priddy on fri 2 jun 00
> And thanks to Elizabeth=3D92s SO. Now, if his theory about micro-magnet=
ic =3D
> fields has any credence, Sir may have to get the old magnetometer out =3D=
> and do a few tests while you, Elizabeth, are throwing. I always say tha=
t =3D
> we should not ignore the phase of the Moon either. =
oh, you can poke fun all you want, but I have seen less s-cracks in
brown iron rich clay than in the white clay I have used. And the forces
that cause those specks to rise and fall in the glaze are asociated
with gravity, which is a gross form of magnetism. Oh yes, I think it IS =
the magnetism....today.
I really do wish anyone with real information onthis would chime in.
I would like to know if this is hoo-ey.
=
respectfully submitted,
elizabeth priddy
priddy-clay@usa.net
http://www.angelfire.com/nc/clayworkshop
____________________________________________________________________
Get free email and a permanent address at http://www.netaddress.com/?N=3D=
1
Michael Banks on sun 4 jun 00
Well, this ties in with RR's comments about Modulus of Rupture (MoR) numbers
for various clays. These are a measure of green strength and correlate to
the strength of the plasticity of the clay before drying. Brown
iron-bearing clays often have higher MoR numbers than light coloured ones.
I believe this is due to the fact that the clay minerals in brown clays have
experienced greater degrees of weathering and mixing with other sediments,
producing finer and more plastic materials.
Such clays are also more likely to have some smectite (e.g: bentonite)
content. Smectites have the highest plasticity's (and green strengths) of
all clay minerals and are rare or absent from whiter clay.
S-cracking IMHO (where not potter induced) is due to a combination of high
drying shrinkage and relatively poor green strength. Good clay makers
counter this by reducing the dry shrinkage, by adding ingredients with a
wide range of particle sizes and maximising green strength by use of
suitable ball clays.
Magnetism may help, but note that the magnetic forces between grains of
magnetite (magnetic iron oxide) are orders of magnitude weaker than the
hydrogen bonding of clay to water molecules (which is the source of clay
adhesion). This can be demonstrated by rolling up a ball of wet magnetite
dust and comparing the strength of this to a similar ball of clay (even a
weakly plastic one).
Michael Banks,
Nelson,
NZ
----- Original Message -----
>> And thanks to Elizabeth=92s SO. Now, if his theory about micro-magnetic =
>> fields has any credence, Sir may have to get the old magnetometer out =
>> and do a few tests while you, Elizabeth, are throwing. I always say that
=
>> we should not ignore the phase of the Moon either.
Elizabeth Priddy wrote:
>oh, you can poke fun all you want, but I have seen less s-cracks in
>brown iron rich clay than in the white clay I have used. And the forces
>that cause those specks to rise and fall in the glaze are asociated
>with gravity, which is a gross form of magnetism. Oh yes, I think it IS
>the magnetism....today.
>I really do wish anyone with real information onthis would chime in.
>I would like to know if this is hoo-ey.
respectfully submitted,
elizabeth priddy
priddy-clay@usa.net
http://www.angelfire.com/nc/clayworkshop
____________________________________________________________________
Get free email and a permanent address at http://www.netaddress.com/?N=1
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