The button is hammered out flat and thin, put in the furnace and kept some time at a red heat; after cooling it off it is rolled up like a quill and heated in a glass vessel containing nitric acid; the acid dissolves the silver and leaves the gold pure and ready to be weighed on its own merits.

Then salt water is poured into the vessel containing the dissolved silver and the silver returns to palpable form again and sinks to the bottom.

Nothing now remains but to weigh it; then the proportions of the several metals contained in the brick are known, and the assayer stamps the value of the brick upon its surface.
The sagacious reader will know now, without being told, that the speculative miner, in getting a "fire-assay" made of a piece of rock from his mine (to help him sell the same), was not in the habit of picking out the least valuable fragment of rock on his dump-pile, but quite the contrary.

I have seen men hunt over a pile of nearly worthless quartz for an hour, and at last find a little piece as large as a filbert, which was rich in gold and silver--and this was reserved for a fire-assay! Of course the fire-assay would demonstrate that a ton of such rock would yield hundreds of dollars--and on such assays many an utterly worthless mine was sold.
Assaying was a good business, and so some men engaged in it, occasionally, who were not strictly scientific and capable.

One assayer got such rich results out of all specimens brought to him that in time he acquired almost a monopoly of the business.