Why is hydrated alumina better than calcined for kiln wadding?
A popular recipe for kiln wadding is 50:50 EPK and hydrated alumina. These bottom two SHAB test bars are the hydrate and calcine versions fired to cone 10 oxidation (the former fire-shrinks 7.5%, the latter 3%). Both produce a workable plasticity with about 20% water and both have a drying shrinkage of about 5%. The top two LDW test samples show the hydrated version has an LOI of 24.5% while the calcined one has 7.5%. Although not as plastic as many other kaolins, EPK is certainly among the stickiest, this makes it well suited for this task (since low drying shrinkage and adherence in the plastic state are important, that-being-said, some people use a dab of white glue to hold the plastic tabs on through drying). However the choice of which alumina is more important. On one hand, the more refractory calcined version seems like it would be better. But that is trumped by a key advantage of the hydrated one: It has a significant firing shrinkage coupled with much higher porosity (25% vs 15%), that helps with release from the vitreous foot rings or bases.