Diane Schanz on fri 21 apr 00
Hey all,
I've got 2 questions regarding glaze fit.
1. I am in the process of learning glaze chemistry and I have a question
that I haven't found an answer to. When trying to find a good glaze fit, how
close should the expansion numbers for clay body and glaze be? How close is
close enough?
2. I am trying to find the best fit for Laguna Aspen porcelain, ^5.
Unfortunately, John Baccini (sp?) from Laguna tells me that they don't run
thermal expansion tests on their clay bodies !??!!??!!! He suggested that I
just buy a bunch of glazes and see what works. Not what I expected to hear.
Does anyone out there have an expansion number for this clay? Or a
spectacularly clear gloss that has been tested by time on this clay body? I
am already testing several recipes, but with a deadline that was just moved
up by a month, I don't have all the free time I'd like to solve this problem.
(BTW, this is an outdoor mural, exposed to temps of 130 in the summer and
below freezing in the winter.)
If you have an answer, could you also post a copy to me privately?
TIA, Diane in Tucson
Michael Banks on sat 22 apr 00
Diane, your best bet with your glaze fit problem (given the outdoor end
use), is to ensure that the porcelain is fired to nil apparent porosity.
That way moisture-expansion crazing will be eliminated. One or two test-tile
firings should establish this total vitrification point, if Laguna can't
tell you (let me know if you need instructions on how to determine
porosity).
As for expansion numbers; try to get hold of the major oxide analysis for
the porcelain and enter it into a glaze calc program. This is a good start
point as the actual expansion of totally vitrified porcelain approaches the
calculated summed oxide expansions (this is not so true for
stoneware -which is less glassy when mature). Then, enter some clear glaze
compositions and look for ones which have higher expansions than the body.
Select several glazes, label (according to relative expansion) and fire on
test pieces. The pieces should have some tightly curved, outside radius
sections (similar to a rounded rim on a coffee mug) to initiate shivering if
possible. Subject these to several freezing/boiling cycles. Examine for
shivering or crazing. Select a composition between the two extremes.
Generally, there are no hard and fast rules to the actual expansion numbers,
even if they provided by the maker. The magnitude of the differences in
expansion between body and glaze depends on the elasticity and tensile
strength of the glaze and body. And the end use of the piece. But ideally
the glaze should have a slightly higher expansion than the body, especially
ware destined to stand hot liquids, oven or outdoor use. This puts the
glaze/body interface under compression. The degree of compression depends
on the size of the expansion contrast. How much compression you can get
away with depends on the strength of the glaze or (if the glaze is thick and
strong), the body strength.
Working with vitrified porcelain, it is easier to get reasonable glaze fit
than with other types of body because the glassy body melds more completely
with the glaze across the boundary layer. Glazes under tension (the
opposite of compression) on porcelain, even quite a lot of tension contrast,
can withstand crazing because of this strong boundary layer. But if I were
you and wished for longevity of your work, I'd try for a bit of compression.
Ware can develop crazing after many years, as very old toilet bowls testify
(I think this is due to slight under-vitrification).
Michael Banks,
Nelson,
NZ
Diane Schanz wrote:
> ----------------------------Original message----------------------------
> Hey all,
> I've got 2 questions regarding glaze fit.
>
> 1. I am in the process of learning glaze chemistry and I have a question
> that I haven't found an answer to. When trying to find a good glaze fit,
how
> close should the expansion numbers for clay body and glaze be? How close
is
> close enough?
>
> 2. I am trying to find the best fit for Laguna Aspen porcelain, ^5.
> Unfortunately, John Baccini (sp?) from Laguna tells me that they don't run
> thermal expansion tests on their clay bodies !??!!??!!! He suggested that
I
> just buy a bunch of glazes and see what works. Not what I expected to
hear.
> Does anyone out there have an expansion number for this clay? Or a
> spectacularly clear gloss that has been tested by time on this clay body?
I
> am already testing several recipes, but with a deadline that was just
moved
> up by a month, I don't have all the free time I'd like to solve this
problem.
> (BTW, this is an outdoor mural, exposed to temps of 130 in the summer and
> below freezing in the winter.)
>
> If you have an answer, could you also post a copy to me privately?
> TIA, Diane in Tucson
>
Paul Taylor on sun 23 apr 00
Dear Dian
Because potters in general do not like arithmetic or chemistry they
are reluctant to use chemical analyses. To encourage people to use the
chemistry or glaze analysis programs those that do uses glaze formulas as
the basis for controlling their glazes and try to encourage others as to the
superiority of it as a methodology. However we have obviously given the
impression that it is such a superior method it will do for every occasion.
