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A chemical substance such as a stop bath or fixer with a pH below 7 that's used to stop film or paper development.
The rating of the acid or alkalinity of a solution in a scale from 0 to 14. It's based on the hydrogen ion concentration in a solution. Chemical solutions with higher PH ratings are increasingly alkaline, while lower ones acid. Distilled water is neutral with a PH rating of 7.
The "brown" in the name of this process gives you some idea of the colour of the finished print -although tones tend to be delicate rather than heavy - so clear, graphic images tend to work best.The process involves combining iron salts with silver nitrate and either oxalic acid for a warm-brown colour or tartaric acid for brown-black result. Tightly-woven fabrics can be a better base than paper, as the finished "print" must be washed in water. Exposure is by contact printing, but, because of the relatively high sensitivity of the medium to light, you should work in dim conditions, and give an exposure of around 1 minute in sunlight.Develop the image in a 1% borax solution, fix it in traditional hypo, and give a final wash of at least 30 minutes.