Half-Crown 1819 - Counterfeit

Thanks for posting!

could be:
a common technique was to "mint" a forgery out of base metal, and then silver plate it - I don't know if they understood electrolysis back then, or had some other technique;
alternatively, they would rub the minted item with mercury, which form a surface-amalgam, with a silver-colour.
The fact that this layer quickly wore away did not concern the forger, who only wanted to spend each item once.

I have found another account, from 1827, of two coiners executed. The ringleader had been convicted 15 years earlier of forging Prussian coins, and sentenced to transportation for seven years. But he returned, and re-commenced coining.
The report says that the dies, presses, etc, cost 1,400 pounds to set up: major organized crime, I would have thought. I haven't chased up reports of the details of the items counterfeited.

Technically, back then, Coining was a form of High Treason, and the report describes the process of "drawing" on a sledge (often misunderstood), and the symbolic waving of a sword over their heads. But ultimately, they were hanged.

reference: The Australian (Sydney, NSW: 1824 - 1848), Wednesday 7 November 1827, page 4
http://nla.gov.au/nla.news-article37074347

Bob, I suspect that both of your items had a layer of mercury at one stage, although I confess that I cannot now see a trace of it on either.
I have a couple of forgeries pretty much identical to yours: apparently brass; correct diameter; almost correct weight. I also have a couple of brass shillings.
The different effigies reflect a change in the genuine coinage.