This was described as a sintering boat but I think annealing is a more likely application: Silicon wafers, which this is obviously designed to hold, are not sintered, but they are annealed after polishing. It's made of fused quartz, presumably to withstand the high temperatures involved. The idea that quartz, the same quartz that crystals are made out of, can be molten and formed like glass never ceases to amaze me.
The photograph shows it holding 25 assorted 4" silicon wafers. The purity listed represents the weight percent of silicon in quartz, which is SiO2.
Source: eBay seller landelec
Contributor: Theodore Gray
Acquired: 11 August, 2004
Text Updated: 29 January, 2009