Refining in general, is the art of purifying any thing; but the term is commonly understood to apply to the purification of metals, particularly gold and silver, from the alloys with which they may be mixed. As gold and silver alone can resist the combined action of air and fire, there is a possibility of purifying gold and silver from all alloy of the other metals merely by the action of fire and air, only keeping them fused till all the alloy be destroyed; but this purification would be very tedious and expensive, from the great consumption of fuel. Silver alloyed with copper has been exposed above sixty hours to a glass-house fire, without being perfectly refined: the reason is, that when a small quantity only of other metals remains united with gold and silver, it is protected from the action of the air, which is necessary for combustion. This refining of gold and silver merely by the action of fire, which was the only method anciently known, was very tedious, difficult, imperfect, and expensive; but a much shorter and more advantageous method has been long practised. This consists in adding to the alloyed gold and silver a certain quantity of lead, and in exposing this mixture to the action of fire.
The vessel in which the refining is performed, is hollow, but shallow, that the matter which it contains may present to the air the greatest surface possible. This form resembles a cup, and hence it is called a cupel. The furnace ought to be vaulted, that the heat may be applied to the surface of the metal during the whole time of the operation. Upon this surface a dark-coloured crust or pellicle is always forming; but when all the other metals are dissipated, the surface of the gold and silver is seen clear and brilliant; which indicates that the metal is free from alloy.