Paul Herman on sun 16 jun 02
First I must confess that I'm one of those dangerously-living fireclay
users (Lincoln #60). As far as I can tell it has been fairly consistant
over more than 25 years. The pit, Lincoln Clay Products, was recently
bought by Gladding McBean. I hope they'll still sell to us potters.
I have noticed several clay recipes that call for pyrophyllite
(Al2O3.4SiO2.H2O) and was wondering why you use it in addition to
silica. To reduce shrinkage? I understand it expands on firing. What
mesh size do you use? Does it have other desirable qualities?
Your posts always are thought provoking, thank you!
> I might add here, that this body is extremely short on silica. You might
> try adding some flint and pyrophyllite.
Ceramic Design Group on sun 16 jun 02
on 6/16/02 2:37 PM, Brandon Phillips at phillipspottery2@HOTMAIL.COM wrote:
> I have a question for you Jonathan. What materials do you recommend for
> potters in place of fireclays? I love using fireclay in my bodies, but
> would I be open to trying something else. Only problem is that I worked
> hard to get this body for consistency in both wood and salt. So what
> materials that are comminly available would you use in place? Fireclays are
> half of my standard clay body. Maybe it would be cheaper than AP Green
> 100 Goldart
> 100 AP Green Fireclay
> 100 OM4 Ball Clay
> 50 Custer Spar
> 50 EPK
> 15 Grog (optional)
Good question and glad you asked. I understand this as a cone 10 clay body.
Understand that there are no other fireclay replacements for fireclays. I
have enjoyed Greenstrip or Sutter. Greenstripe is on its way out ort is
already. I don't know about Sutter.
Take the grog off the top, so to speak and divide by 4 and the body then
25 AP Green Fireclay
25 OM 4 Ball Clay
12.5 Custer Feldspar
100 total with a high quantity of plastics, low on spar, and no flint.
My suggestions would be to use Mulcoa instead of the grog. Gets rid of that
nasty stuff into something very stable.
I would keep the Goldart, maybe add just a touch of Cedar Heights Bonding
clay. Say Goldart at 20 and Bonding at 5, you can vary this around.
I would eliminate the AP Green and add some Spinks Foundry Hill Cream, and
some Tile 6 clay.
OM 4 is a far cry from what it use to be. There are some really nice ball
clays around. I might try C and C Ball Clay or Thomas Ball. If you like OM 4
keep it. Its pretty dirty though.
I might add here, that this body is extremely short on silica. You might
try adding some flint and pyrophyllite.
Cedar Heights Goldart 20
Cedar Heights Bonding 5
Foundry Hill Cream 10
Tile 6 10
Ball Clay of Choice 10
Custer Spar 15
Here you have an acceptable ratio of plastics to non plastics of 70:30. A
ratio of 80:20 works also. This is just a start. The body will be a bit
lighter, but not by much. The grade of Mulcoa you choose will provide the
tooth" in the body for you.
Give it a try and let me know if it gives you the results you want. Note
that with the exception of the Cedar Heights Bonding Clay, all the other
clays are air floated. This could be a nice body. You might have to play
with the relationships between the spar and the flint and pyrophyllite if
the vitrification is not what you need, but without running any dilatometry
and not seeing your fired results, its difficult to ascertain, but this
should be a start for you.
Best of Luck!
CERAMIC DESIGN GROUP
PO Box 775112
Steamboat Springs, CO 80577
Ron Roy on thu 20 jun 02
Some of you may have seen my slide lecture at NCECA - I showed a
dilatometer chart of Greenstripe fire clay fired to cone 10R. I was using
it to show an example of massive cristobalite production in a raw clay - I
have never seen any clay produce cristobalite like that one.
The reason, no doubt, is the high percentage of micro fine SiO2 in that
clay. Perhaps this is one of the reasons it is no longer being mined.
It is interesting to note - it is not the amont of free silica in a clay
that should be our prime concern but rather the size of that silica.
I have also measured Gold Art and Saltlick after a cone 10r firing and
found no cristobalite - a very rare occurance.
Again I must recommend the article by Peter Soghen in Studio Potter - Vol
28 No. 1. For anyone working on functional ware - especially cassarols -
this is a must read because if you are working with a clay that generates
cristobalite - you are going to have to pay the price sooner or later if
you don't learn how to control it.
I should add - if you are working at cone 6 cristobalite is not a concern -
I just can't find a body that has any more than a very small amount. It may
be possible but I can't find any.
The real problem is at cone 10 and up - especially if the firing and/or
cooling is slow - so especially with wood firing because certainly the
cooling is slower. The iron in the body the greater the danger - although I
have seen plenty of cristobalite in light bodies - never in porcelain by
>Good question and glad you asked. I understand this as a cone 10 clay body.
>Understand that there are no other fireclay replacements for fireclays. I
>have enjoyed Greenstrip or Sutter. Greenstripe is on its way out ort is
>already. I don't know about Sutter.
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