Crucibles contamination-free

Crucibles should be clean and free from all traces of contaminants from their manufacture. Pre-treating the crucible by heating over the temperature range of the experiment beforehand is a useful ploy for ensuring the absence of unwanted signals during the experiment. The nature of a crucible surface may be another variable that needs to be considered. There are situations where the crucible may act as a catalyst as in the measurement of oxidation temperatures of oils. 

The crucible is now heated over a small free flame until all initial decomposition is complete. After cooling, the residue is treated with two drops of concentrated sulfuric acid, heated very gentl with indirect flame until fumes of SO3 cease to escape, and finally heated to dull redne. s until the residual sulfate is practically white. (With sodium and potassium sulfates, the heating must be sufficiently low to prevent volatility.) The residual sulfate, that may be contaminated with a trace of sulfide, is best treated with one more drop of H2SO4, and heated to constant weight. 

The normal spectral emissivity Sn was measured by Lange and Schenck at 650 nm for the first time [39]. Since then, over ten measurements have been reported [40-50], as shown in Fig. 4.12. Shvarev et al. [40] reported that thermal emission from molten silicon can be explained by Drude s free-electron model. Pulse lasers have been used to melt silicon. This method has advantages, in that measurement can be carried out in a very short time and a furnace is not required [42-44]. However, it is difficult to measure the sample temperature accurately as long as laser heating is employed. Recently, measurements using a cold crucible or levitator have been attempted these techniques assure measurement conditions without optical contamination, because there is no crucible wall at high temperature, which causes disturbing emission and reflection [47-50]. 

Contamination of a material by its crucible. It is assumed the binary melt-crucible has a 100 kj/mol excess heat of mixing. The lowest point on the free energy curve is the mole fraction of crucible material dissolved in the melt at equilibrium at that particular temperature. At 1500 K, 2 parts per thousand of the crucible material would be in equilibrium with the melt. 

---