This is an interesting process.

The chip is hammered out as thin as paper and weighed on scales so fine and sensitive that if you weigh a two-inch scrap of paper on them and then write your name on the paper with a course, soft pencil and weigh it again, the scales will take marked notice of the addition.
Then a little lead (also weighed) is rolled up with the flake of silver and the two are melted at a great heat in a small vessel called a cupel, made by compressing bone ashes into a cup-shape in a steel mold.

The base metals oxydize and are absorbed with the lead into the pores of the cupel.

A button or globule of perfectly pure gold and silver is left behind, and by weighing it and noting the loss, the assayer knows the proportion of base metal the brick contains.

He has to separate the gold from the silver now.