What does 'vitrified' mean, on the bottom of a plate?!
Answers: That its been heated to a very high temperature to make it look like glass
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Hope this answers your question :
"Vitrified clay pipe (VCP) is pipe made from clay that has been subjected to vitrification, a process which fuses the clay particles to a very hard, inert, glass-like state"
Although this paragraph refers to clay pipe, the process is the same. By compressing the clay to get rid of the air bubbles before baking, the pipe, plate etc becomes more resistant to impact (bumping).
A simple answer is that it is glazed.