JJHerb@aol.com on fri 24 jan 97
For John Weber and other calculating ash glazers: I guess I think of the
need for a hardwood composition to use in my glaze calculation software in
much the same way I viewed the request for the BTU value of cow chips. WHY??
In any event, Conrad, in the much recommended book Advanced Ceramics Manual,
technical data for the studio potter, lists 18 compositions for straw and
wood ash of various kinds. Oak ash is given as: SiO2 - 15.3; Al2O3 - .10;
Fe2O3 - 2.40; CaO - 30.0; MgO - 12.0; K2O - 14.00; Na2O - 9.12; P2O5 - 13.10.
The silica content of the ashes given ranges from 2.65 to 96.00 percent.
This wide range of possible compositions indicates the difficulty of using a
"Theoretical Analysis" for glaze calculation. The actual composition of the
ash you are using will depend on the species of plant, the growing conditions
that year, the soil composition, how the ash was treated during and after
collection (washing, for example), and other things. I have thought that one
of the attractions of ash glazes was the accidental quality of the process.
You find a clay in your back yard, you get some ash from somewhere, you try
to make a glaze. One performs line blends, test fires, makes additions, test
fires, makes adjustments, test fires. On the other hand, I would never stand
in the way of the inappropriate application of technology. For example, I
have gladly accepted the fact that all calculations performed on electronic
hand calculators are accurate to 10 places. When I balance my check book it
is done very accurately, to the millionth of a cent.