Michele Hoskin on wed 26 may 99
Hi everyone.
I have been playing with colouring my clear glaze with oxides and have been
pleased with the results except that I still get small bare patches of clay
coming through the glaze. I occasionally get this with my clear too but it
is not very noticeable so I have been sticking with clear over underglaze
colours. I am really wanting to use coloured glazes and am at a loss as to
why this is happening to me. My hands are clean as is the bisque and the
glaze is a great fit.
Anyone else had this problem? Any suggestions?
Thanks,
Michele Hoskin
Toronto
Erin Hayes on fri 28 may 99
Hi Michele in Toronto!
It might be crawling, especially if your underglazes are on very thick, or
if there's a lot of powdery residue under the glaze. Most times when a
glaze crawls, at least in my experience, the edges surrounding the dry spot
are rounded off.
It could be that your glaze is too thin, and it's "starving." Usually if
there's not enough glaze there is a color change, but not a full enough coat
of glaze to create the shine.
Those are my suggestions. I imagine someone better informed will have
others to think about.
(Go Maple Leafs!)
Erin. (Hoping for a Dallas Stars/Toronto Maple Leafs match-up for the
Stanley Cup.)
Janet H Walker on sat 29 may 99
... I have been playing with colouring my clear glaze with
oxides and have been pleased with the results except that I still
get small bare patches of clay coming through the glaze...
When you say:
...I occasionally get this with my clear too...
do you mean with the clear all by itself on the clay? That might
make a difference. If the clear all by itself has the teeniest
tendency to crawl then that would probably be increased by oxides
(which are somewhat refractory) and by underglaze stains, which can
be somewhat "dusty" in surface on their own.
Find a good diagnostic book (my favorite is "Ceramic Faults and
Their Remedies" by Harry Fraser) and think through all the things
they suggest as possible causes and fixes for crawling. Geez. I
just picked up my copy of Hamer and it fell open at crawling. I've
been meaning to take a look because I'm having some teeny glaze
flaws over some stains that look just like the beginnings of a
crawl. Happens only with my clear not with the white so I've been
wondering.
"Glazes which are prone to crawling are those which have a high
surface tension and are viscous when molten. A high alumina content
makes a glaze viscous.... The supplementary causes which provide
starting points for the areas of crawling must be avoided. See
surface tension..." Hamer
Also "Borocalcite glazes lack sufficent fluxes of the low surface
tension type." The frit 3124 is a borocalcite frit I believe. This
is probably a tough possibility to pursue.
Aha. How about this: "A pre-firing crack [the fine shrinkage lines
in the dried glaze coat] is the commonest cause of crawling."
"Sometimes a glaze begins to crawl when it has previously been
satisfactory. Firing of wet ware should first be considered as
the cause. the second possibility is that a thicker layer of
glaze is being used. Alternatively the bisque ware may be
slightly underfired. this gives a more absorbant bisque which
no only takes a thicker layer of glaze but dries it too quickly
to allow it to become compact... A third possible cause is that
the bisque has become dusty or has been overhandled and become
greasy."
So you see, the cause that is most often cited at the classroom
level (greasy, dusty) is only third in Hamer's list and only in the
paragraph that talks about glazes that have previously been
satisfactory. Probably because in a classroom, no one has precise
enough control over glaze consistency or firing to even bother with
that as a cause.
So, there you are. Straight from the Hamers' mouth. Have fun
speculating and investigating.
Jan (channeling for Hamer) Walker
Cambridge MA USA
Dwiggins, Sandra (NCI) on wed 2 jun 99
Janet--
Thanks for your "channelling" efforts! Those of us who are Hamer-deprived
appreciate your efforts, especially since lots of glazes that usually didn't
crawl are now crawling in my community studio. And, since I thoroughly wipe
each piece to make sure there is no dust or whatever, I had no explanation for
what was happening. The bisque being underfired is a really good explanation.
Our new kilns have been acting strangely...and that is very likely the cause of
the glaze problems.
Sandy
-----Original Message-----
From: Janet H Walker [SMTP:jwalker@world.std.com]
Sent: Saturday, May 29, 1999 11:58 AM
To: CLAYART@LSV.UKY.EDU
Subject: glaze flaws
----------------------------Original message----------------------------
... I have been playing with colouring my clear glaze with
oxides and have been pleased with the results except that I still
get small bare patches of clay coming through the glaze...
When you say:
...I occasionally get this with my clear too...
do you mean with the clear all by itself on the clay? That might
make a difference. If the clear all by itself has the teeniest
tendency to crawl then that would probably be increased by oxides
(which are somewhat refractory) and by underglaze stains, which can
be somewhat "dusty" in surface on their own.
Find a good diagnostic book (my favorite is "Ceramic Faults and
Their Remedies" by Harry Fraser) and think through all the things
they suggest as possible causes and fixes for crawling. Geez. I
just picked up my copy of Hamer and it fell open at crawling. I've
been meaning to take a look because I'm having some teeny glaze
flaws over some stains that look just like the beginnings of a
crawl. Happens only with my clear not with the white so I've been
wondering.
"Glazes which are prone to crawling are those which have a high
surface tension and are viscous when molten. A high alumina content
makes a glaze viscous.... The supplementary causes which provide
starting points for the areas of crawling must be avoided. See
surface tension..." Hamer
Also "Borocalcite glazes lack sufficent fluxes of the low surface
tension type." The frit 3124 is a borocalcite frit I believe. This
is probably a tough possibility to pursue.
Aha. How about this: "A pre-firing crack [the fine shrinkage lines
in the dried glaze coat] is the commonest cause of crawling."
"Sometimes a glaze begins to crawl when it has previously been
satisfactory. Firing of wet ware should first be considered as
the cause. the second possibility is that a thicker layer of
glaze is being used. Alternatively the bisque ware may be
slightly underfired. this gives a more absorbant bisque which
no only takes a thicker layer of glaze but dries it too quickly
to allow it to become compact... A third possible cause is that
the bisque has become dusty or has been overhandled and become
greasy."
So you see, the cause that is most often cited at the classroom
level (greasy, dusty) is only third in Hamer's list and only in the
paragraph that talks about glazes that have previously been
satisfactory. Probably because in a classroom, no one has precise
enough control over glaze consistency or firing to even bother with
that as a cause.
So, there you are. Straight from the Hamers' mouth. Have fun
speculating and investigating.
Jan (channeling for Hamer) Walker
Cambridge MA USA
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