Chris Schafale on sun 1 sep 02
OK, I give up. How does one make a low fire (cone 05, let's say)
glaze that doesn't craze, without using lead? I normally fire to cone
6, but would like a clear that I can fire along with my bisque for
those occasions when a student wants to used some underglazes
that can't go to cone 6.
I've tried to create something in a glaze calc program, but all my
efforts end up with expansions that are 7.5 or greater and sure to
craze on my clay. It seems that I can't add enough silica to get the
expansion down, without having too much to melt at 05. Any
suggestions?
Another question -- how does underfiring affect clay expansion? In
other words, if I take a clay that normally is fired to cone 5 and has
an expansion around 7, and I fire it only to 05, is the expansion any
different than it would be if fired to maturity? If it's different, in
which direction?
Thanks for any help.
Chris Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
Ron Collins on sun 1 sep 02
This is rather elementary answer, but do you let it cool naturally and
completely before you open the kiln? When i was struggling with low fire, I
had glazes that did not craze, but they were usually opaque....clear is
difficult, but putting it on very thinly helps, and some homemade ones, such
as 60 3134 and 40 ball clay worked out better for me, and a little more
silica if needed....all the hobby ceramic glazes I bought here and tried
weren't worth much......Melinda Collins, Antigua, Guatemala
Wayne Bien on sun 1 sep 02
I have used a commercial low fire clear glaze and used to have a crazing
problem too. Someone on this list suggested that I bisque at ^04 if I am
glaze firing at ^06. Also make sure that the work is completely cool
before unloading the kiln. These two suggestions have worked 100% of the
time.
Wayne
Ron Roy on mon 2 sep 02
Hi Chris,
There are significant problems with getting good glaze fit at lower
temperatures - perhaps you can find a solution from understanding what the
problems are. My standard answer is go to cone 6 where the problems are
much easier to solve - especially for functional ware.
Clay bodies at earthenware temperatures are notoriously hard to fit glazes
on - it's because they don't have a high enough coefficient of expansion (
and contraction on cooling) to throw the glazes into compression - which
stops crazing. Also the glaze layer does not typically bond well enough to
the clay and fit problems show up sooner because the glaze is not well
melted into the clay. And to make it all worse - when you do finally get a
glaze that does not craze - after the ware is subject to use - the clay
rehydrates - it gets bigger and the glaze crazes anyway.
One of the best ways to get around this is to use a high Talc body - these
develop steatite with raises the expansion/contraction and also helps
prevent rehydration of the clay later.
I have calculated out a glaze that will be a good starting point - I don't
have much experience with low fire glazes so you will have to be careful
with this - I don't know if it will run and I have no idea if it will
shiver (the opposite of crazing and a dangerous glaze fault) so test for
everything.
This glaze has a low expansion rate - if it needs to be lower add more
silica - if you change bodies test well for fit again - this is probably a
durable glaze if it is melted enough but if you are using colours under it
that are on the inside of pots make sure the stains are mixed with this
same glaze to ensure a good lock up.
Frit 3124 - 80.0
OM#4 - 10.0
Silica - 10.0
Total 100.0
If it runs add more clay till it stops - if it crazes then send it back and
I will try to fix it. If it settles out too fast add 1 or 2 bentonite.
RR
>OK, I give up. How does one make a low fire (cone 05, let's say)
>glaze that doesn't craze, without using lead? I normally fire to cone
>6, but would like a clear that I can fire along with my bisque for
>those occasions when a student wants to used some underglazes
>that can't go to cone 6.
>
>I've tried to create something in a glaze calc program, but all my
>efforts end up with expansions that are 7.5 or greater and sure to
>craze on my clay. It seems that I can't add enough silica to get the
>expansion down, without having too much to melt at 05. Any
>suggestions?
>
>Another question -- how does underfiring affect clay expansion? In
>other words, if I take a clay that normally is fired to cone 5 and has
>an expansion around 7, and I fire it only to 05, is the expansion any
>different than it would be if fired to maturity? If it's different, in
>which direction?
>
>Thanks for any help.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Chris Schafale on tue 3 sep 02
Hi Ron,
Thanks for your reply. I will check out your suggested glaze.
