heavy glaze questions

updated wed 2 jun 04

Paul Raymond on wed 26 may 04

Okay folks, I am going to attempt to ask some questions about a glaze I
am beginning to work with in the theoretical. That is to say, only on a
computer. This is preliminary research to determine what will probably
be the best route to go once I begin to mix and fire.

I got the recipe from Richard Zakin's book, Electric Kiln Ceramics. It
is as follows:

Transparent Mat Glaze
Boron frit 30
Wollastonite 25
Ball clay 40
Dolomite 5

It is described as a mat to satin mat, clear glaze with excellent
durability.

Using Glazemaster I am working with 2 versions. The first has Ferro
3195, the second has 3134. I am using OM#4 in each for the clay.

The biggest differences in the fluxes are with Na, with the glaze
containing 3134 having .126 and 3195 having .094
Mg is .092 with 3134, .113 with 3195. Ca is .772 and .781 respectively
while K is .010 and .012.

The Al in 3134 is .271 while 3195 is .422
Si is 1.938 with 3134 and 2.323 with 3195
Si:Al, 7.15 with 3134 and 5.5 with 3195
Ex Coef is 75.38 with 3134 and 68.37 with 3195

In talking about both glazes I can see that the Si is low in both while
the Al is with acceptable ranges for both. The Si:Al is good as well.
However, when looking more closely I see that the 3195 version with
2.323 Si is pretty close to the lower limits of glass in a glaze and it
is paired with an Al amount of .422 which is well within a good range.
It's ratio of 5.5 puts into the matte side of the glaze texture
spectrum. So, now looking back to the Ca and Mg, I see that the Mg is
quite low for a matte but Ca is just a bit shy. Probably this will
result in something more semi-matte rather than matte.

The version with 3134 has only 1.938 Si and .271 Al, both well below
established norms for durability. Although the Na and Mg levels are
close to being within range the low amount of Si and Al show this glaze
to be of questionable quality.

With that being said, am I right or at least on the right track? If I do
some experimentation with the type of clay I use I could probably alter
the amount of Si to bump it up just a bit to get to 2.5 or perhaps I
should try adding some Si to the original 4 ingredients.

I am posting this as a way to test myself against those who really know
something about glazes. I feel confident in my logic but am very
interested in what you have to say.
Thanks.


Paul Raymond
Franklin, Tennessee
crazyrays@bellsouth.net

Paul Lewing on thu 27 may 04

on 5/26/04 7:03 PM, Paul Raymond at crazyrays@BELLSOUTH.NET wrote:

> With that being said, am I right or at least on the right track? If I do
> some experimentation with the type of clay I use I could probably alter
> the amount of Si to bump it up just a bit to get to 2.5 or perhaps I
> should try adding some Si to the original 4 ingredients.

Paul, I think you're pretty much exactly on the right track here all the way
along. I'd expect both of those glazes to be matte, just because the silica
is so low, and also because the calcium is so high. And I think you're
right that adding a little silica would not be a bad idea for both gloss and
durability.
Good luck, Paul Lewing, Seattle

John Hesselberth on thu 27 may 04

On Wednesday, May 26, 2004, at 11:03 PM, Paul Raymond wrote:

> ith that being said, am I right or at least on the right track? If I do
> some experimentation with the type of clay I use I could probably alter
> the amount of Si to bump it up just a bit to get to 2.5 or perhaps I
> should try adding some Si to the original 4 ingredients.

Hi Paul,

I'm glad to see you are really digging in and trying to understand. A
little background may help. For a number of years the "standard" way to
make matte glazes, particularly at cone 6, has been to lower the silica
level until the Si/Al ratio got down to about 5 or maybe less. This
will indeed give a matte glaze but we now know it is not usually a
durable matte glaze. Oh, it may be durable enough with no or very low
levels of colorants; however it simply will not be with higher levels,
particularly if the colorant is copper. This was not understood by
people working on mid-fire glazes in the 1960s- through about 2000.
Either that or their standards of durability were very much lower than
mine.

The reason matte glaze development moved in that direction was that no
effort was made to control cooling rate in their electric kilns and
electric kilns have usually been very poorly insulated and they cool
very rapidly if just turned off. Low Si/Al levels is the most reliable
way to make a matte glaze with rapid cooling. In reality it is matte
because it was never thoroughly melted in the first place. All that
undissolved or unmelted alumina (clay) is what provides the matte
appearance.

What we proved in the early part of this century is that glazes made
per the above paragraph are, indeed, unstable in many/most situations.
The analytical tools are now available to measure things inexpensively
which had never been measured before. But we also found that really
high levels of calcium (or other alkaline earths) could also give matte
surfaces without the deterioration of durability seen with low Si/Al
glazes IF they were cooled slowly and the crystals were given time to
grow. You still have to work with relatively low Si/Al levels, but not
all the way down to 4 and 5 like Zakin and others used--6-7 will do the
job and you might be able to do it even a little higher.

So you will find that nearly all books published before 2000 (and a few
after) will be filled with recipes that show matte glazes with low
Si/Al and the glazes will not be durable in spite of what they may
claim! That's just the way our craft evolved. So I wouldn't try to
adjust those glazes myself. They are usually not good starting places
if you want durable matte glazes. Rather start with a Si/Al ratio of
6-7 with silica above 2.5, preferably 3.0 and the alumina level above
0.3. Have enough boron or zinc to assure good melting. Then start
increasing the alkaline earth levels, combining that will slow cooling,
until you start to get the level of matteness you want and like. You
will see occasional high calcium (or more frequently high barium or
magnesium) mattes in the earlier literature, but what was not
recognized is that this was really a superior way to make a matte glaze
if you want a durable glaze.

