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refiring an ash glaze. getting the black grunge

updated sat 1 mar 03


iandol on fri 28 feb 03

Dear Friends,
Tony Clennell has asked me to try to put some spin on this topic. In his =
original posting he said ,<with a olive green ash, when refired in heavy reduction in a gas kiln =
come out with the ash deposit a charcoal black. Is it ash trapping =

I think the first thing to consider is the mechanics of this process as =
it is described. It consists of two parts, the first being the =
maturation of the clay and the melting of deposited ash on clay which =
has become wet and sticky so the ash starts dissolving into the vitreous =
phase. Since ash consists mostly of Potassium and Calcium Carbonates =
they will react. CaCO3 changes into lime and the K2CO3 melts. Potassium =
carbonate will be miscible in the melt, possibly takes part in an =
acid-base reaction with Silica in the melt and increases fluidity. Lime =
will dissolve into this solution which now has enhanced solvent action. =
Addition of Calcium ions into the solution will reduce the viscosity. =
Since there is not enough lime in the glaze to crystallise out you get =
cool your green colour which may be due to iron from the clay. All this =
has happened in the 1200-1300 Celsius range.
Now put that back into gas kiln and reheat. Because you are reheating an =
already formed glass it will start to melt at the Glass Transition =
Temperature. This can be as low as 500 Celsius if the glass is high in =
Potassium or Sodium. Now you have a sticky surface. Reduce the kiln =
heavily. There will be a lack of oxygen so you must get uncomubsted =
carbon. This sticks to the surface creating a semi porous structure. As =
melting of the original glaze progresses capillary activity will cause =
the molten glaze to be sucked into the building Graphite contamination. =
The glass protects this silicate wetted carbon preventing it from =
oxidising. So you get a black finish to your pots.
Cure would seem to be to avoid that heavy reducing with its production =
of copious free carbon. I would ask "Why would it be necessary? , what =
is the purpose of refiring.
Sorry to be so long winded, but Tony did ask, and I promised.
You could test this hypothesis out with a few well placed draw rings.
Best regards,
Ivor Lewis. Redhill, South Australia.
Seems I am not the only one with thoughts about the destruction of =
Treasures in the Assyrian Birthplace of Human Culture. Listened to a =
Yankee Professor of Architectural History from Columbia University on =
the radio this morning. There must be a better way than 600 bombs a day =
for weeks on end.