Dr. Bill found the following information somewhere online. His information includes mention of kaolin that of troy had remembered:

"Requires special clay (kaolin)
a special type of clay either white or grey, to which kaolin (a white firing stiff clay) and white China stone (finely decayed granite, washed and prepared as small white blocks) is added. When fired at temperatures of 1,280°C and over (up to 1,400°C was achieved by the Chinese), the body vitrifies, ie it becomes completely impermeable. Glazes can be applied for the first firing, or the vessel can be decorated with a low-firing glaze and put back into the kiln a second time. "

Now perhaps this clay type called kaolin is a silicate of some sort. That would explain the glass and use of the word 'vitreous.'

Edit: Well, it appears all clays are by definition of silicate composition. The vitreous clays, however, appear to be in a group of their own:

http://www.clays.org.au/mins.htm