Lisa on mon 28 oct 96
In our teaching studio, we have 2 glazes that use the same base recipe.
The only difference between them is that one uses 2% copper carb and the
other uses 6%. Here is the recipe; it is ^6 Oxidation
Glossy Turquoise/Metallic Jade
Spodumene 50%
Gerstley Borate 25%
Silica 25%
Adds:
Tin 5%
Copper Carb 2%/6%
These glazes are absolutely gorgeous over the Kickwheel Gray Speck
claybody. The problem is that they shiver off like you would NOT
believe. At this very moment I have an 8# bowl sitting on the floor
behind me making those little "ping" noises that say, "Here comes some
more glaze off the rim..." The only thing we're doing different from
what Kickwheel suggests is that our pieces get bisqued to ^06 rather than
04.
Can any of y'all suggest a solution?
Tia
Craig Martell on tue 29 oct 96
This happened to me a long time ago at cone 6 ox., using a glaze that had a
high % of Lithium carb. Spodumene is a lithium bearing material. Lithium
has a VERY low coefficint of expansion. What is happening, is the clay is
shrinking more than the glaze (much more!). The glaze is being forced into a
state of super compression and cannot adhere to the piece. Your glaze is
quite high in spodumene, so a possible solution might be to lower the amount
of spodumene and replace it with a higher expansion flux such as soda spar,
potash spar, or nepheline syenite. The tradeoff here is that you will lose
some or all of the glazes character. You won't know this, of course without
doing some tests. I am suggesting changes to the glaze because you are using
a commercial claybody that the supplier is probably unwilling to change so
your glaze will fit. I see no reason that changing the bisque temperature
will help in any way. The problem does not develop until the clay and glaze
have matured fully in the glaze firing, and the glaze has fusion bonded to
the clay. So why would a bisque change make any difference?
I hope I've helped you some,
Craig Martell-Oregon
Karen Gringhuis on tue 29 oct 96
With 50% spodumene (a lithium feldspar) the glaze probably has too much Li
which can cause shivering. Try the glaze on every different clay body
you can get your hands on. If they all craze.........ditch or reformulate
the glaze.
Lisa on wed 30 oct 96
Craig Martell wrote:
>
> ----------------------------Original message----------------------------
> This happened to me a long time ago at cone 6 ox., using a glaze that had a
> high % of Lithium carb. Spodumene is a lithium bearing material. Lithium
> has a VERY low coefficint of expansion. What is happening, is the clay is
> shrinking more than the glaze (much more!). The glaze is being forced into a
> state of super compression and cannot adhere to the piece. Your glaze is
> quite high in spodumene, so a possible solution might be to lower the amount
> of spodumene and replace it with a higher expansion flux such as soda spar,
> potash spar, or nepheline syenite. The tradeoff here is that you will lose
> some or all of the glazes character. You won't know this, of course without
> doing some tests. I am suggesting changes to the glaze because you are using
> a commercial claybody that the supplier is probably unwilling to change so
> your glaze will fit. I see no reason that changing the bisque temperature
> will help in any way. The problem does not develop until the clay and glaze
> have matured fully in the glaze firing, and the glaze has fusion bonded to
> the clay. So why would a bisque change make any difference?
>
> I hope I've helped you some,
> Craig Martell-Oregon
Ok, so what I hear you saying is that I need to find out the chemical
composition of Spodumene and get those chemicals from some other source,
substituting the lithium for something else. Is this right? We'll try
it and see what happens. What you are describing is exactly what this
looks like - the glaze has what looks like wrinkles in it.
Thanks.
Karen Gringhuis on wed 30 oct 96
Craig is right - replace SOME of the spodumene w/ a soda spar (neph. sye)
(Kona). You will need to replace soem Al & Si you were getting from
all that spod. but this shouldn't be too diff.
If you work w/ a UMF spodumene's UMF is 1.0 Li, 1.16 Al 4.37 Si.
according to Cushing's Handbook. If you add more clay, be sure not to
take the EPK (wasn't it?) over about 20%; more than that split it between
EPK & calcined kaolin. (I always split even 20% clay half & half.)
Craig Martell on wed 30 oct 96
Lisa: The composition of spodumene (ideally) is: 1molecule of Lithium Ox. to
1 mol. alumina ox. to 4 mols. silicon dioxide. So, you can't really replace
the offending material lithium with the same stuff from another source. You
have to go to a higher expansion flux to make the glaze fit the claybody. I
would suggest that you try adding nepheline syenite in 5% increments until
the glaze stops shivering. Nepeline Syenite is the highest expansion
feldspar that we can obtain, and contains a high volume of sodium in its
composition. Sodium and lithium are said to be quite similar in color
response. Be aware that Neph Sy will begin melting at a lower temp than the
spodumene. Neph Sy is fairly close to spodumene in its unity formula in that
it contains 4.65 mols(ideally) to .75Na2O+.25K20 & 1Al203. I didn't mention
it before but Boric Oxide also has a low coefficient of expansion and is
probably contributing to the shivering as much as the spodumene. If my
memory serves, your glaze also contains Gerstley Borate? You might also try
replacing some or all of the Gerstley Borate with Neph Sy. Hopefully, you'll
get what you are seeking. Who knows, you might even come up with something
that you like better. In case you need this info, I will give you a list of
radical oxides in their order of expansion from the highest to the lowest.
They are as follows:1-sodium, 2-potassium, 3-calcium, 4-barium, 5-lead,
6-lithium, 7-zinc, 8-magnesium, 9-boric ox(boron). Also, when you reach the
point of proper glaze fit remember that shivering and crazing can be delayed
and will at times occur weeks and months after pieces are taken from the
kiln. It might be best to go a little beyond the adjustment point where your
glaze is fitting properly for a bit of added insurance. Anyway, good luck
and I hope I've helped.
Craig Martell-in temporarily sunny Oregon
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