Some bodies cannot be fired to even near zero porosity
Bloating in an over fired middle temperature high iron raw clay (Plainsman M2). It is still stable, dense and apparently strong at cone 4 (having 1.1% porosity). But between cone 6 and 7 (top bar) it is already bloating badly. Such clays must be fired at low enough temperatures to avoid this volatility (if accidentally over fired). This clay only reaches a minimum of 1% porosity (between cone 4 and 5), it is not possible to fire it to zero porosity. This is because of the particulate gas-producing particles (it is ground to 42 mesh only).
Terra cotta gets brittle when over fired
An excellent example that demonstrates the brittleness typical of vitrified terra cotta bodies. This bowl was fired to cone 02 and rung like a tuning fork when struck with a spoon. The body is dense like a porcelain and at appeared to be incredibly strong (this body is much more vitreous than an average terra cotta would be). However after a few more taps with the spoon it broke in two! It is brittle! Very hard, but brittle. At first I thought it might be that the glaze is under compression, but when I dropped the halves they did not shatter in the manner characteristic of compressed glaze, and they broke with razor sharp edges (like a vitrified porcelain does). So firing for this body must stop short of the most dense matrix possible to avoid this brittleness.
A true matte is still matte when you over fire it
The top glaze is VC71, a popular matte cone 6 glaze used by potters. Bottom is G2934 matte, a public domain recipe produced by Plainsman Clays. The latter is a high-MgO matte, it melts well and does not cutlery mark or stain easily. As evidence that it is a true matte, notice that it is still matte when fired to cone 7 or 8. VC71, while having a similar pleasant silky matte surface at cone 6, converts to a glossy if fired higher. This suggests that the cone 6 matteness is due to incomplete melting. For the same reason, it is whiter in color (as soon as it begins to melt and have depth the color darkens).
This is when you should program a firing yourself
This is Polar Ice cone 6 porcelain that has been over fired. The electric kiln was set to do its standard cone 6 fast fire schedule, but a cone in the kiln demonstrates that it fired much higher (perhaps to cone 7 judging by the bend on the cone). This is a translucent frit-fluxed porcelain that demands accurate firing, the over fire has produced tiny bubbles and surface dimples in the glaze. The mug rim has also warped to oval shape. The lesson: If you are firing ware that is sensitive to schedule or temperature, use large cones and adjust if needed. If it fires too hot like this, then program to fire to cone 5 with a longer soak, or cone 5.5 (if possible). Or, program all the steps yourself; that is definitely our preference.
Why is the clay blistering on this figurine?
This is an admirable first effort by a budding artist. They used a built-in cone 6 program on an electronic controller equipped electric kiln. But it is over fired. How do we know that? To the right are fired test bars of this clay, they go from cone 4 (top) to cone 8 (bottom). The data sheet of this clay says do not fire over cone 6. Why? Notice the cone 7 bar has turned to a solid grey and started blistering and the cone 8 one is blistering much more. That cone 8 bar is the same color as the figurine (although the colors do not match on the photo). The solution: Put a large cone 6 in the kiln and program the schedule manually so you can compensate the top temperature with what the cone tells you.