Eleanora Eden on mon 27 mar 06
Hi Ron and all
I have been going around in circles today about my new bag of New
Foundry Hill Creme.
My supplier sold it to me as ball clay but I talked to Laguna and it
is agreed that it is a clay something like Goldart, an all-around
smooth stoneware.
In the Insight materials list the only comment is the following:
Jim Robinson observes that this material holds all the glazes in his
GTS series with shivering, 'probably because cristobalite is not able
to form'
According to Hamer, cristobalite is a good thing, encouraging
compression and, thus, strong pots.
I was thinking about replacing a part of the goldart with foundry
hill just to have more different stuff in the clay.
Any comments would be appreciated.
Eleanora
PS I have a request in to HC Spinks asking for a formula for the New FHC.
Ron Roy on mon 27 mar 06
Hi Eleanora,
Cristobalite is generally not deamed to be a good thing - certainly not for
any oven ware. If there is cristobalite - it has a profound effect on
expansion - and it is not that easy to control - especially when some
variable materials are used. If the cristobalite content is constantly
moving around it will effect glazes fit - not the kind of thing a studio
potter would enjoy dealing with.
I can remember - when I first set up my studio - using a dark stoneware
body - every piece I refired came out of the kiln shivering - refiring
produces more cristobalite - but what did we know of such things then?
Where it can be an advantage - in low fire ware - when crazing can be a
real problem - cristobalite can be introduced into the clay body - raising
the amount of body contraction during cooling and stop crazing. The amount
has to be carefully controlled. Still - this would not be a good solution
if the ware was to be used in a cooking oven where temperatures got over
the inverion points of cristobalite at 200C
Tuckers uses quite a lot of FHC - as you know - they test their raw
materials and it is a very stable clay.
When compared to the other clays we commonly call ball clays - it has more
SiO2 than most.
If fired to cone 10 by itself it will generate cristobalite - but in a body
with the correct amount of spar it will be fine.
It certainly is a much more stable clay than Gold Art - but it does not
give that "greasy" feel to a clay body that Gold Art does.
This is the analysis I have used for years now and have not had any reason
to change it.
SiO2 - 60.28
Al2O3 - 25.00
Fe2O3 - 1.42
TiO2 - 0.80
CaO - 0.20
MgO - 0.10
K2O - 0.80
Na2O - 0.20
MnO2 - 0.10
LOI - 11.10
Total - 100.0
RR
>Hi Ron and all
>
>I have been going around in circles today about my new bag of New
>Foundry Hill Creme.
>
>My supplier sold it to me as ball clay but I talked to Laguna and it
>is agreed that it is a clay something like Goldart, an all-around
>smooth stoneware.
>
>In the Insight materials list the only comment is the following:
>
>Jim Robinson observes that this material holds all the glazes in his
>GTS series with shivering, 'probably because cristobalite is not able
>to form'
>
>According to Hamer, cristobalite is a good thing, encouraging
>compression and, thus, strong pots.
>
>I was thinking about replacing a part of the goldart with foundry
>hill just to have more different stuff in the clay.
>
>Any comments would be appreciated.
>
>Eleanora
>
>PS I have a request in to HC Spinks asking for a formula for the New FHC.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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