iandol on sun 9 mar 03
Dear Ron Roy,
It would be an interesting task to test out the hypothesis that Talc =
decomposes when heated beyond 900=B0C to become Magnesia and Silica. =
Should give a strong alkali response which tested with indicator after =
being dissolved in water.
My understanding, from the Gemstone knowledge base, is that Talc on =
heating to that temperate changes to a material called Enstatite which =
is both hard and heat resistant. I have several Talc carvings which are =
difficult to scratch with steel. Sepiolite is a Talc carving material =
used, because of its heat resistance, to make the bowls of smokers =
pipes. Talc has also been turned to make the insulator shafts for =
sparking plugs. becomes very hard after firing and has heat resistant =
properties.
By the way, are you saying that the clay will craze if talc is not =
included in the body composition or do you mean it should be put in the =
glazes <produce Cristobalite in low fire bodies to help avoid crazing - and also =
to reduce rehydration in low fired bodies.>> I am a bit confused by what =
you are saying here.
Best regards,
Ivor Lewis
Ron Roy on tue 11 mar 03
Hi Ivor,
I have looked this up in 3 texts - I will quote below.
The enstatite is formed on cooling - can even show up in glazes - has a
high CTE and is why it is used in lowfire bodies to combat crazing - and
cuts water absorption of underfired bodies.
What I am saying about crazing in lowfire bodies is - it is difficult to
prevent crazing on them with certain glazes - the talc addition helps by
raising the CTE of the body (50 to 60% Talc is normal) and - because the
rehydration issue is also addressed - you are also dealing with the crazing
due to rehydration.
Cardew - bodies with Talc - Below 1260C the mineral crystallizing will
probably will probably not be corderite but various forms of enstatite
(MgO,SiO2), a mineral which has a high thermal expansion.
Obstler - Out of the Earth into the Fire under Talc - clay bodies - function=
of.
earthenware bodies - enstatite high thermal expansion - reduces crazing and
reduces the post firing moisture expansion ...... and reduces delayed
crazing due to.
Hamer - under enstatite 1180 1180C Magnesium oxide and silica are involved
in a ceramic fusion. During cooling some of the Magnesium oxide and
devitrify to give 3(MgO.SiO2) =3D enstatite - .........
Hope this helps - RR
>My understanding, from the Gemstone knowledge base, is that Talc on
>heating to that temperate changes to a material called Enstatite which is
>both hard and heat resistant. I have several Talc carvings which are
>difficult to scratch with steel. Sepiolite is a Talc carving material
>used, because of its heat resistance, to make the bowls of smokers pipes.
>Talc has also been turned to make the insulator shafts for sparking plugs.
>becomes very hard after firing and has heat resistant properties.
>
>By the way, are you saying that the clay will craze if talc is not
>included in the body composition or do you mean it should be put in the
>glazes <>produce Cristobalite in low fire bodies to help avoid crazing - and also
>to reduce rehydration in low fired bodies.>> I am a bit confused by what
>you are saying here.
>
>Best regards,
>Ivor Lewis
>
>
>
>
>
>
>>
>
>
>
>
>
>Dear Ron Roy,
>It would be an interesting task to test out the hypothesis that Talc
>decomposes when heated beyond 900=B0C to become Magnesia and Silica. Should
>give a
>strong alkali response which tested with indicator after being
>dissolved in
>water.
>My understanding, from the Gemstone knowledge base, is that Talc on heat=
ing
>to that temperate changes to a material called Enstatite which is both
>hard and
>heat resistant. I have several Talc carvings which are difficult to
>scratch with
>steel. Sepiolite is a Talc carving material used, because of its heat
>resistance, to make the bowls of smokers pipes. Talc has also been turned t=
o
>make the insulator shafts for sparking plugs. becomes very hard after
>firing and has heat resistant properties.
>By the way, are you saying that the clay will craze if talc is not inclu=
ded
>in the body composition or do you mean it should be put in the glazes
><<Talc is not a low fire flux - i=
t
>acts in
>a catalytic way to produce Cristobalite in low fire bodies to help avoid
>crazing
>- and also to reduce rehydration in low fired bodies.>> I am a bit
>confused by what you are saying here.
>Best regards,
Ivor Lewis T>>size=3D2>
>
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
iandol on wed 12 mar 03
Dear Ron Roy,=20
Thanks for those notes. I don't thing Obstler is saying anything new. I =
was questioning your claim of Catalytic reaction of the talc when you =
wrote <produce Cristobalite in low fire bodies to help avoid crazing - and also =
to reduce rehydration in low fired bodies.>>
I am highly suspicious of the entry in Hamer's Dictionary. I wonder at =
the truth of his claim that there is total decomposition and ponder on =
the type of reaction which would cause two highly refractory oxides to =
recombine without a high degree of solvation and very slow cooling.
Hope you have a good trip south.
Best regards,
Ivor Lewis
| |
|
