Tom Buck on thu 25 may 00
Hello, Paul (Lewing):
You ask about Bristol glazes. Many years ago (is it really 25??) I
made a study of this type of oxidation glaze and devloped several that
matured at Cone 3 (C2/C4) on a pink- firing body. As Parmelee notes (Pg
167, 1st Edn, "Ceramic Glazes"), the Bristol is an adaption of a "soft"
porcelain glaze, using Zinc Oxide as an active flux at a lower temperature
and thus achieve a lower melting point.
This glaze type originated 100+ years ago in Bristol, England
(hence the name), to replace lead-based glazes on once-fired ware,
terracotta particularly, although it can be adapted to work on bisque-ware
too. One feature that made the Bristol approach attractive to the local
potteries was the relatively high viscosity of the melt at the usual
working Cone 5 (some at C3); good gloss glazes were produced without fear
of pots sticking to the shelves.
The Bristol recipe(s) uses alkalies (K & Na), alkaline earths (Ca,
Mg, Sr, Ba), and Zinc, to flux clay and free silica. The result is a very
tough durable glaze, either opaque or clear, that could tolerate a long
firing schedule (common in 1890s).
Two pioneers, Purdy and Watts, are cited by Parmelee as ceramic
specialists who published data on the Bristol type.
Purdy cited this Seger:
Lowfire Bristol Glaze
0.40 KNaO 0.48 Al2O3 2.64 SiO2
0.20 CaO
0.40 ZnO
If formulated precisely, Purdy said this glaze would mature as
low as C03 (1035 oC, 1900 oF).
Watts' lowfire Bristol:
0.36 KNaO 0.57 Al2O3 3.29 SiO2
0.28 CaO
0.36 ZnO
This too would experience a eutectic (melting point) close to
C03.
Pamelee also lists the following limits for Bristol glazes
to be applied to terracotta (functional, structural, architectual
ware):
0.2-0.4 KNaO 0.2-0.55 Al2O3 2.0-3.5 SiO2
0.1-0.5 CaO
0.1-0.4 ZnO
0-0.2 MgO
0-0.3 BaO
0-0.4 SrO
When I tried to design a base glaze using first Purdy's then Watts'
Seger Formulas, I achieved immature, clumpy glazes. (My raw materials were
different than those used in Bristol, especially particle size ranges).But
with persistence, and by introducing Gerstley Borate (or Colemanite) I
obtained bright, durable C3 glazes. One such mix was:
Bristol milky blue, C3 oxidation
252 Gerstley Borate (analysis 1977)
335 Nepheline syenite
253 Flint fine mesh
51 Zinc oxide
41 Barium Carbonate (now I'd use 31 Strontium Carbonate)
36 Spodumene (Foote Minerals)
11 Whiting (now I'd leave it out, or use Wollastonite)
14 Kaolin (EPK)
7 Titanium dioxide
add 0.5% Cobalt Oxide (or 0.7% Carbonate) and 2% Copper Oxide Red
for mottled blue colour I liked. Or just add more Cobalt Oxide.
I grant you, 11 materials is a lot of weighing, but I did it and
found the result worthwhile.
So, there you have it, Paul. Good tests. BFN. Peace. Tom.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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