John Baymore on sun 25 apr 99
------------------
(snip)
..... .. .. .......this is the only raku glaze I've seen with barium
carbonate: its
name is Middlebrook's Crackle White. 60=25 gerstley borate, 50=25 potash
feldspar, 22=25 barium carbonate, 16=25 silica. It is from Pipenburg's book,
Raku Pottery. ......... ...... ... .. .. .
.............called =22Reliquary Vessel=22, is 42=22H, and covered by a =
beautiful
blue glaze called Barium Matte Glaze -- and looks like it, too, is raku.
......... ...... ... Didn't know that barium could be used in raku, if
....................
Lori and Anne,
The predominant fluxes in the above glaze are CaO and the KNaO from the
spar, along with the lowering of the melting of the glass formers caused by
the high B2O3 content in relation to the SiO2. Barium oxide is not an
active flux at raku temperatures. So it is taking little role in the
glazes you mention. You could probably leave it out altogether and not
notice too much difference in the fired glaze. Maybe add a tiny bit of
zircopax to match the slight opacifying the undecomposed barium carbonate
is probably causing (see below).
In fact, Barium carbonate doesn't decompose to barium oxide and carbon
dioxide untill a much higher temperature than those used for raku, although
it's decomposition is enhanced by reduction so if the kiln is reduced on
the up cycle that may be helping it decompose. So it is likely that most
of or all of the barium carbonate in the aforementioned glaze is remaining
as little tiny specks of uncombined barium carbonate suspended in the glass
matrix.
Dr. Eppler (Glaze Doctor from ACerS) talked about this subject as part of
his presentation at the Columbus NCECA. He said that barium oxide is
basically inert below cone 2. He also cautioned about it's toxic nature in
functional ware glazes and suggested it not be used. He suggested that if
it IS used, it should be sourced in a barium frit, not barium carbonate due
to the fickle decomposition characteristics of BC.
The glaze as stated breaks down roughly as follows with the stock MDT in
Insight:
Barium Raku Glaze
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
GERSTLEY BORATE..... 60.00 40.54=25
G-200 FELDSPAR G200. 50.00 33.78=25
BARIUM CARBONATE.... 22.00 14.86=25
FLINT............... 16.00 10.81=25
=3D=3D=3D=3D=3D=3D=3D=3D
148.00
BaO 0.23=2A 13.19=25
CaO 0.41=2A 8.55=25
MgO 0.00=2A 0.02=25
K2O 0.12=2A 4.16=25
Na2O 0.24=2A 5.49=25
Al2O3 0.19 7.17=25
B2O3 0.94 24.21=25
SiO2 1.66 37.17=25
Fe2O3 0.00 0.03=25
Si:Al 8.80
SiB:Al 13.76
Expan 8.98
If checked against the 05-08 limits (best match there for raku) , it is
slightly oversupplied with BaO, on the low end for SiO2, and the high end
for B2O3. This is a very soft glass that probably leaches barium oxide.
Best,
............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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