Clay cone rating vs glaze cone rating

[jenni keller]

I have some Standard Clay white stoneware slip and it says cone 6. I understand my bisque fire will be lower than that but I'm curious if I can use lower cone glazes on the bisqued clay or if I need to match my glaze to the stonewares cone. 

 

From what I understand it's fire "up to" cone 6, but I am primarily interested in utility ware so I want to make sure it will be strong once done.

[Bill Kielb]

Hopefully this makes sense
Most often you will want to fire the clay to maturity which generally means most dense, strongest and least absorbent for that claybody. So “Up to” is probably not the best way to think of it as it often implies firing lower than maturity make little difference when in fact it will not be as sturdy / durable and likely will be too absorbent. Most potters for functional wares try and fire to clay body maturity for the strongest best product. Most potters try and match their glaze to the mature clay body. The potential exception I can see cited here would be to fire the claybody to maturity, then use a lowfire glaze to say cone 04’ish. Industry often does this to save energy and improve the odds against glazing a piece that fails in midfire / highfire.

So best to fire the clay to maturity and pick a matching glaze that matures with it. Interestingly, for the most part low fire clay will not be water resistant and will weep without the help of low fire glaze to seal it. Functional wares and low fire clay - not so much for many potters.

Incidentally part of the process of establishing successful wares requires testing of the clay and glaze combinations for similar expansion and contraction else defects such as crazing can occur. Matching the cone of both is a good start but not all same cone clay and glaze match expansion wise. Test tiles are quite common, one cannot predict a good match without an actual test of the combination.

Edited June 23 by Bill Kielb

[Jeff Longtin]

Hello Jenni,

If you're using a "cone 6" clay body you want to use a "cone 6" glaze. (Or "cone 5-6".)

You do not want to use a low fire glaze on a pot that will be fired to cone 6. That is a recipe for disaster. (Glaze all over the kiln shelves, in other words.)

 

[Chilly]

On 6/23/2024 at 5:18 AM, jenni keller said:

curious if I can use lower cone glazes

You can try anything you like, but ....

• Clay and glaze need to expand and contract together else the glaze might craze or shiver 
• plus....
• high fire glaze on low fire clay, fired to low temp = glaze probably won't mature - meaning dull rough finish
• high fire glaze on low fire clay, fired to high temp = big messy puddle on kiln shelf as clay melts and collapses
• low fire glaze on high fire clay, fired to low temp = clay not mature, shivering, crazing, leaking
• low fire glaze on high fire clay, fired to high temp = big messy puddle on kiln shelf as glaze runs off pot,.