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"They were called veil respirators, and it was basically pads of cotton waste that were wrapped in gauze soaked in a solution of sodium thiosulphate, which neutralised the effects of low concentrations of chlorine gas," Dr Sturdy explained.

Regarding the sulphate-reducing bacteria, “the bacterial precipitation of gold from the gold–thiosulphate solution is linked directly” to the microbes' ability to metabolize sulphur compounds, says Lengke.

Reith's team has also developed a method based on high-performance liquid chromatography and inductively coupled mass spectrometry to measure the complexed forms — such as gold chloride or gold thiosulphate — in which gold exists in aqueous solutions.

The more modern process of thiosulphate leaching might offer a non-toxic alternative, but it is most effective with very fine particles of gold — and Madre de Dios tends to yield larger, coarser grains.

Oxy-salts of gold are almost unknown, but the sulphite and thiosulphate form double salts.