Ron Roy on mon 30 jul 07
question on COE
Hi Ivor,
We have provided comparisons in our book (chapter 5) with the calculated
expansions (3 kinds) and the real measured expansion of all our fit testing
glazes.
Note that real expansion is usually given as an average within a range of
temperatures. Also dilatometors need to be checked to see if they are
working properly - I run a calibration check every 50 times I use it.
Calculated expansion is just used to show you which way you are altering
the expansion when fixing a glaze - it only applies to glossy glazes and
there are some wrinkles that you have to keep in mind - like the amount
B2O3 that lowers expansion and when over 12% it raises expansion. This has
more to do with elasticity by the way - see page 62 in Ceramic Glazes by
Felix Singer and W L German.
I also find it helps me know what calculated expansion I need in order to
make glazes that won't craze or shiver on the clay I use.
Bailey and Hewitt say to use linear expansion by the way - and my
dilatometer measures linear expansion.
RR
>Perhaps the way values of Expansively are arrived at for contributing
>oxides gives an artificial value and the only firm information would come
>from testing specimens by dilatometry, as done by Ron Roy.
>It is important to ascertain what assumptions were made by those people
>who constructed scales that are employed now in computerised glaze
>calculations.
>Other points to consider: If the glazes is in the viscous state then
>Cubical, not Linear coefficients would apply. As viscosity approaches
>infinity, at some point on the cooling curve a point is reached called the
>Glass Transition Temperature and the substance is rigid. Linear
>Expansivity is not necessarily uniform across the temperature range from
>ambient to Glass Transition point
>It could be suggested that when layered glazes are being created that
>potters might adopt the Glass Founders process of annealing.
>By the way, I do appreciate the idea of using polarised light to examine
>transparent specimens for stress fields.
>Best regards,
>Ivor Lewis.
>Redhill,
>South Australia.
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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