Tom Buck on fri 9 jan 98
Ouch & Groan! I feel like a one-ball juggler who confused up & down
and so dropped the ball.
My picture of what was going on between claybody and glaze
was reasonable, and the numbers in question were ballpark for the
glaze being talked about. But instead of seeing the glaze in tension,
I saw glaze in compression; hence, shivering instead of crazing.
Let's try a new picture. Imagine a test tile a million
millimetres long (ie, 1 kilometre in length). The tile has been
bisqued, and coated with glaze. But this time the glaze is afixed
only to one end and otherwise "floats" atop of the body. The body
expands 5.5 x10-6 units of length per unit of length per degree
Celsius (Centigrade) or 3 x 10-6 L/Lxdeg F.
Now, recall that the glaze doesn't exist until the ingredients
melt, coalesce, and combine into the macro- (huge) molecules, like
extremely tiny pieces of string, and which aggregate to form glass. So
there is no glaze expansion during the rise to fusion temperature, and no
contraction until the glaze solidifies. Let's say the glaze goes solid at
1020 C (1840 F) and we will cool it to 20 C (70F). And also note that the
solid glaze contracts at 8 x10-6 L/Lxdeg C (4.4 x 10-6 L/LxdegF).
Let's say that both body and glaze start to shrink at 1020 C
and do so uniformly til room temperature, 20 C, is reached.
The body (a 1 km tile) contracts 1,000,000 x 0.0000055 x 1000
or 5.5 x 1000 millimetres, 5500 mm, or 5.5 metres.
The glaze contracts 1,000,000 x .0.000 008 x 1000 or 8 x 1000 mm,
ie, 8.0 metres.
So, at one end of this imaginary tile there would be exposed an
unglazed length of 2.5 metres (8-5.5), a quite noticeable gap.
The same shrinkage occurs on a fired-pot as it cools. But because
the glaze melts on, and afixes to, the surface of the pot everywhere it
has been placed, the difference in shrinkage between body and glaze sets
up large stresses (forces that will break physical, chemical bonds). In
turn, these stresses will disrupt the continuity of the glass "strings"
at irregularities in the body/glaze interface (boundary layer). The glaze,
shrinking more, will crack and open up at these irregularities. We say the
glaze "has crazed".
A similar analogy would explain what happens when the glaze
contracts more than the body. Then the glaze bulges up from the boundary
layer, and pieces of the glaze crack off. We say the glaze "has shivered".
My apologies to all for getting it backwards in my earlier post. I
hope I got it right this time. Til later. Tom.
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).
CAROLRATLI on sat 10 jan 98
Ok guys....what if you have a bucket of glaze that is acting like what you
have been discussing .....shivering, cracking off, etc.........in general, is
there something you can add to fix it or does it really depend on the
individual glaze that much? remember, I said, in general. I am just looking
for a "first try rule of thumb" to stash away in my memory banks, and a way to
keep from dumping a gallon of glaze.
carol ratliff
san diego
Craig Martell on mon 12 jan 98
At 06:28 PM 1/10/98 EST, Carol Ratliff wrote:
>----------------------------Original message----------------------------
>Ok guys....what if you have a bucket of glaze that is acting like what you
>have been discussing .....shivering, cracking off, etc.........in general, is
>there something you can add to fix it or does it really depend on the
>individual glaze that much?
Hello Carol:
If you are talking about a post firing problem such as shivering, the
problem is caused by the clay and the glaze having radically different rates
of expansion. Shivering will occur when the glaze is expanding and
contracting less than the pot it's on. When the glaze expands more, crazing
occurs from too much tension. When the glaze is under slight compression,
you get optimum glaze fit. Too much compression causes the glaze to shiver.
The fault can lie in either the clay or the glaze. One needs to look at both
to determine where the appropriate adjustments need to be made. But, lets
say that your clay is well balanced and not developing cristobalite, which
can cause shivering, so we are going to work on the glaze to make them
compatible. Look at the glazes Seger formula and the expansion. See if the
oxides are within, or close to appropriate limits and see how the silica and
alumina stack up against the fluxes. Is there enough silica and alumina?
If the glaze shivers, you need to raise the coefficient of expansion. This
can be done by adding high expansion oxides to the glaze or lowering low
expansion oxides. If you have Hamer's dictionary, this is dealt with under
"glaze fit", "shivering", "crazing", and other headings relating to these
types of faults. I would mix a series of 200 gram tests in search of the
correct expansion coefficient. I would also calculate formulas as I go along
to be sure that I'm making balanced glazes. Once you have found the optimum
blend that is aesthetically what you want, and fits the clay, you can adjust
the already mixed glaze. Use Brogniart's formula to determine the amount of
raw materials in the glaze suspension and you can then weigh out and add the
oxides that will cure the problem. There is a thorough explanation of
Brogniart's formula and how to use it in Hamer's dictionary. If you don't
want to dink around with this, just totally dry the glaze and weigh what you
have.
This is probably more than you wanted but there really isn't a quick, easy
answer to these problems. It takes time and effort but when you're
successful in curing the problem, you have a glaze that works and you've
learned a lot.
regards, Craig Martell-Oregon
Ron Roy on mon 12 jan 98
First thing to always remember - it is always the glaze and the clay
together that make fit problems - changing to another clay with a lower
expansion/contraction might be the solution for this glaze - remember - it
will probably make some of your other glazes craze though.
Another option would be to run a line blend with a crazing glaze at one end
and the shivering glaze at the other. If the crazing stopped at 20% of the
crazing glaze and 80% of the shivering glaze it would be a simple matter to
calculate the recipe for the new glaze.
The third option is to recalculate the glaze - substituting high expansion
fluxes for low and/or taking out some alumina and silica. This last option
may scare the pants off some of you - but you should think about it - I'm
no genius - had a hard time getting out of high school - and I can do it.
>----------------------------Original message----------------------------
>Ok guys....what if you have a bucket of glaze that is acting like what you
>have been discussing .....shivering, cracking off, etc.........in general, is
>there something you can add to fix it or does it really depend on the
>individual glaze that much? remember, I said, in general. I am just looking
>for a "first try rule of thumb" to stash away in my memory banks, and a way to
>keep from dumping a gallon of glaze.
>carol ratliff
>san diego
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Kathynina on mon 26 jan 98
Carol R. regards to recycling glazes. 2 suggestions look up specific glaze
faults in a reserch book and add raw materials to Fix it. Or just add clear
glaze you can start with a test batch of 20% clear up to 50 %. I' recycled
glazes for years , remember you end up with something that most likley can't be
reproduced, Joy In Tucson
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