Crucibles are thick, cup-like containers that come in many different shapes and sizes and are widely used in applications from industrial to chemical, commercial to residential. They can be fabricated with or without a removable lid and occasionally with a spout, which eases the pouring process.
In terms of structure, graphite crucibles may be barrel, cylinder or conical shaped, wide-form with a shallow, low profile, or high-form, with high, tapered walls. Crucibles are made of metals like copper, platinum, quartz or porcelain, but graphite is a cost saving alternative to these more expensive materials. Graphite is a chemically inert, temperature resistant mineral that fares well in ovens and furnaces. It resists thermal shock, oxidation and abrasions well, and won’t melt, burn or chemically change in extreme conditions.
It is able to maintain its structure and form even at temperatures as high as 5000°F and so can be used in furnaces and high heat processes. Graphite crucibles have many applications within the foundry, laboratory testing, jewelry making, plumbing fixtures, valves, sprinklers, and other heavy-duty products. They are further used in chemical analysis, to store materials and samples during fusion and mixing processes, in ash content determination, to melt precious metals, and in foundries to melt many different metals.
The graphite used for the fabrication of graphite crucibles or other graphite products may be regular or high purity. Even less pure graphite mixtures still usually contain a high percentage of graphite, which is often mixed with clay; however the crucible does undergo high purification before being used in a process. Crucibles of any size or material are measured in milliliters and they are always portable and handled with tongs and great care. Most crucibles, unless specifically designed, have a smooth surface and they are all dark grey or black in color.
To eliminate the risk of cross-contamination and a difficult cleaning process, many graphite crucibles are one-time-use disposable products. Due to it being mixed with clay, the temperature limits of most graphite crucibles are lowered, as it the durability. After graphite reaches its maximum temperature, it tends to start slowly oxidizing and if left in a furnace too long, may erode and cause contamination.
Graphite crucibles are not very porous, and won’t absorb much of the material being held. Many sizes are available, and they may be custom made to fit certain manufacturing requirements for a higher cost, or come in standardized sizes. The machined graphite is mixed with refractory clay, grog and other additives, and then formed into the crucible shape by extrusion or die casting, and heat treated in an oven where strengthening takes place.