C J TRIPP on sat 8 apr 00
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I need some help with glaze chemistry. I just started making my own glazes
a few months ago and one of the first I tried was by Ron Roy =96 his =233 =
Black,
=5E6 ox. It worked very well on my clay (Tuckers Mid Red) and in my kiln =
with
a slow glaze fire schedule (50C/hour from 700 to 900C and 60C/hour up to
1200C with a 20 minute soak and a fire down of 100C/hour to 900 and 50C/hour
to 700. (My kiln is only 2 =BD=94 thick, 3.9 cu ft and the temp drops like=
a
rock if I don=92t fire down.)
As this glaze worked well, I decided to use it as a base:
Custer Feldspar 22
Whiting 4
Talc 5
Frit 3134 26
EPK 17
Silica 26
Bentonite 2=25
I did some line blends and the best results were=3B
1) 7.5=25red iron oxide and 5.5=25tin oxide gave a rich caramel brown with=
a
smooth high gloss surface
2) 8=25 red iron oxide for a rich deep brown with a smooth high gloss =
surface
3) 4=25 red iron oxide and 3.5=25 titanium dioxide gave a deep brown =
background
with a blue haze that looks like a brilliant starry sky =96 really pretty =
BUT
pinholes of two sorts=3B either large craters on larger test pieces or lots =
of
tiny ones on small test tiles.
For the next round of testing, I used Tony Hansen=92s 5x20 =5E6 glossy as =
the
base:
Wollastonite 20
Frit 3134 20
EPK 20
Silica 20
Custer Feldspar 20
I did some line blends as above and the best were=3B
1) 6.8=25 red iron oxide and 5=25 tin gave an olive green /brown with brown
specks, smooth, high gloss surface
2) 8=25 red iron oxide was olive greenish brown, smooth, high gloss surface
3) 6=25 red iron oxide and 6=25titanium dioxide gave a lighter brown =
background
with some blue =93stars=94 , but at all as striking as the rio/td mix in the=
RR
base. BUT no pinholes either on the smooth high gloss surface.
So, what are the questions?
-What is it that gives the TH 5x20 base glaze the greenish cast in these
tests ? Is it the high calcium content?
-Why are there pinholes in the RIO/ titanium dioxide RR glaze and none in
the TH glaze? There are no pinholes in the other RR glaze tests so I
imagine that the titianium dioxide has something to do with it. But, how
come nearly double the amount of titanium dioxide fits into the TH glaze
without a problem?
How can I get rid of the pinholes in the RR base plus RIO and titanium
dioxide?
Please point me in the direction I need to test. Thanks.
Carol
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