[Figs. 78.] [79.] [80.] are cupels of various sizes, to be used in the furnace. They are the same as those used by assayers in their ordinary furnaces.
[Figs. 81.] and [82.] are views of the hand-shovels, used for filling the furnace with charcoal; they should be made of such size and form as to fit the opening h, in [figs. 70.] and [71.]
The smaller pincers or tongs, by which the assays are charged into the cupels, and by which the latter are withdrawn from the furnace, as well as the teaser for cleaning the grate of the furnace, are similar to those used in the British Mint.
In the furnace of the Mint above described, the number of assays that can be made at one time, is 45. The same number of cupels are put into the muffle. The furnace is then filled with charcoal to the top, and upon this are laid a few pieces already ignited. In the course of three hours, a little more or less, according to circumstances, the whole is ignited; during which period, the muffle, which is made of fire-clay, is gradually heated to redness, and is prevented from cracking; which a less regular or more sudden increase of temperature would not fail to do: the cupels, also, become properly annealed. All moisture being dispelled, they are in a fit state to receive the piece of silver or gold to be assayed.
The greater care that is exercised in this operation, the less liable is the assayer to accidents from the breaking of the muffle; which it is both expensive and troublesome to fit properly into the furnace.
The cupels used in the assay process, are made of the ashes of burnt bones (phosphate of lime). In the Royal Mint, the cores of ox-horn are selected for this purpose; and the ashes produced are about four times the expense of the bone-ash, used in the process of cupellation upon the large scale. So much depends upon the accuracy of an assay of gold or silver, where a mass of 15lbs. troy in the first, and 60lbs. troy in the second instance, is determined by the analysis of a portion not exceeding 20 troy grains, that every precaution which the longest experience has suggested, is used to obtain an accurate result. Hence the attention paid to the selection of the most proper materials for making the cupels.