Expert: Sam Kelly - 7/15/2010

Question
I've begun experimenting with new product design crafted in ceramic, specifically biscuit/bisque-fired earthenware, and quite like it's textural properties in the unglazed state. The product roughly resembles a deep soup bowl no larger than 12cm in diameter with a wall thickness of about 3mm.

My question would be, how does biscuit earthenware compare with other ceramics in terms of toughness when dropped? Would a similar piece in stoneware, porcelain or another ceramic better withstand impact? Please describe their strength. Is their an additive used to produce a more resilient ceramic?

Thank you!
Silas

Answer
Hi Silas.

Bisque is the least strongest and unglazed is highly absorbent, therefore only good for looking at. Bisque is the FIRST firing and therefore not fired to maturity so it will absorbe a glaze when applied prior to final firing.

Stoneware and porcelain fired to bisque are also absorbent, bisque temperatures being the same for all.

If the above 3 clays are fired to maturity(vitrification), wether there is a glaze applied or not earthenware is the softest.

Fired to maturity earthenware will still absorbe moisture over time as its porosity(up to 12%) is much higher than Stoneware or Porcelain(up to 2%)

With Stoneware and porcelain firings being higher a change of the clay occurs, this is the fomation of Mullite Crystals, these are needle shaped, dense and add strenghth. Earthenware does not fire high enough for these Mullite Crystals to fully form.

If you are not going to use glaze, fire to the top temperature, not bisque.

Sam