[Clayart] Microwave heating

[Girrell, Bruce]

Vince,

Much of my experience related to clays comes from a petrophysical background (oil and gas exploration). I may be using terms that are different.

Consider an open pore structure as you would have with dried green clay. Sinter it as we do in our firings. At first just the points of contact will fuse and the pores decrease slightly in size. As the temperature rises and the melt proceeds, the walls of the pore spaces close in, collapsing them. This is vitrification. But not all of the gas will be expelled from some pores before they are sealed off by the advancing melt. If you examine obsidian under magnification you will see that even with a full melt, as in lava, gas pockets still remain. Those become closed pores. No further fluid interactions can occur with closed pores. Since we normally fire just to the edge of vitrification in order to maintain mechanical stability of the pottery, many pores remain slightly open and somewhat connected. Those are the open pores and are the ones involved with the hot cup in the microwave issues. Also, any crazing in a glaze would harbor water that could be quickly converted to steam - possibly the bigger culprit.

Bruce Girrell

-----Original Message-----
From: Clayart [mailto:clayart-bounces at lists.clayartworld.com] On Behalf Of Vince Pitelka
Sent: Wednesday, November 09, 2016 7:58 PM
To: 'Clayart international pottery discussion forum' <clayart at lists.clayartworld.com>
Subject: Re: [Clayart] Microwave heating

I have never heard anything about closed pores and open pores.  The reality is that water can be impacted fairly permanently into any claybody that has any porosity, and it is enough to make the pot get hot in the microwave, and it is enough to make the pot explode violently if someone tries to refire it in a normal glaze firing.  I have seen this happen, and people on Clayart have reported it happening.  When a glaze-firing is heated quickly, the impacted moisture cannot escape as fast as the steam is formed, and steam can build up enormous pressure. 
- Vince 

Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka at dtccom.net  
https://sites.tntech.edu/wpitelka/