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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