The present invention relates generally to cupels and, more particularly, to cupels used in the fire assaying of metal ores.
Conventional cupels, well known in the art of cupellation, or the distillation of pure metal from an ore sample by exposure to high temperature, include a cylindrical body made of a porous material and have a single recess for holding a predetermined amount of partially refined metal secured from the ore which is to be assayed. Heat is applied to the cupel by placing it in a muffle or furnace having a temperature in excess of 300 degrees Centigrade and sufficient to oxidize the impurities so that a quantity of pure metal, known as the "dore button", is left in the cupel.
The process of cupellation has been practiced for many years. It is utilized primarily in connection with the fire assaying of precious metals but is not necessarily limited to such metals. One of the first steps in the process of fire assaying is combining a determined quantity of rock ore sample with certain pre-mixed chemicals such as litharage, soda ash, borax glass or silica. The sample and chemicals are then heated in a crucible and reduced into a "uniform melt". The addition of finely divided carbon is further reduced by the application of heat to a "final melt" which includes a quantity of lead as well as other impurities. The final melt is allowed to cool in a conically shaped mold, with the molten lead, containing the noble metal, flowing to the tip of the inverted cone shaped mold. As the final melt cools, the impurities and metal ore tend to separate, and when cooled the small end of the resultant cone, called a "bullion head", is broken off. The bullion head contains the metal ore that was contained in the original ore sample, it having been separated from a significant proportion of the impurities present in the original rock ore sample.
At this stage the bullion head is placed in a cupel which has been heated to a pre-determined temperature for quick melting of the bullion head. Conventional cupels are generally comprised of a cylindrical body made of a porous material such as bone ash, and have a single hemispherical shallow recess for holding the bullion head. The cupel with bullion head is placed in a muffle which allows air to pass over the cupel, and allows the molten lead in the bullion head to burn off or oxidize, part of which is carried off by convection currents as a gaseous waste and the remainder to be asorbed in the body of the cupel.
In the process of cupellation, it is very important that the muffle be maintained at a temperature high enough to keep the oxidation process going, this temperature being known as the "kindling temperature". The kindling temperature will be considerably above the melting point of lead (approximately 300 degrees Centigrade). As the lead content becomes proportionately less through oxidation, the proportionate amount of pure metal increases. Consequently, the kindling temperature needed to maintain the cupellation increases in direct proportion to the amount of lead oxidized. Unfortunately, by utilizing previously known cupels, one effect of the proportionately higher temperatures required to oxidize the lead is an accompanying loss of precious metal. Conventional cupels do not overcome or even mitigate this problem and given the present day cost of precious metals, considerable dollar amounts of ore are lost by excessive temperatures during cupellation.
Should the temperature of the muffle fall below the kindling temperature needed to maintain cupellation, a lead oxide skin forms on the surface of the precious metal-lead alloy in the cupel. In order to evaporate the lead oxide skin and re-start cupellation, the temperature of the muffle (and consequently the cupel and its contents) must be raised far above the temperature normally required to commence cupellation. This higher temperature results in an even greater loss of precious metal.
The value of precious metals is such that even the loss of a fraction of a troy-ounce in the assaying process can amount to a significant sum. Accordingly, there is a need for a more efficient apparatus for practicing the cupellation process. As will become apparent from the following, the present invention fulfills that need, and further provides other related advantages.