The unfortunate truth is that, it is only another tool in the kit. The
most useful, but not an essential. For a start it difficult to use unless
you have an idea of what the analyses of your materials are. If you got the
material dug locally. You can only guess at the analyses unless you get it
analyzed fortunately some minerals are nearly standard. Also some recipes
are a bit vage. Recipes that you use to take make formulas from are a bit
difficult to analyze correctly. Many recommend ball clay with out telling
you what sort. Ball clays can have a difference of 20% in their silica
amounts.
The same is with thermal expansion figures; it is not that to work out
the thermal expansion of a fully matured glaze is impossible but the
difficulty is with the body. Your manufacturer porcelains thermal expansion
will chang very little but a stoneware clays expansion rate can change a lot
with a different temperature and the rate of clime and the atmosphere it
is fired in.
Glaze fit at cone five is not easy because two of the fluxes you are
using are at opposite end of the expansion chart so a change in their
relative amounts can make a big difference. I am not surprised manufactures
are reluctant to supply expansion figures at this temperature because the
firing range of clays that vitrify at this temperature is smaller than say
cone 8 or 9 which gives a greater range of thermal expansion over the
temperatures that the wayward potter might use it at.
A good glaze will be under compression not tension . A glaze can
take ten times the compression as it does tension ie it is more likely to
craze than shiver that is why crazing is so common and shivering so rare.
If he is any good ,the manufacturer will supply you with a clear
glaze that will not craze on your porcelain unless you fire it much below
1225 centigrade or above 1290 centigrade the most expensive porcelains have
a slightly better range.
The reason potters make their own glazes is because we need glazes that
are different to industry but a clear glaze is the only one we want the
same. I would doubt if I can improve on an industrial glaze for Quality but
I can on price. In your post you say that their is a time problem. So get
the clay supplier to supply you with enough glaze that he knows fits your
porcelain. I have nearly thirty years experience making glazes but if I had
to fire a strange clay with a clear glaze I would buy an industrial standard
glaze from the same supplier and sleep without worry. If something went
wrong I would
also have someone to shout at as well as the customer shouting at me.
As to thermal expansion I think the glaze calculation programs give you a
figures. I am not quite sure of the scale they use . I decided that thermal
expansion was too much arithmetic and I reckon they would only confirmed
what experience told me . Also some glazes are more elastic than others.
When I started to study which ones the variable were so mind numbing that I
decided not to bother . And the same for me with thermal expansion because
the only glaze it seemed to work for was a clear glaze. Mat glazes seem
better behaved as long as they are not over fired.
I use published body expansions rates as a rough guide not for their
accuracy but to give me an idea of the relationship of one body to the next.
I know what glaze fits on my stone ware so if for some reason I use another
clay I can make an intelligent guess as to the adjustment to make to the
glaze.
I always supposed my figures to be linear and a percentage. If you
look at the figures I use, most bodies have an expansion from0. 27 raku
and .354 for bone china . However most porcelains are some ware near the
average 0.308. Stone ware about .328 at vitrification point. Now the rub is
that the vitrification point is decided by the manufacturer and he does not
give any other information apart from his reckoning on the vitrification
temperature. You get no indication of the rate of clime this point was
reached at.
The thermal expansion of the bodies I used is 0.328 my porcelain
will also take the glaze without crazing if I fire it to the same
temperature it has a published expansion of 0.308 but the glaze will craze
on a more expensive porcelain given the same thermal expansion but this
porcelain fires 5 degrees higher yet it's recommended minimum temperature
is the same. So for porcelain I substitute some corniest stone for feldspar
because I like the glaze to have a little compression . If I add !% iron to
my glaze it will some times craze on my stoneware so I also use a little
more cornish stone in the recipes but not as much as for the porcelain.
So to conclude Buy enough glaze for the tiles. You will need very
little and it will not go off in the bucket it will be a good thing to have
around for the future.
When you make your own glazes and calculate the thermal expansion of
your glaze make sure the figures is bellow the thermal expansion of the
glaze because any vagaries in the firing compression will forgive but
tension will not . Do not go less than 0.02. or you might get shivering
(That is a guess supported only by observation).