Looking at the analysis, I will be very interested to see if it melts.
Alumina at .38 and silica at 3.34 looks a lot higher than I thought I
had to aim at. Published limits I had seen said that alumina should
be below .25 and silica below 2.5. If this melts, then I have a lot
more room to play with than I thought I did.
Could you address the second question? What is the effect, if any,
on expansion of the clay, when you underfire? That is, how would
you expect underfiring to affect glaze fit?
Changing clay bodies is not an option here, at least at the moment.
Chris
Thanks for your help.
Date sent: Mon, 2 Sep 2002 22:35:29 -0500
Send reply to: Ceramic Arts Discussion List
From: Ron Roy
Subject: Re: low fire glazes and crazing
To: CLAYART@LSV.CERAMICS.ORG
> Hi Chris,
>
> There are significant problems with getting good glaze fit at lower
> temperatures - perhaps you can find a solution from understanding what the
> problems are. My standard answer is go to cone 6 where the problems are
> much easier to solve - especially for functional ware.
>
> Clay bodies at earthenware temperatures are notoriously hard to fit glazes
> on - it's because they don't have a high enough coefficient of expansion (
> and contraction on cooling) to throw the glazes into compression - which
> stops crazing. Also the glaze layer does not typically bond well enough to
> the clay and fit problems show up sooner because the glaze is not well
> melted into the clay. And to make it all worse - when you do finally get a
> glaze that does not craze - after the ware is subject to use - the clay
> rehydrates - it gets bigger and the glaze crazes anyway.
>
> One of the best ways to get around this is to use a high Talc body - these
> develop steatite with raises the expansion/contraction and also helps
> prevent rehydration of the clay later.
>
> I have calculated out a glaze that will be a good starting point - I don't
> have much experience with low fire glazes so you will have to be careful
> with this - I don't know if it will run and I have no idea if it will
> shiver (the opposite of crazing and a dangerous glaze fault) so test for
> everything.
>
> This glaze has a low expansion rate - if it needs to be lower add more
> silica - if you change bodies test well for fit again - this is probably a
> durable glaze if it is melted enough but if you are using colours under it
> that are on the inside of pots make sure the stains are mixed with this
> same glaze to ensure a good lock up.
>
> Frit 3124 - 80.0
> OM#4 - 10.0
> Silica - 10.0
> Total 100.0
>
> If it runs add more clay till it stops - if it crazes then send it back and
> I will try to fix it. If it settles out too fast add 1 or 2 bentonite.
>
> RR
>
>
> >OK, I give up. How does one make a low fire (cone 05, let's say)
> >glaze that doesn't craze, without using lead? I normally fire to cone
> >6, but would like a clear that I can fire along with my bisque for
> >those occasions when a student wants to used some underglazes
> >that can't go to cone 6.
> >
> >I've tried to create something in a glaze calc program, but all my
> >efforts end up with expansions that are 7.5 or greater and sure to
> >craze on my clay. It seems that I can't add enough silica to get the
> >expansion down, without having too much to melt at 05. Any
> >suggestions?
> >
> >Another question -- how does underfiring affect clay expansion? In
> >other words, if I take a clay that normally is fired to cone 5 and has
> >an expansion around 7, and I fire it only to 05, is the expansion any
> >different than it would be if fired to maturity? If it's different, in
> >which direction?
> >
> >Thanks for any help.
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
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> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
Autumn Downey on tue 3 sep 02
Hi Chris,
I'm not sure if I'm right about this or not but sometimes with bigtime
fritted glazes, looking at the analysis with the boron changed to a flux
seems more accurate as to the melting powers.. Your is as follows
according to my Insight. It doesn't appear to have that low an expansion
though. Not that everyone else will agree but I remember that some old
recipes with only GB, silica and kaolin were great non crazers. (I suspect
GB/boron is elastic??) One such (Les Lawrence's with about 50% GB) was all
that worked on a cone 6 porcelain, though it was a bit yellowish and the
red underglazes disappeared. Variations on this were used for lowfire.
