I haven't found the article, only a reference but Captain T.M. Bagnold in an address or letter to the Society of Arts circa 1851 notes that the Bombay tube makers attempting to make tubes from iron hoops from British casks would use a mixture of mud, clay and cow or horse dung to prevent oxidation or used as a flux. They also soaked the tube in vinegar or a sulphate of iron solution.

Kind Regards,

Raimey
rse