chris clarke on fri 8 mar 02
How do you tell if a clay is over fired?? Brittle? Melted?
chris
temecula, california
chris@ccpots.com
www.ccpots.com
Ron Roy on sat 9 mar 02
Tricky question - first sign might be no absorbency - although porcelains
are usually taken to at least that point. Excessive shrinkage - warping -
ware becomes covered with small bubbles (blebbing) bubbles get bigger and
ware starts to get bigger cause clay is expanding from gases.
If over fired the clay - when broken looks glassy inside - again OK for
certain types of porcelain but stonewares are melting quickly at that point
- ware is brittle.
Lids tend to stick on, ware tends to stick to kiln shelves - off gassing
will produce blisters and pin holes in some glazes.
I'm no expert on this subject but there are many on this list who can give
you first hand observations about what happens.
>How do you tell if a clay is over fired?? Brittle? Melted?
>
>chris
>
>
>temecula, california
>chris@ccpots.com
> www.ccpots.com
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Wanda Holmes on sun 10 mar 02
Chris, I make a series of test bars - .5" thick by 4.5" long by 1" wide. I
fire 4-10 of these bars at successive cones. For a mid-range clay I'd
probably fire at 4, 5, 6, and 7. I then break each one under controlled
conditions expecting to see the strength numbers climb (getting stronger as
it nears vitrification) and suddenly drop off. So, for example, if the
strength increases from cone 4 to cone 5 and from cone 5 to cone 6 and drops
off at cone 7, I know that cone 6 is the optimum firing point for that body.
The mechanism I use for strength testing is an arbor press with a torque
wrench attached. I measure the torque required to break the bar.
Wanda
>How do you tell if a clay is over fired?? Brittle? Melted?
>
>chris
>
>
>temecula, california
>chris@ccpots.com
> www.ccpots.com
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