Ruth Ballou on tue 20 dec 11
Hi all,
Yellow Salt
71.6 Neph Sey
23.6 Dolomite
4.8 Ball Clay
18 Zircopax
1.1 Red Iron Oxide
The results are in! Anyone who would like to see the grids can email and I
will send the jpegs for best viewing. I took the pics at maximum resolution
so the files are large. I'll also send the Excel files with recipe details
for each glaze in the grid.
I'm gonna rename this glaze "BumbleBee Yellow" 'cause there is no way it
should be a glaze. Mole % of silica is only 54% and then there's all that
Zircopax.
Frequently when doing a grid, the actual glaze doesn't show up on the grid.
In this case, however, #26 is right on. You can see what happens as the
clay is increased from bottom to top, and the silica increases from left to
right. Top right hand corner B (#5) has the highest alumina and silica
(.824/7.046). Diagonally opposite corner C at the bottom has the lowest
alumina and silica (.42/1.609). The glazes in the far left column have less
than 60% silica, with the exception of #5, which has 61%. In the second
column, the silica content ranges from 60 to 66%. You can see the analysis
for the rest of the grid in the Excel files.
The grids were done on 4 clay bodies: Tom Coleman porcelain, Tony Clennell
stoneware (no grog), Brown Sculpture clay and a white stoneware from Star
Clays in NC.
I looked at the tiles with a lighted 12.5 power hand lens. Viewing the the
results with this lens revealed that there is a lot more pinholing
happening than is visible with the naked eye. In addition, many if not most
of the glazes either specks of unmelted material (zircopax?) or bubbles.
For those of you using the glaze, I recommend getting a lighted hand lens
to see if you observe the same thing in your results.
Clay body has a big impact on this glaze. The smoothest surfaces were on
the porcelain; the roughest, most pinholed were on the Brown Sculpture.
Glazes that appear smooth and creamy appear on each of the tiles, in
different places. Most of them are still pinholed when viewed with the
lens. Harry Fraser writes in Ceramic Faults and Their Remedies that any
test that influences the severity of a a defect should be considered
significant. He later observes that dolomite is well known for the amount
of gas it releases during firing and recommends replacing the dolomite with
a mixture of wollastonite and talc. Both wollastonite and talc contain
silica, so that is difficult to do with this low silica glaze. Also, the
mag carb light generally available in the US can cause crawling problems
in large amounts in a glaze. Other parts of the world can get heavy mag
carb. Does anyone know of a supplier in the US?
So, you might want to try one of the smooth looking glazes on the grids,
being mindful of your clay body. Reducing the zircopax is probably also a
safe bet. I've reformulated the glaze with mag carb and whiting, but this
may well result in other problems. I've arbitrarily reduced the zircopax to
10%.
72.3 Neph Sye
10.1 Magnesium Carbonate Light
13.0 Whiting
4.5 Ball Clay
10 Zircopax
1.1 Red Iron Oxide
Any comments or observations appreciated.....
Ruth Ballou
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