RR's with boron as flux:
FRIT 3124........... 80.00 80.00%
OM #4 BALL CLAY..... 10.00 10.00%
silica............... 10.00 10.00%
========
100.00
CaO 0.45* 11.45%
MgO 0.00* 0.04%
K2O 0.02* 0.68%
Na2O 0.18* 5.14%
TiO2 0.00 0.12%
Al2O3 0.23 10.84%
B2O3 0.35* 11.10%
SiO2 2.22 60.53%
Fe2O3 0.00 0.11%
Cost/kg 3.44
Si:Al 9.48
SiB:Al 10.98
Expan 7.08
Autumn Downey
Yellowknife, NWT
At 06:10 AM 2002-09-03 -0400, you wrote:
>Hi Ron,
>
>Thanks for your reply. I will check out your suggested glaze.
>Looking at the analysis, I will be very interested to see if it melts.
>Alumina at .38 and silica at 3.34 looks a lot higher than I thought I
>had to aim at. Published limits I had seen said that alumina should
>be below .25 and silica below 2.5. If this melts, then I have a lot
>more room to play with than I thought I did.
>
>Could you address the second question? What is the effect, if any,
>on expansion of the clay, when you underfire? That is, how would
>you expect underfiring to affect glaze fit?
>
>Changing clay bodies is not an option here, at least at the moment.
>
>Chris
>
>Thanks for your help.
>
Stephani Stephenson on tue 3 sep 02
Ron, this glaze
Frit 3124 - 80.0
> OM#4 - 10.0
> Silica - 10.0
> Total 100.0
is similar, if not exactly like(pardon the fogginess, my glaze notebook
is at the studio)
to the cone 04 Carleton Ball base I have often used to test colorants .
I also highly recommend the approach Ivor suggests. When staring form
scratch at this temperature,
I made small dishes , then compacted and inverted small 'buttons' of
various frits into the dishes
(press frit into 1/4 tsp. spoons, invert, and voila ,you have little
domes or buttons of frit)
At this temperature it will be in your best interest to know your frits.
I bought a sampling of frits and tested ALL of them
with this button test first. I like Ivor's suggestion of line blending
them with your clay or feldspar.
The next step for me was to add 10% clay and 10 % silica to each frit. I
should note that some frits. such as Ferro 265 and 3124 are almost
complete ,white, glazes at cone 04, without any additions. 3190 will
give you a hard clear melt and is a good base for adding ingredients.
3134 has a wide melting range surface qualities of it's own.
The next step for me was to take these basic glazes and glaze large 4" X
8" test bars, with test stripes of all the different stains and coloring
oxides, to see the different color responses.
Of course from there you get into issues of opacity, texture, surface
etc., but this glaze is a good starting point.
With regard to the issue of clay rehydration and also interaction
between clay and glaze.
The original question dealt with cone 05. If you could push it to cone
04 I would recommend this. To a lowfire person, the difference between
04 and 06 is substantial, though a 'high firer'might not notice! you
just get a bit more melt, a bit more interaction, a bit more
vitrification of materials.
Clay rehydration can definitely be a problem at this temperature.
Lowfire clays are usually higher priced than high fire clays. In
addition to talc bodies, some clay bodies, (even tile bodies with grog
), also contain added body fluxes such as frit , to ensure a more
vitreous and dense body at lower temps. Once we tested some clay which
had glass cullet added to it instead of frit. the supplier was
attempting to find a lower cost body flux.
SO your clay body will make a HUGE difference in how it responds to
glazes at lower temps. I believe that the fluxing/vitrifying occurring
in the clay body essential for good glaze/body bond and good glaze melt
/fit at lower temps.
In layman's terms, the clay has to reach out and grab the glaze as much
as the glaze has to reach in and cling to the clay!
So if you have a hot spicy glaze that wants to dance the rumba , and a
stiff dry uppercrust clay that wants to stand in the corner and snub the
dancers, well, nothing fun is going to happen. Have to loosen that dry
dusty claybody up a little and get it to start shaking it's molecules
around if you want to have a dance party on the surface of your pot.. I
know Ron is CRINGING at this!
