These two unglazed porcelain tiles appear to have a similar degree of vitrification, but do they? I have stained both with a black marker pen and then cleaned it off using acetone. Clearly the one on the right has removed better, that means the surface is more dense, it is more vitreous. In industry (e.g. porcelain insulators) it is common to observe the depth of penetration of dye or ink into the matrix as an indication of fired maturity.
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