ronroy@CA.INTER.NET on sat 25 aug 12
Hi Ivor -
I have measured lots of glazes (probably over 100) and have never seen
any quartz except in one, obviously underfired, glaze.
Do you have any evidence of quartz in any glazes?
RR
Quoting ivor and olive lewis :
> Dear Susan Fox Hirschmann,
>
> On the technical side, glaze and commercial glass are materials with
> differing processing methods. They have different Expansion Coefficients.=
A
> glaze may have a quartz inversion
>
> Glass must be annealed, that is, held at a particular temperature or a
> length of time. A glaze must be cooled slowly through the Quartz inversio=
n.
>
> Without knowledge of these times and temperature you may get serious
> problems. Getting the right instructions may not be possible.
>
> I would err on the side of caution unless you student can provide rock so=
lid
> information relating to the annealing of her glass and you glazes a free
> from Quartz inversions.
>
> Sincerely,
>
> Ivor Lewis,
> REDHILL,
> South Australia
>
ivor and olive lewis on sun 26 aug 12
Ron,
Slim odds I agree. But given the size of the recipe pool from which we make
our choice of glazes, such an event though improbable it is not an
impossible chance.
Even if I was incorrect in my statement there is still the question of
annealing the glass.
All the best,
Ivor.
Ron Responded
> Hi Ivor -
>
> I have measured lots of glazes (probably over 100) and have never seen
> any quartz except in one, obviously underfired, glaze.
>
> Do you have any evidence of quartz in any glazes?
>
> RR
>
>
Des & Jan Howard on sun 26 aug 12
Ron
I'm being a bit picky, I guess. Bear with me.
Do you mean you have measured lots of glazes & not seen
the effects of quartz?
Or, have you examined lots of glazes & not seen any quartz?
The reason I ask is because of the reported causes of
the appearance of "good" celadons resulting from
suspended gas bubbles &/or quartz particles.
Des
On 26/08/2012 3:09 AM, ronroy@CA.INTER.NET wrote:
> I have measured lots of glazes (probably over 100) and have
> never seen any quartz except in one, obviously underfired, glaze.
--
Des & Jan Howard
Lue Pottery
Lue NSW
Australia
2850
02 6373 6419
www.luepottery.hwy.com.au
-32.656072 149.840624
ronroy@CA.INTER.NET on sun 26 aug 12
Hi Des, Hi Jan,
When I say I have not seen quartz in glazes I mean - looking at a
dilatometer chart I have not seen that telltale change at 573C that
says there is quartz present. It's easy to see in clay bodies - always
there.
Quartz in a glaze is converted from the crystalline form to the
amorphous and therefore does not go through any inversions.
Recrystallized glazes are another matter - the crystals we then have
to deal with are not just quartz but combinations of materials - that
is why the calculated expansions of those glazes is unreliable - who
knows what crystals and how they expand and contract.
When you look at the dilatometer chart of a glaze you see a steady
rise from room temperature to a transition point (which varies with
each glaze) - at that point you see a sharper rise until the line
stops rising and bends over. That is the melting point and - because
the rod (the glaze) is now getting shorter because it is melting - the
chart indicates less expansion.
Hope I am answering your question.
Happy to send you a PDF of a glaze chart if you want to see what I am
trying to explain.
RR
Quoting Des & Jan Howard :
> Ron
> I'm being a bit picky, I guess. Bear with me.
> Do you mean you have measured lots of glazes & not seen
> the effects of quartz?
> Or, have you examined lots of glazes & not seen any quartz?
> The reason I ask is because of the reported causes of
> the appearance of "good" celadons resulting from
> suspended gas bubbles &/or quartz particles.
> Des
>
> On 26/08/2012 3:09 AM, ronroy@CA.INTER.NET wrote:
>> I have measured lots of glazes (probably over 100) and have
>> never seen any quartz except in one, obviously underfired, glaze.
>
>
> --
> Des & Jan Howard
> Lue Pottery
> Lue NSW
> Australia
> 2850
>
> 02 6373 6419
> www.luepottery.hwy.com.au
> -32.656072 149.840624
>
Des & Jan Howard on mon 27 aug 12
Ron
Thanks for the reply.
I seem to remember quartz beyond a certain particle
size does not transform from beta quartz to beta
cristabolite, Thus, taking part in the higher temp beta
to alpha quartz cooling inversion, but not in the beta
to alpha cristabolite cooling inversion. Do you
recollect the size range?
If quartz in a glaze converts from crystalline to
amorphous, what is the purpose of using already
amorphous flint in low-fired glazes?
Des
On 27/08/2012 5:50 AM, ronroy@CA.INTER.NET wrote:
> When I say I have not seen quartz in glazes I mean -
> looking at a
> dilatometer chart I have not seen that telltale change
> at 573C that
> says there is quartz present. It's easy to see in clay
> bodies - always
> there.
>
> Quartz in a glaze is converted from the crystalline
> form to the
> amorphous and therefore does not go through any
> inversions.
>
> Recrystallized glazes are another matter - the crystals
> we then have
> to deal with are not just quartz but combinations of
> materials - that
> is why the calculated expansions of those glazes is
> unreliable - who
> knows what crystals and how they expand and contract.
--
Des & Jan Howard
Lue Pottery
Lue NSW
Australia
2850
02 6373 6419
www.luepottery.hwy.com.au
-32.656072 149.840624
ronroy@CA.INTER.NET on tue 28 aug 12
Hi Des,
Tying to get in under the wire with this.
Micro fine is all I know - 200 mesh quartz used to have a high
percentage of 200 mesh - now only about 8% - because of the newer
grinding machines the rest is all microfine - at least in North
America. Also there is a fine particle ejected from clay as it
transforms into mullite and those particle are seeds for cristobalite.
If you have enough KNaO around to melt them they cannot so having
enough spar is the key.
Not sure what the amorphous silica advantage is - perhaps it melts
easier? I do know that many low fire glaze do not "freeze" until after
the quartz inversion so there is no help, in that case, to counter
crazing. Some earthenware makes add cristobalite (carefully) to help
counter crazing - lets hope they don't in the case of ovenware.
Hope I have answered the right questions - I think I could poke around
a bit and fine the exact size of quartz that converts to cristobalite
if it's needed.
RR
Quoting Des & Jan Howard :
> Ron
> Thanks for the reply.
> I seem to remember quartz beyond a certain particle
> size does not transform from beta quartz to beta
> cristabolite, Thus, taking part in the higher temp beta
> to alpha quartz cooling inversion, but not in the beta
> to alpha cristabolite cooling inversion. Do you
> recollect the size range?
>
> If quartz in a glaze converts from crystalline to
> amorphous, what is the purpose of using already
> amorphous flint in low-fired glazes?
> Des
>
> On 27/08/2012 5:50 AM, ronroy@CA.INTER.NET wrote:
>> When I say I have not seen quartz in glazes I mean -
>> looking at a
>> dilatometer chart I have not seen that telltale change
>> at 573C that
>> says there is quartz present. It's easy to see in clay
>> bodies - always
>> there.
>>
>> Quartz in a glaze is converted from the crystalline
>> form to the
>> amorphous and therefore does not go through any
>> inversions.
>>
>> Recrystallized glazes are another matter - the crystals
>> we then have
>> to deal with are not just quartz but combinations of
>> materials - that
>> is why the calculated expansions of those glazes is
>> unreliable - who
>> knows what crystals and how they expand and contract.
>
>
> --
> Des & Jan Howard
> Lue Pottery
> Lue NSW
> Australia
> 2850
>
> 02 6373 6419
> www.luepottery.hwy.com.au
> -32.656072 149.840624
>
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