P.S. This may be what was meant by using a special clay body. Still you aren't using a glaze so fit isn't an issue and as long as you don't go from one extreme temperature to another using terra cotta shouldn't be a problem. I also wouldn't just chuck your piece. You can always use it as a test piece and surely there must be other uses for it if you decide not to use it for the oven? You can still glaze it even though it has been burnished if that is what you would like.
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(digitalfire is a must have link) Ovenware Ovenware (and flameware) clay bodies have a lower thermal expansion so they can withstand more sudden changes in temperature. Ovenware bodies should thus have much lower free quartz content and employ low expansion minerals like mullite, pyrophyllite, petalite, kyanite and spodumene. While many potters make ware for use in the oven using standard clay bodies, ovenware manufacturers (like Corning) would object to calling this 'ovenware'. This is because they dedicate considerable resources to producing bodies and glazes that have a much lower thermal expansion and therefore are much more suitable. Potters are able to get away with using standard bodies and glazes by making sure the glaze fits well (no crazing), avoiding high-quartz and highly vitreous bodies, firing evenly to reduce built-in stresses, maintaining an even cross section, avoiding angular contours and larger sizes with broad flat bottoms and telling customers to be careful about subjecting ware to sudden temperature change.
Glaze fit is a major problem in designing an ovenware body since common glazes will craze. It is much easier to make a low expansion clay body than a glaze, thus it is normal to compromise the lowest possible expansion on the body in order to get a reliable glaze fit. Low expansion glazes typically employ lithia and high silica and alumina and avoid sodium and potassium. It is much easier to make a low expansion glaze at high temperatures where silica and alumina can be higher.
There are two mechanisms to creating a low expansion body: By firing to form crystalline minerals that have low expansion or by employing mineral particles and fluid glasses in the body recipe have low expansion. The former produces a more vitreous body and requires much more expertise and test equipment. The later is a bit of a 'crowbar' approach and is dependent on not firing to full maturity (otherwise mineral species can be dissolved by the feldspar in the body and the low expansion effect lost). The latter creates a bit of a 'tug-of-war' in the body since some particles (like quartz) want to expand on heating and contract on cooling whereas others (like kyanite, mullite) want to remain stable. Furthermore, the glass that glues all the particles together introduces a third expansion dynamic to the matrix.