Carol Tripp on tue 12 nov 02
Ron Roy wrote:
>Crystal growth is favoured by silica - because when crystals form they need
>extra silica - there is a section in Taylor and Bulls - "Ceramic Glazes"
>which explains how crystals rob the surrounding glaze of silica - sometimes
>to the extent that the surrounding glaze is less durable because of the
>reduced SiO2.
>
>Alumina inhibits crystal growth (except in the case of alumina mattes -
>Al2O3 oversupplied) - Custer has more silica and less alumina than G200.
>
>Slower cooling at the "right time" will give bigger and more crystals by
>the way.
>
OK, here goes. Months ago, I made up some Tenmoku Gold and it crazed. (I
also subbed copper carb for RIO for a batch and cobalt carb for a batch -
all crazed.) So, I started altering the recipe to bring down the CoE
(before I read John H's "COE calculations are only relieable for glossy
glazes. Once crystals start to form all bets are off." 7sept02) I noticed
that as the silica increased the crystal formations changed from tiny stars
spread out to a fairly closely packed field of tiny crystals. The crazing
remained. Never mind that, because I use this family of glazes on
decorative things. Now, Ron's info explains the increase in crystals but I
want to know what is in this glaze that causes the crystals in the first
place? And would someone like to suggest tests I could run to find out the
answer?
Details are:
^6 ox, Tuckers smooth white, mid red and a mix of these
^6 is fully down, 40 min soak and slow fire down to 800C
5.50 Lithium Carb
28.0 Flint
30.0 Custer Feld
11.0 EPK
4.50 Magnesium Carb
3.50 Fusion Frit F-12 (like 3134)
15.50 Wollastonite
2.00 Bentonite
I add 10% red iron oxide for Tenmoku Gold (brown glossy with gold crystals,
3% copper carb for green glossy with silver crystals,
1.5% cobalt carb for blue glossy with silver/lavender crystals
I regard this glaze as A DECORATIVE GLAZE ONLY and I have not and do not
intend to send it for testing.
I am just interested to know what causes the crystals.
Best regards,
Carol
Dubai, UAE
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June Perry on tue 12 nov 02
Carol, you have several things in there that would form crystals with your
firing down schedule, i.e. calcium, magnesium, lithium; and if you add iron,
you can get iron crystals forming as well.
Regards,
June Perry
http://www.angelfire.com/art2/shambhalapottery/index.html
June Perry on tue 12 nov 02
Carol, I ran that glaze through Insight 5.0 and it has quite low expansion
(6.38).
You might try lowering the silica to 22 which would still give you a gloss
glaze. As it is, your silica:alumina ratio is around 11. Lowering the silica
to 22 would give you a silica:alumina ration of 10 (still a gloss).
Lowering the silica to 22 would raise the expansion to 6.58. You'd still be
within C6 silica limits. That might keep down the crystal growth and might
solve the crazing. Maybe what you think is crazing is not. It may be a
result of the glaze having too low a co-efficient of expansion for the
claybody.
I'll be interested to see what some of our expert glaze people have to say
about this.
Regards,
June Perry
http://www.angelfire.com/art2/shambhalapottery/index.html
Carol Tripp on wed 13 nov 02
Hi June,
I don't want to get rid of the crystals. I want to know why/how they are
caused in this glaze. As for the CoE, I get 6.38 Insight too. Before I
modified this glaze, the CoE was was 7.07 Insight. I made lots of tests on
the way down from 7.07 and the crazing never went away. I got fed up and
decided to live with it. Now I just want to understand the mechanics of the
crystals because I like them.
Best regards,
Carol
June wrote:
>Carol, I ran that glaze through Insight 5.0 and it has quite low expansion
>(6.38).
>You might try lowering the silica to 22 which would still give you a gloss
>glaze. As it is, your silica:alumina ratio is around 11. Lowering the
>silica
>to 22 would give you a silica:alumina ration of 10 (still a gloss).
>Lowering the silica to 22 would raise the expansion to 6.58. You'd still be
>within C6 silica limits. That might keep down the crystal growth and might
>solve the crazing. Maybe what you think is crazing is not. It may be a
>result of the glaze having too low a co-efficient of expansion for the
>claybody.
>I'll be interested to see what some of our expert glaze people have to say
>about this.
>
>
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John Hesselberth on wed 13 nov 02
Carol/June,
I once found I had to go the wrong way to get rid of a dunting problem
on a matte glaze. You might find you have to go up in calculated
expansion to get rid of the crazing. As I said earlier, all bets are
off on the calculated expansion numbers when crystals form.
Regards,
John
On Wednesday, November 13, 2002, at 12:00 AM, Carol Tripp wrote:
> I don't want to get rid of the crystals. I want to know why/how they
> are
> caused in this glaze. As for the CoE, I get 6.38 Insight too. Before I
> modified this glaze, the CoE was was 7.07 Insight. I made lots of
> tests on
> the way down from 7.07 and the crazing never went away. I got fed up and
> decided to live with it. Now I just want to understand the mechanics
> of the
> crystals because I like them.
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ron Roy on thu 14 nov 02
Hi Carol,
Nothing much unusual about this glaze - high in Lithium. I supect it is
slow cooling that is letting the crustals grow - if you fast cooled they
would not be there.
The expansion is quite low to start with by the way so I am puzzeled as to
why it is crazing - perhaps you should describe the crazing - does it
happen after the pots are out of the kiln or are they crazed when they come
out?
I am beginning to suspect it is the Lithium Carb - somewhat soluble - that
is doing it.
