fran johnson on sun 28 oct 07
I have a question about using/subbing kaolin for epk
in glaze recipes. And epk for kaolin.
I know epk is a more plastic form of kaolin but have
been unable to find anything other than theoritical
formulas for clay in any books I have.
What I am wondering is if kaolin and epk are
interchangable in glaze recipes. I've seen some
recipes listing kaolin as an ingredient, others epk.
Is there a difference in glaze results using one or
the other?
Thanks for any help,
Fran
P.S. Itoo like the bumper stickers.
Timothy Joko-Veltman on mon 29 oct 07
On 10/28/07, fran johnson wrote:
> I have a question about using/subbing kaolin for epk
> in glaze recipes. And epk for kaolin.
> I know epk is a more plastic form of kaolin but have
> been unable to find anything other than theoritical
> formulas for clay in any books I have.
> What I am wondering is if kaolin and epk are
> interchangable in glaze recipes. I've seen some
> recipes listing kaolin as an ingredient, others epk.
> Is there a difference in glaze results using one or
> the other?
Kaolin is just a generic material name, while EPK is a specific
kaolin. The best ways to find out how your kaolin compares to EPK are
1) get a formula for both, or 2) try it and see (make small batch
(ie., 100g) of each, fire them and compare). The second is probably a
lot faster, unless you happen to have the analysis of your kaolin on
hand.
Regards,
Tim
Daniel Sommerfeld on mon 29 oct 07
Fran,
I've learned to laugh when I see a recipe calling for "kaolin". I believe
there are two types of people who write recipes using the word "kaolin":
One - a person who has used a couple kaolins and believes them all to be
the same, or two - a person who "gladly" hands out a recipe using the
word "kaolin" knowing the majority of people will assume it's EPK when in
fact they used Grolleg (or one of another dozen common types).
"Kaolin" listed as an ingredient is like using "feldspar" without knowing
which type (custer/potash, soda, petalite). Though many recipes will end
up working because kaolin content is usually pretty low in a glaze recipe
every kaolin is individual from their physical properties to their
chemistry, fired whiteness/impurities to their translucency.
As an example we'll look at four kaolins that I have recently tested in a
clay body. (Fluxes have been summed)
EPK
Silica 45.73
Alumina 37.36
Flux 0.91
Iron 0.79
Titania 0.37
LOI 14.84
Grolleg
Silica 48.00
Alumina 36.50
Flux 2.35
Iron 0.70
Titania 0.03
LOI 12.42
Kaopaque
Silica 45.17
Alumina 39.40
Flux 0.38
Iron 0.25
Titania 0.96
LOI 13.84
Snocal
Silica 65.41
Alumina 23.12
Flux 0.91
Iron 0.20 (way off in my personal opinion)
Titania 0.20
LOI 10.16
(I have found these numbers on the respective companies websites when
available, but Snocal's came from Laguna - the kaolin was murky red which
I've taken to suggest much higher Iron)
Now why did I spend all this time writing these out:
Looking at the fluxes first, there is a range of .38 to 2.35 which may
seem close, but consider that the Grolleg has six times the flux of
Kaopaque. When developing a clay body the substitution of these kaolins
would require an adjustment in fluxes (probably changing the feldspar
percentage.)
Looking at the Titania and Iron content next, they range from 0.03 to 0.96
and 0.2 to 0.79, respectively. In a clay body you may get a good
translucency with Grolleg over the Kaopaque (which by the name is quite
opaque), but I have found the Kaopaque to be whiter. I believe I remember
Ron Roy stating that in his tests EPK is consistantly whiter than Grolleg.
Finally, look at Snocal compared to any of the other examples. It stands
out because it's Silica/Alumina ratio is out of whack compared to the
others, but even excluding the Snocal you can see the three percent swing
of Alumina and Silica between Kaopaque and Grolleg which may shift a
recipe's balance.
Now, after all my ranting I should tell you that I began by trying out a
recipe, and I DID use EPK when it said "Kaolin". It is a starting point.
If the recipe doesn't do what you thought it should, then try a different
kaolin like Grolleg (no guarantees). In fact you should consider a fun
experiment to find a high kaolin content glaze recipe and substitute
different types to see the results.
That may sound like torture to you, but I'd love to see the range.
Good luck,
Dan Sommerfeld
www.byhand.us
Alisa Clausen on mon 29 oct 07
On Mon, 29 Oct 2007 09:55:28 -0500, Daniel Sommerfeld
wrote:
>Fran,
>
>I've learned to laugh when I see a recipe calling for "kaolin". >
>"Kaolin" listed as an ingredient is like using "feldspar" without knowing
>which type (custer/potash, soda, petalite).
Now, after all my ranting I should tell you that I began by trying out a
recipe, and I DID use EPK when it said "Kaolin". It is a starting point.
If the recipe doesn't do what you thought it should, then try a different
kaolin like Grolleg (no guarantees). In fact you should consider a fun
experiment to find a high kaolin content glaze recipe and substitute
different types to see the results.
Hi Dan
I was glad to read this at the end of your post. We potters are all over
the world and have different materials available locally. Therefore I
often write Feldspar or Kaolin because it is up to the person making the
glaze, to make it work for them. Find out what their materials contain.
It will get them to try the local materials they have on hand. I think it
is a good idea to get people to make their glazes work using local
materials or ones that are at least available by means of average costs.
Best regards,
Alisa in Denmark
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