Putting the ware out of doors will also exasperate any crazing problems
another reason to have the glaze under slight compression. The danger comes
not from the heat but the cold according to Ron Roy who can probably give
you a more confident answer to the thermal ex question. He suggested I put
my Guan glaze in the freezer to help get the ink in it -that reminds me to
thank him and the others that replied to my post.
Please tell me if I am wrong. I wrote a reply not because I am a
glaze expert but to give you a hint of how to cope with not being one.
Regards Paul Taylor
----------
>From: Diane Schanz
>To: CLAYART@LSV.UKY.EDU
>Subject: How close is close enough? Glaze fit, clear gloss for Aspen
Porcelain
>Date: Fri, Apr 21, 2000, 8:13 pm
>
>----------------------------Original message----------------------------
>Hey all,
>I've got 2 questions regarding glaze fit.
>
>1. I am in the process of learning glaze chemistry and I have a question
>that I haven't found an answer to. When trying to find a good glaze fit, how
>close should the expansion numbers for clay body and glaze be? How close is
>close enough?
>
>2. I am trying to find the best fit for Laguna Aspen porcelain, ^5.
>Unfortunately, John Baccini (sp?) from Laguna tells me that they don't run
>thermal expansion tests on their clay bodies !??!!??!!! He suggested that I
>just buy a bunch of glazes and see what works. Not what I expected to hear.
>Does anyone out there have an expansion number for this clay? Or a
>spectacularly clear gloss that has been tested by time on this clay body? I
>am already testing several recipes, but with a deadline that was just moved
>up by a month, I don't have all the free time I'd like to solve this problem.
> (BTW, this is an outdoor mural, exposed to temps of 130 in the summer and
>below freezing in the winter.)
>
>If you have an answer, could you also post a copy to me privately?
>TIA, Diane in Tucson
Ron Roy on tue 25 apr 00
A couple of comments about calculation of glazes and clay.
Calculated expansion of re-crystallized glazes (and clay) is not reliable.
Calculation of expansion is most accurate when applied to shiny glazes.
I calculate the expansion of both glazes and clays but the numbers never
give any true indication of compatibility between the two. Clays always
come out way lower than glazes.
On top of that, calculation cannot take into account either free quartz or
cristobalite in a clay body and both have a profound effect on
expansion/contraction.
I have found it more difficult to fit glazes to porcelain than stoneware -
Glazes on porcelain demand lower expansion rates than on stoneware - again,
- partly because of the influence of free quartz and cristobalite. I have
never measured a porcelain body that did not have some free quartz (I'm not
saying there is no such thing) but the amount is always less than in
stoneware.
Free quartz in a body helps to avoid crazing because - at 573C the free
quartz in a clay body gets smaller - affecting the clay and making it get
smaller - this helps to put the glaze into compression and sometimes enough
to avoid crazing of the glaze. That is why - one of the cures for crazing
is adding silica to a body.
If crazing is to be avoided - a series of glazes should be tried until you
have a shiny glaze that crazes and one that does not - it is possible then
to find the number (calculated expansion) and use it to construct other
glazes that will not craze on that body.
There is a faster way - I can measure the expansion of your clay - compare
it to the hundreds of other bodies I have measured and get close to the
right calculated expansion for glazes that will fit that clay. It is then
simply a matter of testing to see which will be truly craze free.
RR
>----------------------------Original message----------------------------
>As for expansion numbers; try to get hold of the major oxide analysis for
>the porcelain and enter it into a glaze calc program. This is a good start
>point as the actual expansion of totally vitrified porcelain approaches the
>calculated summed oxide expansions (this is not so true for
>stoneware -which is less glassy when mature). Then, enter some clear glaze
>compositions and look for ones which have higher expansions than the body.
>Select several glazes, label (according to relative expansion) and fire on
>test pieces. The pieces should have some tightly curved, outside radius
>sections (similar to a rounded rim on a coffee mug) to initiate shivering if
>possible. Subject these to several freezing/boiling cycles. Examine for
>shivering or crazing. Select a composition between the two extremes.
>Working with vitrified porcelain, it is easier to get reasonable glaze fit
>than with other types of body because the glassy body melds more completely
>with the glaze across the boundary layer. Glazes under tension (the
>opposite of compression) on porcelain, even quite a lot of tension contrast,
>can withstand crazing because of this strong boundary layer. But if I were
>you and wished for longevity of your work, I'd try for a bit of compression.
>Ware can develop crazing after many years, as very old toilet bowls testify
>(I think this is due to slight under-vitrification).
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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