Cor, (Hi!) I think Jerry is addressing a different issue here.... (OTOH, I might be dreaming... I am so sleepy.) --- On Thu, 2/24/11, C.Breukel@xxxxxxx <C.Breukel@xxxxxxx> wrote: From: C.Breukel@xxxxxxx <C.Breukel@xxxxxxx> Subject: [pure-silver] Re: Thiourea and Thiourea dioxide To: pure-silver@xxxxxxxxxxxxx Date: Thursday, February 24, 2011, 2:58 PM 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