Crazing After Hot Liquid Is Poured

[leslieponder]

howdy.  I have been experimenting with low fire glaze that is safe for food use with stoneware clay.

 

I am firing bisque at cone 05 and glaze firing to cone 05.  My mugs look great and then I add in hot water and they start to craze.  the craze is faint but, I can see lines.  

 

Should I bisque fire to 04 and keep glaze to 05?   

 

I have an old kiln so, I don't really want to use any high fire glazes so, this works best for me.  Any advice will help.  

[neilestrick]

Have you tried using a low fire clay? Low fire glaze on stoneware will usually craze, in my experience.

[glazenerd]

Correct Neil.

 

Porcelain and Stoneware bodies actually expand at bisq temperatures: in part due to their higher silica content. Both bodies begin to close at 2025/50F range as spinel (metakaolin) begins to form. In this case, the clay is still expanding from 05 to 04. So a low fire clay would be the best solution. You can always try firing in the cone 2-3 range with extended holds: much less wear and tear on the elements.

 

Nerd

 

Cone 04 is 1950 +/- pending temp. rate/climb.

Cone 3 is 2106 +/- pending temp. rate/climb.

 

http://paragonweb.com/files/manuals/Orton_Temps_degreeF1.pdf

[oldlady]

nerd, 05 to 04?  lots of inexperienced people don't know the numbers run 022 to cone 1 upwards in temperature.

[leslieponder]

I am trying to sell my stuff.  Would a low fire clay be sufficient for selling?  

[Mark C.]

You need to use the right clay which will vitrify at the glaze temps you are firing to.

Thats cone 05 so you need clay made to fire to cone 05.

Clay that fires hotter than cone 05 should not be used as it will cause troubles like you are seeing.

[neilestrick]

I am trying to sell my stuff.  Would a low fire clay be sufficient for selling?  

 

I prefer vitrified clays for functional work. If you're using commercial clay bodies and/or glazes, that means cone 5/6. There are some terra cotta bodies that can be pretty tight at cone 3, but you won't find commercially available glazes for cone 3. That said, there are folks who make functional low fire pots. But you'll need to use a low fire clay since a good clay/glaze fit is super important. If the glaze crazes, liquids will seep into the wall of the pot where you can't get them out. I've seen black gunk growing underneath a low fire glaze.

[glazenerd]

About the only clay that will remotely vitrify (psuedo) at 04 is frit ware. The high glass content substitutes the normal glassy matrix in cone 5/6 or above bodies. That said, I would not be willing to classify it as functional ware.

 

Nerd

[What?]

Firing in this range poses a lot of problems for functional wares. Not impossible though. If your wares are iffy get product liability insurance if you have assets you want to protect and this may not protect you if you are aware of the issue. Functional pottery should be safe and functional.

[leslieponder]

thanks guys.  I guess i'm going to have to start work with high fire glazes, at least for functional ware.