I have reduced the Lithium Carb - replaced it with boron mostly - expansion
is up but it still should not craze - I have no idea if the crystals will
be there but we do need to know if this glaze crazes.
Lith Carb - 2.0
Silica - 25.0
Custer - 33.0
EPK - 10.0
Mag carb - 5.0
F12 - 8.0
Wollas - 17.0
Bentonite - 2.0
Total - 102.0
You should also mix equal parts by volume - of this one and the original -
again to see if the crazing stops and/or the crystals develop.
RR
>OK, here goes. Months ago, I made up some Tenmoku Gold and it crazed. (I
>also subbed copper carb for RIO for a batch and cobalt carb for a batch -
>all crazed.) So, I started altering the recipe to bring down the CoE
>(before I read John H's "COE calculations are only relieable for glossy
>glazes. Once crystals start to form all bets are off." 7sept02) I noticed
>that as the silica increased the crystal formations changed from tiny stars
>spread out to a fairly closely packed field of tiny crystals. The crazing
>remained. Never mind that, because I use this family of glazes on
>decorative things. Now, Ron's info explains the increase in crystals but I
>want to know what is in this glaze that causes the crystals in the first
>place? And would someone like to suggest tests I could run to find out the
>answer?
>Details are:
>^6 ox, Tuckers smooth white, mid red and a mix of these
>^6 is fully down, 40 min soak and slow fire down to 800C
>
>5.50 Lithium Carb
>28.0 Flint
>30.0 Custer Feld
>11.0 EPK
>4.50 Magnesium Carb
>3.50 Fusion Frit F-12 (like 3134)
>15.50 Wollastonite
>2.00 Bentonite
>
>I add 10% red iron oxide for Tenmoku Gold (brown glossy with gold crystals,
>3% copper carb for green glossy with silver crystals,
>1.5% cobalt carb for blue glossy with silver/lavender crystals
>I regard this glaze as A DECORATIVE GLAZE ONLY and I have not and do not
>intend to send it for testing.
>I am just interested to know what causes the crystals.
>Best regards,
>Carol
>Dubai, UAE
>
>
>
>
>
>
>
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Carol Tripp on thu 14 nov 02
Hi John,
Ok, I'll give that a try.
Best regards,
Carol
Dubai, UAE
Shucks, I had planned to make test glazes today but my son is now marrooned
on the couch with a bucket beside him and I can't very well lock myself up
in the studio...I might miss the cries of Mom Mom I'm going to be eeuuiigh!
John wrote:
>I once found I had to go the wrong way to get rid of a dunting problem
>on a matte glaze. You might find you have to go up in calculated
>expansion to get rid of the crazing. As I said earlier, all bets are
>off on the calculated expansion numbers when crystals form.
>
>Regards,
>
>John
>On Wednesday, November 13, 2002, at 12:00 AM, Carol Tripp wrote:
>
>>I don't want to get rid of the crystals. I want to know why/how they
>>are
>>caused in this glaze. As for the CoE, I get 6.38 Insight too. Before I
>>modified this glaze, the CoE was was 7.07 Insight. I made lots of
>>tests on
>>the way down from 7.07 and the crazing never went away. I got fed up and
>>decided to live with it. Now I just want to understand the mechanics
>>of the
>>crystals because I like them.
>>
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Carol Tripp on thu 14 nov 02
Hi Ron,
The original glaze was:
61 cornwall stone
8 whiting
3 frit 3134
5.5 lithium carb
7 dolomite
5.5 silica
10 RIO
This ran and crazed fresh out of the kiln (150C). Ping ping. The crystals
were fine and spread out.
Then Cindy Strnad posted this:
10 whiting
5 lithium
4 dolomite
21 silica
37 custer
8 epk
2 magnesium carb
3 Frit 3134
10 RIO
This also crazed fresh from the kiln but was less runny and the crystals
were pinhead sized and a bit denser than the first glaze.
Then I revised the recipe to come up with the one I posted and it crazed
fresh from the kiln too. The crystals were denser still. I was surprised to
see the crazing at such a low CoE. I'm using Tuckers, not some weirdo
stuff. I should add that I tested all these glazes with 10% rio, or 2.5%
copper carbonate, or 1.5% cobalt carbonate. Everything pinged.
So, I will try out what you suggest and report back. There will be a delay
as a tummy bug is gripping my son and I can't disappear to test anything.
Thanks for helping.
Best regards,
Carol
Ron Roy wrote:
>Nothing much unusual about this glaze - high in Lithium. I supect it is
>slow cooling that is letting the crustals grow - if you fast cooled they
>would not be there.
>
>The expansion is quite low to start with by the way so I am puzzeled as to
>why it is crazing - perhaps you should describe the crazing - does it
>happen after the pots are out of the kiln or are they crazed when they come
>out?
>
>I am beginning to suspect it is the Lithium Carb - somewhat soluble - that
>is doing it.
>
>I have reduced the Lithium Carb - replaced it with boron mostly - expansion
>is up but it still should not craze - I have no idea if the crystals will
>be there but we do need to know if this glaze crazes.
>
>Lith Carb - 2.0
>Silica - 25.0
>Custer - 33.0
>EPK - 10.0
>Mag carb - 5.0
>F12 - 8.0
>Wollas - 17.0
>Bentonite - 2.0
>Total - 102.0
>
>You should also mix equal parts by volume - of this one and the original -
>again to see if the crazing stops and/or the crystals develop.
>
>RR
>
>
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