[Clayart] Low fire, functional, and vitrification

ronroy at ca.inter.net ronroy at ca.inter.net
Tue Nov 8 11:45:20 EST 2016

My understanding is - to stop clay from leaking you need a certain  
absorption number and that also is in relation to the test that you use.

Some clays need more vitrification to not leak. I don't have specific  
info on that. It is true that porcelain needs a lower absorption rate  
to not leak.

Room for some studies and an article here. If anyone is interested in  
doing that I can help.


Quoting Paul Gerhold <gerholdclay at gmail.com>:

> Since vitrification is to make like glass and 1.5% or 2% water  
> absorption in clay is nowhere near the water absorption of glass I  
> am wondering where the standards for absorption in clay actually  
> come from.  Are these just arbitrary numbers or is there a  
> functional reason.  Will pottery seep water at 2.5% and not at 2%.
> Paul
> Sent from my iPad
>> On Nov 7, 2016, at 10:13 PM, Robert Harris <robertgharris at gmail.com> wrote:
>> Hi Deb - in short, yes you are confused.
>> Only stoneware and porcelains vitrify. Earthenware (whatever temp it is
>> fired to) is never vitrified. Vitrification literally means to make like
>> glass. In other words the particles melt together enough to eliminate any
>> porosity. In modern parlance we measure the amount of porosity by water
>> absorption. A fully vitrified clay has less than 1.5%  absorption of water.
>> (Although I know Ron Roy is more generous than me and allows 2%+ or so).
>> Earthenware (which includes your mica clay) is always porous ... its
>> vitrification temperature is about the same as its slumping temperature.
>> When we are making things from stoneware then the cone rating on the side
>> of the box ought to be the temperature at which vitrification is such that
>> water absorption is less than 1.5%. That is the crux of the current
>> argument about "Cone 6-10" clays. There is no way that a clay can have 1.5%
>> water absorption at the lower boundary and not melt or bloat at the upper
>> boundary. Therefore these clays are unlikely to be properly vitrified at
>> Cone 6.
>> I hope that makes sense.
>> Robert
>>> On Mon, Nov 7, 2016 at 7:16 PM, Deborah Thuman  
>>> <debthuman at zianet.com> wrote:
>>> I thought that clay was vitrified when fired to the cone for which the
>>> clay was designed. ^6 for ^6 clay, ^04 for^04 clay. Or am I confused?
>>> I have a mica clay bread pan that I dearly love. Because mica melts at
>>> about ^2 or so, this is ^04 clay. I glazed the inside of the pan because I
>>> tried, several times, making bread in an unglazed ceramic bread pan. It can
>>> be done, but a generous helping of C-4 is needed to get the baked loaf out
>>> of the pan. I didn’t glaze the outside of the mica clay pan because I like
>>> how mica clay looks. I’ve been baking bread in that pan for…. maybe 4 years
>>> now??? I bake in it, I clean it in the dishwasher, the glaze is as good as
>>> the day it came out of the kiln, the pan is holding up well. I’ve put that
>>> pan in an oven already preheated to 425 degrees. I’ve put that pan in the
>>> microwave/convection oven and preheated to 425 then set the timer for 30
>>> minutes. Either way, I get great bread. I’ve made rice pudding in that pan
>>> (I had made a cover for the pan) and taken pan and pudding to pot luck
>>> meals.
>>> Deb Thuman
>>> debthuman at zianet.com <mailto:debthuman at zianet.com>
>>> https://debthumanblog.wordpress.com <https://debthumanblog.wordpress.com/>
>>> You can’t have a light without a dark to stick it in.
>>> Arlo Guthrie
>>> -------------- next part --------------
>>> An HTML attachment was scrubbed...
>>> URL: <http://lists.clayartworld.com/pipermail/clayart/
>>> attachments/20161107/1cf3db68/attachment.html>
>> --
>> ----------------------------------------------------
>> -------------- next part --------------
>> An HTML attachment was scrubbed...
>> URL:  
>> <http://lists.clayartworld.com/pipermail/clayart/attachments/20161107/8ac6c565/attachment.html>

Ron Roy
ronroy at ca.inter.net
Web page ronroy.net

More information about the Clayart mailing list