I am trying to make a porous ceramic disk about 3 mm thick. It
must be permeable to water, but the pores must be small enough that the
water's surface tension in the ceramic will exclude air at one
atmosphere differential pressure. A second requirement is that
distilled water passing through must evaporate from one surface of the
ceramic without leaving a residue behind. I used a commercial slip,
Ceramic Source Corp's "Premier". When fired at 1915F (1046C, cone 05)
the porosity was right but a residue formed after a month which sealed
off the surface. The residue problem disappeared when the cast slip was
fired to 2079F (1137C, cone 01), but pore size increased with firing
temperature and air entry pressure was reduced to about 1/2-atmosphere.
The residue is white; some of it is in very fine crystalline whiskers.
It does not dissolve in concentrated HCL and shows no characteristic
color in a flame test or borax bead test. Ceramic Source Corp is not
willing to divulge the recipe, but they do say the slip contains talc
but not feldspar.
What might the residue be? And why does porosity increase with
firing temperature. Should I be casting with a different slip? Are
there books or experts I can turn to?

Thanks,
Henry Hetzel

I think that your problem is the low temperature you use. I don't know what kind of material you are working, but if you can, you must use more temperature than to firing and you'll see how the pore size decrease.

Other possible solution is to increase the time of firing at 1137 until the equilibrium is reached.

Good luck

The formula for the slip you are describing is a talc based
casting slip for earthenware. This slip is probably based
on anywhere from 50:50 to 40:60 clay:talc ratio. The clay
portion is usually a good ball clay such as KT OM#4 or P.V. or
S.G.P. or even a combo of these clays. The temp range you are
firing to suggests that the slip is close to it's maturing point.
If I am correct in my guessing, I suggest firing to cone 2 or 3
to lower the porosity. (Any higher and you are beyond the melting point)

Just a thought, have you considered a porcelain or stoneware slip??

Hope this helps.

Robin B. Thacker