Crucibles are used to melt crude bullions from the gold recovery process and cast into bars. Crucibles are made of fireclay, graphite mixed with fireclay, carbon bonded silicon carbide, ceramic bonded clay graphite or graphite lined with clay. Detachable clay liners are used in some occasions or a clay pot may be installed inside a graphite guard vessel. The size of the crucibles and the weight of the charges of bullion vary greatly, but in most cases, the metallurgists consider that an average gold charge is 18-20 kg. Obviously, there are smaller and bigger crucibles and the final election must be based on the estimated gold production.
It is important to indicate that crucibles must be thoroughly annealed before being used; otherwise the contained moisture being suddenly converted into steam when the crucible is heated rapidly, the container cracks. Basically, the crucible is kept on a shelf near the flue or in a specially heated chamber for as many days as necessary before being used. It is then placed on the top of the furnace and also, it is good practice to place a newly annealed crucible in a cool furnace or in one that is being heated from cold. The right place must be warm. Some crucibles needs a less degree of care in annealing because they are annealed before being commercialized and in some cases have been treated to full working temperature by the manufacturers. In all circumstances is important to avoid excessive temperature, thermal shock, mechanical shock or damage to crucible glazes.
The crucible in an oil furnace rests on a firebrick about 75 mm thick, which is laid on the bottom. The fuel is built up round the pot until it reaches to its rim, and the fire urged until the whole vessel is at full red heat. During the melting process, when the temperature is appropriate, borax is added into the crucible by means of a scoop to slag off metallic oxides and help the melting operation. A pure borax slag is often too thin when molten and tends to follow the metal into the mold. It can be thickened with additions of sodium carbonate and silica so that it can be rolled back and remain behind. In order to protect the crucible, large charges are added with crucible tongs. The cover that must have been previously heated is usually kept on the crucible during melting as much as possible.
Crucible tilting furnaces fired by oil or gas are also used for melting precious metals. These furnaces operate more quickly than stationary furnaces. One of the their advantages is that the pot remains in the furnace during pouring and recharging and is less cooled down than pots which are lifted out before being poured. Also, the reduction in the amount of alternate heating and cooling gives a longer life to the crucible. During operation the crucible must be inspected for damage after each heat and slag inside the vessel must be removed. If the crucible does not look good, it is good idea to change the crucible in order to avoid any unexpected problem.