Anyway, the rehydration which causes crazing is difficult to eliminate
completely. Delayed crazing is a problem, as a glaze may look fine, but
6 month later develop crazing. Of course, some people love a craze or
crackle glaze, so you may find this is a flaw you can live with, unless
you are making sanitary ware or dishware.
It is also a problem for tilemakers. Tile makers have a final back up
solution, which does not absolve them from using stable, well fitting
glazes, but it never hurts... that is the use of moisture barriers in
the installation process. The traditional way to set tile is to use a
complete bed of thinset on the backside, which acts as a water barrier,
to prevent water from seeping into the back of the tile, through the
unglazed side. Use of penetrating sealants on grout and tile and even
top sealants , prevent further moisture seepage from top and sides.
I have made tile used to replace damaged tile in many 100 year old
fireplaces. Water seepage had occurred from the back, sometimes from
outside the house, through the walls, and either deposited efflorescence
of salts on tile surfaces or caused severe crazing of glaze.
Nowadays, installation materials can help with post firing seepage
problems.
Bye now
have a good rumba, or at least a waltz in the glaze room, just take
notes.....
Stephani Stephenson
steph@alchemiestudio.com
Martin Rice on tue 3 sep 02
Ron wrote:
> One of the best ways to get around this is to use a high Talc body - these
> develop steatite with raises the expansion/contraction and also helps
> prevent rehydration of the clay later.
I am using a clay body that I bisque at ^04 and have been glaze firing at
^06. It's
70 Ball Clay -- using KY-TN #4
10 Talc
10 Calcium Carbonate
10 Feldspar
The glazing is at ^06 because I've been using prepared glazes from Duncan;
Concept series for colors and GL612 Diamond Clear Gloss Glaze - Transparent.
Thus far I've had no trouble, though in tests if I put on the clear glaze
very thickly it crazes.
But I would like to start making my own glazes for this range -- assuming I
can get enough materials here in Costa Rica to do it -- and I was wondering
whether I could adjust my claybody with more talc to perhaps help improve
the behavior of glazes I might make? I have no idea about how much more talc
to add, if any, which of the other ingredients to reduce, and whether this
would still be a ^04 bisquing claybody after the changes.
Any information would be appreciated.
Thanks,
Martin
Lagunas de Barú, Costa Rica
http://www.rice-family.org
Ron Roy on fri 6 sep 02
Hi Chris,
Your orginal post got by me - perhaps when I was away - anyway - the
Alumina is within limits whenyou use the Cooper and Green limits for boron
glazes - although the silica is higher - if need be we can lower the silica
but the expansion will rise - maybe you should do one with 85 frit, 10 epk,
and 5 silica - the expansion is up a bit but it will help us do the next
one if needed.
Keep in mind - frits are already melted so fitted glazes have a big
advantage in that some of the heat work is done - and you can have more
refractories because of this. Especially so when the glaze is almost all
frit.
I don't really know how under firing affects fit - I suppose some day I
will have enough time to do the experiments - but I would say the expansion
will not be significantly changed - lower means some more free quartz I
suppose but I would guess not enough to make a real difference.
You are going to get all kinds of opinions on this - I just don't know.
I suggest you do some experiments - the hard part is getting the
application thickness the same for different kiln loads - just have to find
a way to dip 5 tiles at the same time I guess.
Authum said about Boron - elasticity - and I think she is right - BUT -
only if the % of Boron is less than 12 - over that % and boron starts to
loose it's anti craze property.
RR
>>Hi Ron,
>>
>>Thanks for your reply. I will check out your suggested glaze.
>>Looking at the analysis, I will be very interested to see if it melts.
>>Alumina at .38 and silica at 3.34 looks a lot higher than I thought I
>>had to aim at. Published limits I had seen said that alumina should
>>be below .25 and silica below 2.5. If this melts, then I have a lot
>>more room to play with than I thought I did.
>>
>>Could you address the second question? What is the effect, if any,
>>on expansion of the clay, when you underfire? That is, how would
>>you expect underfiring to affect glaze fit?
>>
>>Changing clay bodies is not an option here, at least at the moment.
>>
>>Chris
>>
>>Thanks for your help.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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