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lowering the expansion on cone 5 clear

updated sat 29 nov 97


Lance Philip Strugar on tue 25 nov 97

I've been trying to develop a good clear glaze for a white
stoneware clay rated at cone 4 - cone 8. The manufacturer
is L=26R SPECIALTIES Inc in Nixa Missouri. I bisque to =5E06
and glaze to =5E5.

I started out with the famous IMC cone 6 clear=3B
20=25 each Wollastonite, Ferro Frit 3134, EPK, Flint, and
Custer Feldspar. After applying the glaze to 6 cm high
by 6 cm diameter cups and firing to cone 5, crazing was
quite pronounced. (I also fired some 5 cm tiles, but
they didn't craze, even using the freeze/boil test.)
The original glaze had an expansion of 7.14 according to
=22Insight5=22. I was able to get the crazing to go away only
after reducing the expansion to 5.84 using the following:
FRIT 3134.......... 18
EPK KAOLIN......... 20
FLINT.............. 26
SPODUMENE.......... 26

I don't understand how this worked at cone 5, because
the oxides seemed to be closer to cone 10, but the
glaze was quite even. (By the way, I reduced the
expansion by about .2 for each blend before arriving
at one that didn't craze.)

The trouble is, this glaze turns Mason stain 6006 to
gray. I applied some slip made from 3 parts
claybody (dry) and 1 part 6006.

I'm sure that I'm seeing the same problem Tony talked
about in Magic of Fire, pages 100 to 103. He says that
the CaO must be between 10 and 15 =25 by weight, and
the CaO:B2O3 ratio must be at least 3:1.

How can I keep the expansion down and the CaO up?
Wollastonite has a very high expansion, as does
3134. Are these mutually exclusive objectives?

Adding Flint will reduce the expansion, but how
much Flint can I add before driving the melt
temperature too high?

Tony, can you help out a frustrated engineer??
(electrical, not chemical)

Lance Philip Strugar

Tony Hansen on fri 28 nov 97

> How can I keep the expansion down and the CaO up?

The 5X20 glaze has a CaO:B2O3 ratio of 4:1 so there is room
to increase B2O3 to 30 and still be well under the 10% thresh
hold where it begins to increase glaze expansion.
The greater fluidity afforded by
this will allow you to increase kaolin to 25 (to raise alumina
to prevent boron-blue crystals) and to raise flint as high as
30. This lowers the expansion considerably and pinks should still
You might try getting a thermal expansion curve on that
clay body, perhaps you could get another body with a high

> Wollastonite has a very high expansion, as does 3134.

Wollastonite is a source of CaO which has a relatively low
expansion compared to Na2O and K2O and it is a source of
very low expansion SiO2.
Frit 3134 is a source of aluminaless boron which means you
can add lots of kaolin to bring expansion way down.

T o n y H a n s e n
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