Tom Buck on wed 28 sep 05
David Woof:
the main difference between unwashed and washed woodash is the
removal of solubles from the woodash when treated with water several
times.
the chief materials removed by the water are sodium (Na) and
potassium (K) compounds, likely the hydroxide and carbonate, perhaps some
oxide would survive in a dry climate. The flux oxides that remain are
calcium (Ca) and magnesium (Mg) compounds, largely carbonates which are
almost insoluble.
also remaining are compounds of silicon, phosphorus, and aluminum,
chiefly in the oxide form. iron oxide would also be present in a small
amount.
Joseph Grebanier in his 1975 book, "Chinese stoneware glazes",
cited two analyses of a Japanese woodash ("Issu-wood"). Patterson, JBE,
gave this for a fully washed ash:
SiO2, 71.96%; Al2O3 0.63; P2O5 0.42; Fe2O3 0.28; CaO 15.95; K2O 0.84; Na2O
0.0; MgO 1.57; and Loss on Ignition 8.29 (chiefly carbon dioxide)
Grebanier also cited an analysis by Herbert Sanders for the same woodash,
but this time likely not washed (although Sanders didn't say so):
SiO2 34.60; Al2O3 4.38; P2O5 3.93; Fe2O3 0.49; CaO 47.71; K2O 2.51; Na2O
0.06; MgO 5.99; LOI not given, cited as zero.
Grebanier notes the differences and says "wood ash" is not a standardized
material even when it comes from the same wood.
He cites a "common" wood ash composition, unwashed presumeably:
SiO2 30.99; P2O5 1.91; Al2O3 8.91; Fe2O3 3.04; CaO 22.42; MgO 3.30; K2O
3.91; Na2O 2.33; MnO 1.26; LOI 21.44.
Grebanier then calculates the ash's Seger Formula, which solely includes
active oxides in the melt, and he uses it to back-track, and calculates
the following "recipe/analysis":
bone ash 4.09; whiting 35.73; magnesium carbonate 6.86; potspar (he used
Buckingham spar, popular in the 70s) 24.35; kaolin 10.39; flint 10.11;
soda ash 3.90; manganese dioxide 1.32; and red iron oxide 3.00.
As you can see, David, this is a very simplified overview of the chemical
compositions of unwashed and washed woodash. what it mainly suggests is
this: if you want the strong alkali flux oxides to enhance melting of the
glaze, then use unwashed woodash and exercise due caution when handling
this slurry. if you want to convert a base clear to a matt, especially a
dry matt, then use washed woodash.
good pots peace Tom B.
Tom Buck ) -- primary address.
"alias" or secondary address.
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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