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Nancy S.

Standard 630 - Glazes Crazing

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I've been throwing with Standard Clay 630 (^6 white stoneware) this year and have really been enjoying using it. However, I used a few different commercial glazes with it and have found that some of them are crazing. For example:

 

Amaco HF-9 Zinc Free Clear

Amaco PC-40 Celadon

Amaco Sahara Yellow

Coyote Light Blue Gloss

 

All of these are translucent, so maybe that's the common thread? Because Amaco's Ultramarine, Amaco's Amethyst, and Coyote's Buttercup didn't craze at all. But I tried my local studio's zinc-free clear, and it didn't craze.

 

It's not the cooling, because nothing is being fired any differently than other pieces that don't craze. So I'm thinking it's a COE / glaze fit issue...but I am a total newbie at glaze chemistry, so I bow down to the collective knowledge of the forums and ask if there's anything I can do to my commercial glazes to make them fit.

 

Other than changing clays, of course. Which is probably what I'll have to do, but I like the 630 the best out of the ones I've tried and I really love the celadon and light blue glazes...

 

Also, how serious is this breed of crazing? Is it really *that bad* when it comes to using them for food/drink??

 

 

(Regarding clays.....I know some of you swear by clays made by Highwater, Laguna, etc., but I live in Pennsyltucky and getting anything other than Standard isn't an option right now. Also, I am a hobby potter and porcelain is out of my current league.)

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Guest JBaymore

If a glaze is crazing... there is ONE cause.  A differential between the COE (sometimes said CTE) of the glaze and the clay body.  If the differnce is only slight.... you get delayed crazing....which usually shows up when the object is slightly stressed via usage....or over a long time.  If it is larger, it shows up as the kiln cools and slight un-evennesses in dropping temparature of different parts of the pieces or airflow across the surfaces stress the pieces.

 

Fixing commercial glazes is a crap shoot.  The starting place will be to take a MEASURED amount of the liquid glaze and check the weight of a pint of it.  The add a measured amount of dry flint powder (SMALL amount) and add that to the test batch.  Then apply and fire.  See if it fixes things without changing the look of the glaze.  If the crazing is not gone, but is improved (less craze lines per square inch) AND the glaze appearance has not changed...... add a bit more flint and test again.

 

This approach (increasing SiO2 -silica- content via adding flint) has its limits... and can only address SLIGHT differences.

 

The other approach to matching the COEs is altering the clay body........ more difficult.  And again... commercial body.... even MORE difficult.

 

One of the drawbacks to using commercial clay bodies and glazes.

 

best,

 

....................john

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This is one of the problems with commercial clay and glazes. If you're using both, you have no opportunity to tweak either one. We use a lot of 630 in my studio, and don't have any problems with it. It's a wonderful clay to work with.

 

From a hygiene/food safety standpoint, the crazing is not really an issue since the clay is vitrified.

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The studio bisques at ^05. They used to do ^06 but had bloating on one of the dark Standard bodies. So that could be a possible cause. On to the experimentation!

 

Thanks for your thoughts and suggestions! :D

 

(and yay that I can still use my bowls in spite of the crazing!)

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