Thermal expansion mismatch

[LaniJones]

Hi guys 

 

RE my last post, I was wondering if anyone has used a combination of a low fire clay and low fire transparent glaze which has worked with no shivering (glaze peeling off) if so, would they be willing to share? Each combination I try results in shivering, despite the thermal expansion of both matching in theory 

 

many thanks 

[Min]

16 hours ago, LaniJones said:

Each combination I try results in shivering, despite the thermal expansion of both matching in theory 

 

@LaniJones, I had a look at the Gedeo modelling clay you posted you referenced to using (this thread) and I'm not seeing any coefficient of thermal expansion (COE or CTE) figure for it. Even if they did post it that doesn't mean it's totally accurate, when clay suppliers post a claybody COE figure it can get you in the ballpark of whether a low, medium or high expansion glaze has a probability of fitting a claybody with a similar COE but testing is still required using your firing conditions. The Colorobbia Superclear Low-Fire Dipping Glaze posts " This glaze is suitable for the most common ceramic bodies having a coefficient of thermal expansion of 59-60 x 10-7..."  

Just because both the clay and glaze are lowfire does not mean they have a matching COE. When people talk about expansion figures in relation to glaze fit a simple analogy is to think of clothing. If I wear a size 10 jean and my daughter wears a size 2 there will be problems if I wear her jeans or vice versa. Size 2 jeans won't fit my body, they would tear and rip to get on, this is crazing in glaze terms. Now if my daughter wears my size 10 jeans they will be too large and bag out or fall off, this is shivering in glaze terms. Just as you need to try on jeans to see if they fit you need to test glazes to see if they fit claybodies.

Ask your supplier for a lowfire claybody suitable for use with your Colorobbia clear glaze then test it well before you make a load of work.

 

[Hulk]

Stellar analogy!

Clay COE figures likely helpful for comparing clays - especially from the same supplier, if perhaps less helpful when comparing clays from differing suppliers (and many do not publish COE figures).

Similarly, calculated glaze COE numbers don't always compare well when the recipes/ingredients are very different, however, when trying to drive COE up or down by changing the ingredient ratios, or changing an ingredient or two, the number can be helpful.

Where the analogy could be stretched (hmm) - one jean vendor's 34 inch waist equals another's 32, yet another's 34 does match, but the fabric shrinks on washing to a 31, yet another sizes by letter - XL, L, M - and etc.

Per my reading on this forum, and elsewhere, and experience as well...