Вот контекст:
Gold occurs mixed naturally with silver, and gold is sujected to a second purification process-cementation-to remove the silver. This process is called tabkh or tas'id. According to al-Hamadani, thin plates of gold were interleaved with the cementation compound, known as the dawa'. This consisted of a mixture of vitriol, salt and ground brick. The whole was then strongly heated. The mixture evolved the vapours of sulphuric and hydrochloric acids. These did not attack gold but converted the surface silver and copper into chlorides which could be scraped off.

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