Paul Borian on mon 22 nov 10
i have some dunting issues with my latest clay body and it does not seem
like the firing process can be changed enough to eliminate the problem, so =
i
need to adjust the body.
the most common problem is rim cracks on platters which happen during
cooling in the bisque firing.
i could add a little more feldspar to help but i also need to figure out, o=
f
the clays i use, which ones have the most free quartz after an 04 bisque
firing.
can this information be determined just by looking at the chemical analysis
for each material and seeing how high the percentage of silica is?
for example, i use foundry hill creme, which is 59% silica and 26% alumina;
and hawthorn bond which is 50% silica and 31% alumina.
would the foundry hill necessarily have more free quartz after bisque firin=
g
then?
i mention these two clays because i want to increase the hawthorn and
decrease the foundry hill for other reasons as well but was wondering if
that could also help with this issue.
does anyone know the answer to this?
thanks,
paul
Ron Roy on wed 24 nov 10
Hi Paul,
If you look at the analysis for kaolins - they have the least silica -
so other types of clays do have more free silica and you can tell by
looking at the analysis. What you can't tell is - particle size range
of the silica - you have to see a particle distribution analysis to
see that.
Finer particles are going to be converted to non crystalline silica
more easily than larger size quartz during the bisque firing.
You are right - Hawthorn does have less free silica then FHC but check
out particle size before making a decision.
RR
Quoting Paul Borian :
> i have some dunting issues with my latest clay body and it does not seem
> like the firing process can be changed enough to eliminate the problem, s=
o i
> need to adjust the body.
> the most common problem is rim cracks on platters which happen during
> cooling in the bisque firing.
> i could add a little more feldspar to help but i also need to figure out,=
of
> the clays i use, which ones have the most free quartz after an 04 bisque
> firing.
> can this information be determined just by looking at the chemical analys=
is
> for each material and seeing how high the percentage of silica is?
> for example, i use foundry hill creme, which is 59% silica and 26% alumin=
a;
> and hawthorn bond which is 50% silica and 31% alumina.
> would the foundry hill necessarily have more free quartz after bisque fir=
ing
> then?
> i mention these two clays because i want to increase the hawthorn and
> decrease the foundry hill for other reasons as well but was wondering if
> that could also help with this issue.
>
> does anyone know the answer to this?
>
> thanks,
> paul
>
ivor and olive lewis on wed 24 nov 10
Dear Paul Borian,
The information you give...
"I use foundry hill creme, which is 59% silica and 26% alumina; and hawthor=
n
bond which is 50% silica and 31% alumina."
tells us very little about your raw clay. The important quandary is to know
how those Al and Si values are allocated across the mineral assay. Without
that knowledge the first step in arriving at a solution cannot be taken.
Good technical problem solving arrises from knowing how to obtain additiona=
l
knowledge. In this case the starting place would be an up to date assay of
the mineral content of each clay and of the other contributing materials
that might be in your clay batch.. I would need to know how the Aluminium
Oxide and Silicon dioxide are distributed as minerals, eg Kaolin, Mica,
Felspar, Arkose, Quartz and so forth. . An appraisal of molecular content i=
s
inadequate
I would reject both of these. Both offer potential problems for the Studio
Potter or the ceramic artist. Both might be classed as Fireclays. Free
quartz, (Foundry Hill Cream has a very high value for molecular Silicon
Dioxide) in both will always make them suspect.
Regards,
Ivor Lewis,
REDHILL,
South Australia
| |
|
