Faraday to William Tierney Clark   2 August 1831¹

Royal Institution | August 2nd 1831

My dear Sir

I have now examined the substance produced by the decay of your Copper Boiler and have no difficulty arriving at a conclusion respecting the nature of the injury the boiler has sustained².

The substance is almost entirely sulphate of copper or blue vitriol. It has been produced from the action of the sulphur of the coals forming sulphurous acid under the influence of which the Copper has been oxidized far beyond what it would have been by heat and air alone - the Oxide has combined with the acid and which at the same time has become sulphuric acid and a sulphate of Copper has been the consequence. This salt by swelling has risen up from the Metal (or rather in the position of the Boiler has swelled downwards) and has left new surfaces for the action of successive portions of the sulphurous acid vapours³.

You have in fact been making blue vitriol from the boiler by a process nearly the same as one expressly adopted for that purpose. Plates of copper are made hot in a Fire and then sprinkled with sulphur.

Galvanic action has had nothing to do with the decay. Had any galvanic action taken place it would have been to the preservation of the Copper. But the injury is referable to the cause I have described.

I am My Dear Sir | Very truly Yours | (signed) W. Farraday

W.T. Clark Esq