Paul on wed 1 oct 03
Is there any risks involved in using silica sand instead of grog in a
stoneware body? The only thing i have heard is that it could have an affect
on expansion but i don't know to what extent this would be a problem. I
would like to hear if anyone has tried both and found a considerable
difference between the two. thanks,
Paul B
Falmouth, Ky
John K Dellow on thu 2 oct 03
I have found in at least terra-cotta clay the addition of sand will
allow water to be
absorbed quickly leading to slumping.
I had a coarse kaolin ( for theAusies ,one from Home Rule)
added to the body I use fore my terra-cotta wears .Its marked as Mid
Fire Red.
Paul wrote:
>Is there any risks involved in using silica sand instead of grog in a
>stoneware body? The only thing i have heard is that it could have an affect
>on expansion but i don't know to what extent this would be a problem. I
>would like to hear if anyone has tried both and found a considerable
>difference between the two. thanks,
>Paul B
>Falmouth, Ky
>
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William Lucius on fri 3 oct 03
For years I used a fine beach sand in my earthenware and stoneware pots
(free from an old Cretaceous beach in Northern Colorado). In some of the
several native clays that I use it causes cracking in the bisque fire range.
This is undoubtedly due to the quartz inversion problem, where the little
spheres of primarily quartz initially expand during heating, pushing the
clay away. On the way back down the temperature curve they shrink a bit,
leaving a small void. All of this movement tends to result in a myriad of
tiny cracks that seriously compromise the structural strength of the clay
body. And the problem just gets worse as the cracks start running from
heating and cooling. If the pots make it through the bisque and are then
subjected to stoneware temperatures, the problem goes away when the clay
sinters and shrinks, effectively erasing the tiny cracks. However, most of
my native clays shrink so much that you can see each sand grain on the
surface of the pot, a not too pleasant decorative effect. I have abandoned
the sand and now follow the lead of the prehistoric Pueblo potters of the
American Southwest, who avoided using sand and preferred crushed igneous
rock (diorite) or ground up pots (grog). Trouble is, grinding up rocks or
sherds is hard work with a mano and metate. Anyone out there have a cheap
hammer mill (I will accept donations)?
William A. Lucius, Board President
Institute for Archaeological Ceramic Research
845 Hartford Drive
Boulder, CO 80305
iacr@msn.com
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iandol on sat 4 oct 03
Dear William Lucius,=20
An interesting description of your work. I am unsure if you need =
comments about the way you are using this Cretaceous Sand but I was =
interested in your comment about this influence of quartz inversions.
Perhaps is not universally recognised but when a clay body dries =
additives such as sand and grog remain at a constant size while the clay =
fraction shrinks as water evaporates. As there is no adhesive to fix =
clay to these additives many fractures form and it is possible that =
every non plastic particle is, to some degree, detached from the clay. =
This is more noticeable where mesh size is large. Paradoxically, when =
stress develops due to drying and other causes, blocky fragments tend to =
arrest the motion of a crack as it propagates through the material. =
However, the final integrity of any clay body depends on the proportions =
of plastic to non plastic materials and the mesh sizes involved.
Best regards,
Ivor Lewis. Redhill, South Australia
Ron Roy on tue 7 oct 03
Hi William,
I have a "trick" for you!
I make my own grog for certain projects - crush up dried trimmings - with a
rolling pin - then sieve different sizes. Then bisque it.
that way you have all the advantages of grog but it is the same as the body
so there is no mis fit possible.
A side advantage - you get lots of dry powdered body - handy if you are
making deflocced slip.
It's a dusty business - the sieving part so it should be done outside or in
a well ventilated area with a good dust mask on.
RR
>For years I used a fine beach sand in my earthenware and stoneware pots
>(free from an old Cretaceous beach in Northern Colorado). In some of the
>several native clays that I use it causes cracking in the bisque fire range.
> This is undoubtedly due to the quartz inversion problem, where the little
>spheres of primarily quartz initially expand during heating, pushing the
>clay away. On the way back down the temperature curve they shrink a bit,
>leaving a small void. All of this movement tends to result in a myriad of
>tiny cracks that seriously compromise the structural strength of the clay
>body. And the problem just gets worse as the cracks start running from
>heating and cooling. If the pots make it through the bisque and are then
>subjected to stoneware temperatures, the problem goes away when the clay
>sinters and shrinks, effectively erasing the tiny cracks. However, most of
>my native clays shrink so much that you can see each sand grain on the
>surface of the pot, a not too pleasant decorative effect. I have abandoned
>the sand and now follow the lead of the prehistoric Pueblo potters of the
>American Southwest, who avoided using sand and preferred crushed igneous
>rock (diorite) or ground up pots (grog). Trouble is, grinding up rocks or
>sherds is hard work with a mano and metate. Anyone out there have a cheap
>hammer mill (I will accept donations)?
>William A. Lucius, Board President
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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