These two mugs are made from the same material: Ravenscrag Slip plus 20% Ferro Frit 3134. The one on the right has been bisque fired to 1550F. The one on the left has been clear glazed and fired to cone 03 (1950F). That means that this body vitrifies well below cone 03, likely well below cone 06. Thus strength, maturity, vitrification are not a matter of temperature, they are a matter of how much flux is available in the body to mature it to a dense, strong matrix.
Fluxes are the reason we can fire clay bodies and glazes in common kilns, they make glazes melt and bodies vitrify at lower temperatures.
The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.