Jon Pacini on mon 10 may 04
-----Original Message-----
From: Jon Pacini [mailto:jpacini@lagunaclay.com]
Sent: Friday, May 07, 2004 8:57 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Shell temper and lime pops
Grettings All, Vince wrote---
> I know that lime inclusions need to be a certain size before they
cause lime
> pops. I do not know the specific size, but when crushed seashells
were used
> by tribal potters as tempering material, the coarser particles were
sifted
> out, and only the finer material was mixed into the clay - thus no
problem
* with lime pops.
I thought I d throw in my two bits here---------
Some fireclays and grogs tend to exhibit this particular difficulty with
amazingly frustrating irregularity. Fortunately these materials are
available in various mesh grinds and you can pretty much predict which
ones are going to have the occasional lime pop and which won t by the mesh
size.
If you get lime that is 35 mesh size or larger, it s likely to cause a pop
out on pit-fired ware, low fired Raku and soft bisque. 50 mesh and finer
you will rarely see a problem, occasionally on very soft fired ware.
Low fired wares, Raku and bisque are most susceptible to the problem due to
a lack of strength in the ware. When the lime expands due to rehydration,
the clay can t resist/contain that expansion. You will rarely see once
fired/high-fired stoneware/porcelain with lime problems. At this point the
clay has greater strength and/or the lime has been assimilated into the body
for the most part.
Course quartz will also give you grief, particularly that which is contained
in flint fireclays and grogs. This is can be a problem at 20 mesh sizes and
larger. It exhibits a similar pop off, but appears as a shiny crystal in the
clay rather that the flat white granule of lime.
So, if you re going to add ground shell to your clay, or any inclusion, pay
attention to the temperature you re firing at and test with a couple of mesh
sizes to see what is going to cause you grief and what won t.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co
Ron Roy on tue 11 may 04
In the case of high fired stoneware - lime can still be a problem if the
clay is not properly vitrified of course - so we are back to that again!
The problem is still there with even vitrified stoneware - some of the
chunks close to the surface will still rehydrate.
The solution is to screen the clays (particularly the fire clays and grog)
through 35 mesh - finer for low fired bodies
RR
>Low fired wares, Raku and bisque are most susceptible to the problem due to
>a lack of strength in the ware. When the lime expands due to rehydration,
>the clay can t resist/contain that expansion. You will rarely see once
>fired/high-fired stoneware/porcelain with lime problems. At this point the
>clay has greater strength and/or the lime has been assimilated into the body
>for the most part.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Jon Pacini on thu 13 may 04
Greetings All --------
-Ron wrote-----The solution is to screen the clays (particularly the fire
clays and grog)
through 35 mesh - finer for low fired bodies
This would work fine unless you happen to want courser particles than 35
mesh in your clay.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co
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