Robert Cope on wed 26 oct 05
Hello, I wonder if someone can help me with the composition of a cone 6
clay body. I'm using:
Kentucky stone 50
OM4 10
Greenstripe Fireclay 10
Red Art 17
Silica 15
Neph. Syn. 8
Talc 5
Total 115
Very good throwing and handbuilding. Lots of green strength. I bisque to
cone 04 electric.
OK. If the clay is fired to cone 6 electric with no glaze or other
treatment, does fine. If terra sigillata, Hopper slip, or glaze (from
Hesselberth and Roy) are used thinly, its fine. But if TS or glazes are
thicker or applied twice i.e. one dip, wait, then second dip (but still not
heavy) I get small numerous bumps in the body and a few larger ones that
almost look like trapped air in the body--but I'm pretty sure its not that.
The small bumps are quite small. It certainly appears to be related to the
thickness of the glaze or TS, and it does not seem to be outgassing from
the body of any type. I soak for 20 minutes at 2232 F. It seems like
something in the body that is expanding under these conditions. The body
seems quite vitrified. Also, a thin soda ash wash has the same effect on an
even samller scale (smaller bumps).
Any clues? I thank you for your assistance. Bob Cope
Ron Roy on fri 28 oct 05
Hi Bob,
I don't have an analysis for Greenstripe by the way - I just used Hawthorn
fire clay - if you have an analysis for Greenstripe send it to me and I'll
check it with that.
The bubbles in the clay - thats called blebbing - it's a sign your clay
body is overfired and is beginning to break down producing gas which
results in the bubbles. This type of body needs a clean bisque firing for
starters because of the iron involved.
I've compared your clay body with others that I work on - it's also way
over fluxed for cone 6 - and half of it is Kentucky Stone - which is not a
very reliable clay - in other words it is not consistent in it's
refractoriness. Part of the flux is Neph Sy - not the best thing because it
will probably deflocculate the body over time.
Take out all the Neph Sy and Talc and add in 5 G200 or Custer and test it
for absorbency - it may still be tight - in that case take out 5 Kent Stn
and add in 5 EPK - test again - if it still has zero absorbency - take out
5 more Kent Stn and add 5 more EPK - etc. - you are looking for an
absorbency of about 2%.
Let me know if you need to know how I test for absorbency.
Let me know if you have any questions - RR
>Hello, I wonder if someone can help me with the composition of a cone 6
>clay body. I'm using:
>
>Kentucky stone 50
>OM4 10
>Greenstripe Fireclay 10
>Red Art 17
>Silica 15
>Neph. Syn. 8
>Talc 5
>Total 115
>
>Very good throwing and handbuilding. Lots of green strength. I bisque to
>cone 04 electric.
>
>OK. If the clay is fired to cone 6 electric with no glaze or other
>treatment, does fine. If terra sigillata, Hopper slip, or glaze (from
>Hesselberth and Roy) are used thinly, its fine. But if TS or glazes are
>thicker or applied twice i.e. one dip, wait, then second dip (but still not
>heavy) I get small numerous bumps in the body and a few larger ones that
>almost look like trapped air in the body--but I'm pretty sure its not that.
>The small bumps are quite small. It certainly appears to be related to the
>thickness of the glaze or TS, and it does not seem to be outgassing from
>the body of any type. I soak for 20 minutes at 2232 F. It seems like
>something in the body that is expanding under these conditions. The body
>seems quite vitrified. Also, a thin soda ash wash has the same effect on an
>even samller scale (smaller bumps).
>
>Any clues? I thank you for your assistance. Bob Cope
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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