text: Crucible is used in the laboratory to contain chemical compounds when heated to extremely high temperatures.  Crucibles are available in several sizes and typically come with a correspondingly-sized lid. When heated over a flame, the crucible is often held inside a pipeclay triangle which itself is held on top of a tripod.
  Crucibles and their covers are made of high temperature-resistant materials, usually porcelain, alumina or an inert metal. One of the earliest uses of platinum was to make crucibles. Ceramics such as alumina, zirconia, and especially magnesia will tolerate the highest temperatures. More recently, metals such as nickel and zirconium have been used. The lids are typically loose-fitting to allow gases to escape during the heating of a sample inside. Crucibles and their lids can come in high form and low form shapes and in various sizes, but rather small 10 to 15 ml size porcelain crucibles are commonly used for gravimetric chemical analysis. These small-size crucibles and their covers made of porcelain are quite cheap when sold in quantity to laboratories, and the crucibles are sometimes disposed of after use in precise quantitative chemical analysis. There is usually a large mark-up when they are sold individually in hobby shops.
  In the area of chemical analysis, crucibles are used in quantitative gravimetric chemical analysis (analysis by measuring mass of an analyte or its derivative).  Common crucible use may be as follows.  A residue or precipitate in a chemical analysis method can be collected or filtered from some sample or solution on special "ashless" filter paper.  The crucible and lid to be used are pre-weighed very accurately on an analytical balance.  After some possible washing and/or pre-drying of this filtrate, the residue on the filter paper can be placed in the crucible and fired (heated at very high temperature) until all the volatiles and moisture are driven out of the sample residue in the crucible.  The "ashless" filter paper is completely burned up in this process.  The crucible with the sample and lid is allowed to cool in a desiccator.  The crucible and lid with the sample inside are weighed very accurately again only after it has completely cooled to room temperature (higher temperature would cause air currents around the balance giving inaccurate results).  The mass of the empty, pre-weighed crucible and lid is subtracted from this result to yield the mass of the completely dried residue in the crucible.
  A crucible with a bottom perforated with small holes which are designed specifically for use in filtration, especially for gravimetric analysis as just described, is called a Gooch crucible after its inventor, Frank Austin Gooch.
  For completely accurate results, the crucible is handled with clean tongs because fingerprints can add a weighable mass to the crucible.  Porcelain crucibles are hygroscopic, i. e. they absorb a bit of weighable moisture from the air.  For this reason, the porcelain crucible and lid is also pre-fired (pre-heating to high temperature) to constant mass before the pre-weighing.  This determines the mass of the completely dry crucible and lid.  At least two firings, coolings, and weighings resulting in exactly the same mass are needed to confirm the constant (completely dry) mass of the crucible and lid and similarly again for the crucible, lid, and sample residue inside.  Since the mass of every crucible and lid is different, the pre-firing/pre-weighing must be done for every new crucible/lid used.  The desiccator contains desiccant to absorb moisture from the air inside, so the air inside will be completely dry.