Among the equipment necessary for fire assay of gold ores, of course the first that requires a description is the furnace. For most purposes the muffle furnace suffices, and it is the only one used for gold and silver ores. The muffle furnace, which can be procured in most of the cities, and are made of very firm and fire-proof siliceous clay, the same material as that used for some crucibles and its size depends on the number of samples to be assayed daily. The muffle has some holes opening upwards towards the interior and their number of course varies according to the size of the muffle, which again should depend on the quantity of assays expected to be performed at once. The back and the mouth of the muffle are equal in size.
The muffle furnace consists of three chief parts, the chimney, the inner part and the door. Small cracks in the muffle should be mended with coarsely powdered burnt clay. The door is of steel, with an inside coating of fire-clay, an inch and a quarter thick, kept in place by iron bolts, thus protecting the assayer from the radiating heat of the iron, which would otherwise soon be white-hot. Below the plane at the bottom of this is flat and has the opening to take out the ashes. To introduce a regular and sufficient quantity of atmospheric air the operator can regulate an aperture.
After having thus given a description of the muffles used in the fire assay process, other important equipment is the balance. There ought to be two in use, one very accurate (0.00001 g exact), the other less so, for more common purposes, as weighing off the ingredients or mixtures, such as pure lead, borax, or other fluxes. The scale with more exactitude may be used to weigh the small buttons of gold and silver procured. Before weighing the balance must be tested as to its equilibrium. The substance to be weighed should be cold, and must not be placed directly upon the scale, but upon suitable smaller pans, watch glasses, etc. The pans should then be changed in order to test the correctness of the weight. The weighed substance is removed from the pan. In the choice of a scale for weighing larger quantities (e.g. fluxes).
It is important to mention the smaller and less costly utensils, but all of which are necessary for the accomplishment of the operations: the crucibles, scorifiers, cupels or cups, and tongs. The first ones should be made from accurate descriptions by a potter, of pure clay, containing as little silica mechanically combined as possible. Cupels are very important and must be sufficiently porous to allow the oxidizing metals (mainly lead) to enter, which would, as will hereafter be seen, become a great drawback to the gold assay. Some hundreds must always be kept on hand, for they can only be used once, as the lead, almost always present, glazes them immediately. A cupel has a concave inner surface that should not be too shallow, thus letting the contents run over; nor, on the other hand, too deep, in which case too small a surface would be exposed to the oxidizing power of the heat. It is made in great part of bone-ashes, being among the most porous substances that can stand a high temperature. Some labs prepare their own cupels in wooden moulds, using a rod, shaped like a pestle, to make the hollow at the top. The mass consists of 4 parts of wood-ashes previously freed of the potash by filtration, 1 part of lime, and 1 part of bone-ash.
Muffle Furnace and Scorifiers