Kathi LeSueur on tue 20 feb 07
There have been many posts in recent years concerning surface problems
in fired glazes. The bulk of these seem to be pitting problems with
glazes using titanium or rutile, especially on iron bearing clays. I,
myself, have such a glaze that I've used for over thirty years. About
tne years ago it started having problems. Nothing I tried worked except
to use it on white stoneware.
It occurs to me that there is some impurity that wasn't present in the
past in the titanium and rutile. And, that this impurtiy burns out
during the glaze firing. So, the question is: would calcining the
titanium or rutile before adding it to the glaze possible solve the problem?
Thanks,
Kathi
Ben Shelton on wed 21 feb 07
Great question!
I was having this discussion last night with 2 potter friends. They each use
the same rutile based glaze and have each had various amounts of
pinholing/blistering recently. I too have a different rutile glaze that has
the same problem. It happens less so on white clays but still sometimes
pinholes. I remember a discussion on clayart in about '98 or '99 that
suggested a contaminant in the rutile.
My question is, if there is a contaminant in the rutile that is outgassing
at cone 10, how high will we have to calcine the rutile to get rid of the
offending contaminant?
Also,
could ball milling distribute the contaminant so that it would ne be such a
problem?
Thnks for the question Kathi.
Ben
Richard Aerni on wed 21 feb 07
Kathi,
Well, I'm not Ron Roy, but I guess if you had wanted this for him alone you
would have emailed him directly. So...my limited response.
Rutile, as I'm sure you know, has a large amount of small impurities. I
have collected numerous analyses of the material over the years, which all
pretty much show the same chemicals in the same proportions, but which
provide markedly different test results in my studio. So, it will be hard
to prove what is causing your problems. But, as you also surely know,
rutile in amounts over 5 or 6 percent in glazes makes them prone to
pinholing and other problems.
I did an awful lot of research into the various rutiles available to potters
when I was having problems, which would have been eight or ten years ago. I
published a number of posts about my travails and discoveries on clayart,
which should be in the archives.
A salient point...the rutile coming out of what used to be TAM in Niagara
Falls, NY (they've been bought a couple of times now, don't know their
current name), and which is principally used in welding rods, but also
supplies a significant portion of the potting world in North America, goes
by two names...dark rutile and ceramic rutile. They are both exactly the
same material. The only difference is that the ceramic rutile has been
calcined. I'm not sure how high the calcining temp is...the engineer I
talked to mentioned 1000 degrees Centigrade.
So, hope this helps in some small way. If you wish to read all of my
earlier writings, and can't find them, let me know and I'll send them to me,
or if you've patience, when my website goes up (hopefully in the near
future) those writings and others of similar ilk will be posted there.
Good luck,
Richard Aerni
Rochester, NY
On Tue, 20 Feb 2007 23:06:15 -0500, Kathi LeSueur
wrote:
>There have been many posts in recent years concerning surface problems
>in fired glazes. The bulk of these seem to be pitting problems with
>glazes using titanium or rutile, especially on iron bearing clays. I,
>myself, have such a glaze that I've used for over thirty years. About
>tne years ago it started having problems. Nothing I tried worked except
>to use it on white stoneware.
>
>It occurs to me that there is some impurity that wasn't present in the
>past in the titanium and rutile. And, that this impurtiy burns out
>during the glaze firing. So, the question is: would calcining the
>titanium or rutile before adding it to the glaze possible solve the problem?
>
>Thanks,
>Kathi
>
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Ron Roy on thu 22 feb 07
Hi Kathi,
That should be easy to prove - just fire some side by side tests with both
kinds of rutile and you will see.
It could also be that the problem is really the high iron clays - they must
be fired in such a way as to burn any carbon and not reduce the iron. Which
means slow enough and with plenty of oxygen present to support complete
combustion.
I could expand on this a bit better if I knew what temperature you were
firing to and if you are reducing.
I can also check out your glaze to see if it is contributing to the problem
but I need to see the recipe.
It may also be true that the dark clays are overfired - it is quite common.
Tom Wirt has been testing some of my rutile blue revisions and is having
problems with them over one of the clay bodies he is using.
Have you tested the clays you use to see how vitrified they are? The fact
that it still works on the white clay is a clue here.
RR
>There have been many posts in recent years concerning surface problems
>in fired glazes. The bulk of these seem to be pitting problems with
>glazes using titanium or rutile, especially on iron bearing clays. I,
>myself, have such a glaze that I've used for over thirty years. About
>tne years ago it started having problems. Nothing I tried worked except
>to use it on white stoneware.
>
>It occurs to me that there is some impurity that wasn't present in the
>past in the titanium and rutile. And, that this impurtiy burns out
>during the glaze firing. So, the question is: would calcining the
>titanium or rutile before adding it to the glaze possible solve the problem?
>
>Thanks,
>Kathi
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ron Roy on thu 22 feb 07
Hi Ben,
Easy enough to test for in each case. If you don't have a pinholing glaze
to start with I'm sure there are many who can donate one.
Sounds like a good start of an article that many potters would be interested in.
RR
>Great question!
>I was having this discussion last night with 2 potter friends. They each use
>the same rutile based glaze and have each had various amounts of
>pinholing/blistering recently. I too have a different rutile glaze that has
>the same problem. It happens less so on white clays but still sometimes
>pinholes. I remember a discussion on clayart in about '98 or '99 that
>suggested a contaminant in the rutile.
>
>My question is, if there is a contaminant in the rutile that is outgassing
>at cone 10, how high will we have to calcine the rutile to get rid of the
>offending contaminant?
>
>Also,
>could ball milling distribute the contaminant so that it would ne be such a
>problem?
>
>
>Thnks for the question Kathi.
>
>Ben
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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