I know I didn't answer your question directly, but I hope this
long-winded explanation helps.

Regards,

John

John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com

Ivor and Olive Lewis on fri 28 may 04

Dear Paul Raymond,
There are times when I wonder what the Title of a Glaze means, and
this is one of them.
Here we have a Transparent Mat Glaze.
To me, Mat tells me that the glaze has no specular reflections, that
any white light from any direction relative to my viewing point is
reflected to me as a white or slightly grey surface causing the mass
of the glaze to seem opaque.
Similarly, Transparent tells me that any light coming from the glaze
from any direction allows me to see the surface of the clay/glaze
interface, so that underglaze decoration would be clearly visible.
Now a simple analogy would be the surface of a Pond. Without a ripple
you can see clear to the bottom. With a wind the ripples obscure your
vision of the fish causing me to ask, what is your ambition in
relation to the Richard Zakin's Recipe. Your stated objective is to
test your abilities as a logician. My suggestion if you are wishing to
invent a Transparent Mat Glaze is that you return to first principles.
Whose? Your own, those of Ron and John, those of Rhodes or is it now
Hopper-Rhodes. Define your needs and use their Canons.
Apocryphal Tale. Last night I was asked to make a light supper. The
request was for Malibu Bananas. The usual recipe calls for a basting
of equal parts Malibu Liqure and Maple Syrup, Canadian of course!
Knowing Olive's "Less that sweet tooth" I altered the proportion to
give 3 parts liqure and 1 part Maple Syrup. The change was
appreciated.
Moral. The only way to know is to test (taste)
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
Ps. I refrained from commenting about the maturing temperature. No
doubt some other perceptive person will remind you about posting this
important detail.
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.

Ivor and Olive Lewis on fri 28 may 04

Dear John,
I note you deal with creating a mat surface. But as I recall, the
original post said the Zakin Glaze was a Transparent Mat.
Does this qualification, insisting on transparency, change in any way
your advice about efficient melting, SiO2/Al2O3 ratios and cooling
rates?
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member

John Hesselberth on fri 28 may 04

Hello Ivor,

I probably should have said a matte glaze instead of matte surface. But
I have never seen a true transparent matte. Translucent, yes, in a
satin matte. I am of the opinion that "transparent matte" is an
oxymoron. Have you seen one?

If what is wanted is a translucent matte then, no, it does not change
my advice.

Regards,

John
On Friday, May 28, 2004, at 01:32 AM, Ivor and Olive Lewis wrote:

> Dear John,
> I note you deal with creating a mat surface. But as I recall, the
> original post said the Zakin Glaze was a Transparent Mat.
> Does this qualification, insisting on transparency, change in any way
> your advice about efficient melting, SiO2/Al2O3 ratios and cooling
> rates?

John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com

Joyce Lee on fri 28 may 04

ok
----- Original Message -----
From: "John Hesselberth"
To:
Sent: Friday, May 28, 2004 3:58 AM
Subject: Re: Heavy glaze questions


> Hello Ivor,
>
> I probably should have said a matte glaze instead of matte surface. But
> I have never seen a true transparent matte. Translucent, yes, in a
> satin matte. I am of the opinion that "transparent matte" is an
> oxymoron. Have you seen one?
>
> If what is wanted is a translucent matte then, no, it does not change
> my advice.
>
> Regards,
>
> John
> On Friday, May 28, 2004, at 01:32 AM, Ivor and Olive Lewis wrote:
>
> > Dear John,
> > I note you deal with creating a mat surface. But as I recall, the
> > original post said the Zakin Glaze was a Transparent Mat.
> > Does this qualification, insisting on transparency, change in any way
> > your advice about efficient melting, SiO2/Al2O3 ratios and cooling
> > rates?
>
> John Hesselberth
> http://www.frogpondpottery.com
> http://www.masteringglazes.com
>
>
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> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>

Paul Raymond on fri 28 may 04

<if you want durable matte glazes. Rather start with a Si/Al ratio of
6-7 with silica above 2.5, preferably 3.0 and the alumina level above
0.3. Have enough boron or zinc to assure good melting. Then start
increasing the alkaline earth levels, combining that will slow cooling,
until you start to get the level of matteness you want and like.>>

So, John,

Does this mean to start with numbers and work towards ingredients? By
that I mean should I fiddle around with Glazemaster trying this clay
with that frit, etc., until I get into the parameters that you suggest
above. If that is the case I think I would start with 3195, some silica,
some clay, perhaps a bit of barium and/or magnesium. Then for stability
I would need some dolomite or wollastonite. What do you think?

Also, what is the downside of starting with that Zakin recipe and
altering it until I get the above numbers. The version with 3195 is
almost there already so why start over? Just curious about your opinion
on this. Thanks again

Paul Raymond
Franklin, Tennessee
crazyrays@bellsouth.net

Ivor and Olive Lewis on sat 29 may 04

Dear John,
Fair comment with which I agree. I almost wrote Oxymoron in my reply
to the original post.
<But I have never seen a true transparent matte. Translucent, yes, in a
satin matte. I am of the opinion that "transparent matte" is an
oxymoron. Have you seen one? >>
No John, I have never seen a Transparent Mat. always found that if you
were developing a mat then translucency came hand in hand and a good
mat was usually almost opaque.
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member