fade the it, or the ferrous salts would eventually be washed away, or the ferrous ions would be reoxidized. But since Herschel recognized that the iron(II) is easily oxidized to iron(III), which is iron's more stable oxidation state, he was able to use this property to good advantage. The iron oxidation could be coupled with the reduction of silver ions to silver metal. (Remember that for every chemical oxidation there must be a reduction reaction.) Ag+ + Fe2+ ---> Ag + Fe3+

Notes For more information: · L. P. Clerc, Photography Theory and Practice, Pitman Publishing Co., NY, 1954 (a technical book) · Making Kallitypes a Definitive Guide , Dick Stevens ISBN 0-240-80181-4 · The Keepers of the Light, A history and working guide to early photographic processes, William Crawford ISBN 0-87100-158-6 · The New Photography, Catharine Reve & Marilyn Sward 0-306-80295-3 processes
(hand made emulsions)




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