[pure-silver] Re: Thiourea and Thiourea dioxide
- From: <C.Breukel@xxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 24 Feb 2011 15:58:36 +0100
I do not believe that is true in this case, Jerry, its not about pH but
about development:
I quote Tony again:
Thiourea dioxide, also known as formamidine sulphinic acid (FSA for
short) is a common chemical used in the textile industry as a bleach in
the
dyeing process. It has replaced the more toxic hypochlorites, as it is
safer and does less damage to the environment.
The process is simple and the chemistry relatively safe. In fact
thiourea dioxide is much safer than its chemical cousin, thiourea (as
used
in odourless sepia toning kits), which is a suspected carcinogen. The
technique is applicable to both resin coated and fibre based prints and,
because the result is a print which is composed of just colloidal
silver,
the image can be subsequently toned or returned to its original state.
One
major advantage of FSA over the usual bleach and tone formulae is that
there is no loss of image density, if used with the correct beach. In
fact,
there is an intensification of the shadow tones with no loss of
highlight
density.
FSA is a powerful reducing agent whose application to monochrome
printing was discovered by George Wakefield a couple of decades ago.
When
mixed in solution with an alkali, it will reduce the rehalogenized image
back to metallic silver with variable particle size, and therefore
colour.
Best,
Cor
________________________________
From: pure-silver-bounce@xxxxxxxxxxxxx
[mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of Gerald Koch
Sent: donderdag 24 februari 2011 15:37
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: Thiourea and Thiourea dioxide
Sodium bisulfate (sodium hydrogen sulfate) can be substitued for
sulfuric acid in many photo formulas. It has the afvantage of being a
solid and hence safer to use than the acid. It can be obtained from
many sources. It ir sold as pH-Minus for adjusting the pH of swimming
pools.
Jerry
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