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repost for ron roy about bubbles/expansion

updated sat 28 feb 98


John Post on fri 27 feb 98

Hi Ron,

>OK - there are two aspects to answer here. First of all you are probably
>working with one clay and have a much wider set of parameters than I have
>to work with.

You're right. I'm only working with one stoneware body. I'm using
Standards #112 Brown Speckle. Before I was using this body I was working
with a high fireclay buff body. Both have worked well with my glazes.

>I have checked the Insight
>numbers and notice some differences in proportion - The Insight numbers
>have SiO2 and B2O3 as having the same value. I don't know what to do now -
>I am including the numbers I use so you can come to your own conclusions.

I started to look through my expansion numbers (I'm using Glazchem) and
realized that I wasn't able to make any easy comparisons to yours. It
seems the results of my firings and testings are the best guide when it
comes to evaluating glaze fit.

Tony Hansen said in his post...
>Sometimes I wonder whether we have created a monster with this expansion
>calculation thing. I know it works and I use the system all the time
>but I think we all need to appreciate that it is a general guide only.
>It gives direction but the burden of determining a glaze's suitability
>must be entirely on testing.

I'm inclined to agree with him about this. I've tested my "safe range"
glazes using the freezing in fridge/boiling water method & they have been
fine. They have passed the vinegar test with flying colors too. Yeah, I
could use a send them to a lab for testing, but since I use a liner white
inside my food vessels I'm not too worried about it at this point.

I only started taking a more careful look at expansion numbers after
reading the thread on the high expansion of Gibby's Wild Rose on clayart.

>Now I am wondering what conclusion others have come to. Do any of you have
>target calculated expansion rates for your glazes? - does anyone use
>different sets for different clays? Anyone willing to do some testing on

Glazchem has a database feature that allows glazes to be sorted in many
different ways. I sorted my glazes according to expansion. I found all of
my best fitting glazes to have an expansion somewhere in the range 68.0 -
75.0 x 10e-7 per degree C. I wasn't originally formulating glazes for this
range. I was just testing and altering glazes based on limit formula
guidelines. Now that I know that this is the range that works with my
claybody, I pay more attention to this when developing and altering glazes.

Thanks for all the help on this subject Ron, I do appreciate it.

John Post
Sterling Heights, Michigan