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boron expansion,crazing breaks!?

updated sun 5 apr 98


maggie j jones on sat 4 apr 98

Hello Ron...sounds like NCECA was a success...
Here is the formula that I was using that was not compatible after
Brian added Pyrophilite to the brownstone ^4-6 that he has.
I began backing off on silica and there seemed no change at all. I
tried your formula too. So, I rubbed iron ox. into the cracks and noticed
that the less SiO, the more cracks all over the pot, smaller and smaller
network. The more SiO, the larger the cracks and significantly fewer, ALL
broke.... So, I bagan adding SiO to where it is now almost doubled and
the pots survive! I think that considering the forms, the tension is so
strong, the crazing breaks the piece,. The pieces are what we call "slab
pots" because they are rolled out like pie dough onto textured fabric and
draped into a form. Some are deep bowls with wavy edges. They are only
glazed on the inside and it rolls over the top edge. Pie plates are made
this way also. Over the past 18 yrs i have used variations on this GB
glaze that I have never been able to find in books, everyone uses whiting
which changes my colors, I brush decorations on top. Now there is this
"Floating Blue " thread on the list which is very similar!
Below are the before and after formulas:

Used for several yrs, no prob till pyrophilite added to clay:
Neph Sye. 44
Spodumene 8.33
Gerst. Borate 6.2
Ferro 3134 10.4
Ferro 3195 10.4
SiO 10.4
Kaolin 10.4
tin 5.
zircopax 5.

Your form: still broke
NS 40
G200 7.27
GB 6.36
F3134 9.09
F3195 9.09
SiO 8.18
Kao 10.91
tin 4.55
zirco 4.55

The following is successful:
These percentages are the results of taking the formula you gave me
which had more kao in it, used spod instead of G-200, cause thats what I
have, and added Sio to the test batch in increments of 2.5% at a time,
so, as i added SiO, the ratio makes the other materials go down of
course. Note that I changed from frit 3195 to 3124.
I would be curious as to what you observe. I have frozen , washed in
hot, soapy water and put in the microwave. i have had to test the
bisqueware that was packed away in boxes .....because test tiles would
not give the same results!

NS 37
spod 6
GB 6
3134 8.5
3124 8.5
SiO 15
Kao 10
tin 4
zirco 4

Maggie Jones , Turtle Island Pottery, Black Mtn. NC

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