Snail Scott on thu 12 sep 02
At 12:16 PM 9/12/02 -0700, you wrote:
>...The smaller the grog,
>the less it functions as "grog" and the more it becomes like calcined clay.
>So where does grog cease being "grog" as a purposeful additive???
I'd consider calcined clays to be a variety of grog.
They still perform the function of reducing shrinkage,
even if they have a negligible effect on the working
properties of the clay body.
-Snail
Alan D. Scott on thu 12 sep 02
What is the functional limit for grog in a clay body? The smaller the grog,
the less it functions as "grog" and the more it becomes like calcined clay.
So where does grog cease being "grog" as a purposeful additive??? 60 mesh?
80 mesh? 100 mesh???
Alan Scott
The 3rd Cup Clayworks
Parker, AZ
Dave Finkelnburg on thu 12 sep 02
Alan,
This is an excellent, excellent, excellent question!
I am sure others on the list have more practical experience concerning
the answer to this question.
The answer, by the way, depends on whether you are adding grog to affect
the wet structural properties of the clay body, or to affect the drying
properties.
In the latter case, I can tell you from theory (notice I've already
hedged and qualified this answer!) that since clay particles are around 1
micron, approximately, in size, and 100-mesh particles (let's say they are
spheres) are nominally 150-microns in diameter, then a 100-mesh particle has
roughly 3.4-million times the volume of a clay particle. Even 325-mesh
particles have 85,000 times the volume of clay. Soooooo, it seems to me
that in theory 100-mesh grog will have a big effect, and even 325-mesh grog
will have a serious effect on drying.
As for making the clay handle better for throwing large or tall, in my
limited experience I have found that the utility of the grog (or sand)
diminishes proportionally beginning around 35 mesh and working down. If the
coarsest material in a body is 100 mesh, the body may dry very fast and be
very open, but it's not going to be great for throwing large. Coiling and
handbuilding with it could work great, though.
Regards,
Dave Finkelnburg
----- Original Message -----
From: "Alan D. Scott"
> What is the functional limit for grog in a clay body? The smaller the
grog,
> the less it functions as "grog" and the more it becomes like calcined
clay.
> So where does grog cease being "grog" as a purposeful additive??? 60
mesh?
> 80 mesh? 100 mesh???
iandol on fri 13 sep 02
Dear Alan D. Scott'
You ask ..<< What is the functional limit for grog in a clay body? The =
smaller the grog, the less it functions as "grog" and the more it =
becomes like calcined clay. So where does grog cease being "grog" as a =
purposeful additive??? 60 mesh? 80 mesh? 100 mesh???>>...
First of all, "Grog", in a ceramic sense, is fired clay. So it is =
already calcined. Sometimes it is high fired, sometimes it is just =
ground bisque. A second point is that what you may buy form a supplier =
will be <#A>#B, that is, it will have a size range. The source of this =
material may also be important. Should it be refractory, eg Fireclay or =
Molochite? Other questions to consider are colour and absorbency, even a =
non clay filler such as sand.
I think a full answer lies in an interpretation of "purposeful". Just =
what are you expecting it to do by adding grog. Stiffen the clay? Adsorb =
excess water from a sticky mixture? Have clay that dries quickly? Add =
texture? Give it teeth so that you can feel it as it is thrown? Reduce =
shrinkage? Enhance vitrification? Reduce Pyroplasticity?=20
Of one thing I am certain. Regardless of the opinions of other authors =
or commentators, increasing the volume of non plastic additives by a =
significant proportion will decrease the plastic strength of mouldable =
clay. It will also increase the rate at which clay adsorbs water as it =
is thrown.
As Dave said, an excellent question. Perhaps it is important to ask two =
more questions. What makes clay plastic? and, How do you increase =
plastic strength?
Best regards,
Ivor Lewis. Redhill, South Australia
Alan D. Scott on fri 13 sep 02
Snail:
Thanks for that clarification! I'm thinking about the functional end of
things, so what is the smallest size grog that impacts working properties?
(I know, test and discover for myself...)
Alan
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Snail Scott
Sent: Thursday, September 12, 2002 9:54 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Size Limits for Grog???
At 12:16 PM 9/12/02 -0700, you wrote:
>...The smaller the grog,
>the less it functions as "grog" and the more it becomes like calcined clay.
>So where does grog cease being "grog" as a purposeful additive???
I'd consider calcined clays to be a variety of grog.
They still perform the function of reducing shrinkage,
even if they have a negligible effect on the working
properties of the clay body.
-Snail
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Snail Scott on sun 15 sep 02
Ivor wrote:
>>... Just what are you expecting it to do by adding grog.
>>Stiffen the clay? Adsorb excess water from a sticky mixture? Have clay
>>that dries quickly? It will also increase the rate at which clay adsorbs
>>water as it is thrown.
Fred wrote:
>After reading this I had a thought -- what would happen if the grog were
>treated with a water repellent like the new "Water Warden"?
Why? Most grog is already non-absorbent, though it does
add its surface area for moisture to surround. Groggy
clay dries (and wets) quickly because grog DOESN'T absorb
moisture, not because it does. (That, and the micro-
cracking it causes.) Grog seems to provide 'conduits'
for moisture to move around it, NOT through it.
-Snail
Ron Roy on sun 15 sep 02
Rhodes said 30 mesh was the optimum for improving throwability - difficult
to find all 30mesh - it's mostly 30M and smaller.
Running 30 mesh grog through a 30 mesh screen will tell you what percent is
30M.
5 to10% in a body by weight is the amount I think is most benificial. To
much fine does not add structure and also lets water in faster - no science
here - just my experience.
RR
>What is the functional limit for grog in a clay body? The smaller the grog,
>the less it functions as "grog" and the more it becomes like calcined clay.
>So where does grog cease being "grog" as a purposeful additive??? 60 mesh?
>80 mesh? 100 mesh???
>
>Alan Scott
>The 3rd Cup Clayworks
>Parker, AZ
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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