Sentence:  When the water in the <WSD>copper</WSD> boils, the arsenic and tartar, well pounded, is put into it, and kept boiling till the liquor is reduced to about half. 
Meaning of the word marked with <WSD> tags:  A large pot, often used for heating water or washing clothes over a fire. In Australasia at least, it could also be a fixed installation made of copper, with a fire underneath and its own chimney. Generally made redundant by the advent of the washing machine.