Brian Molanphy on tue 9 oct 01
hello vapor-firers,
yesterday i unloaded a soda kiln with pleasing results. pots look good. one
thing that i would like to avoid in future firings is claybody color
'wash-out', for lack of a better term. red clays like amador and death
valley sometimes turn grey, at least on edges and knobs, sometimes whole
pots. i use blair meerfield's yellow slip on white clays like bmix and
soldate, but it sometimes goes grey too. jeff zamek wrote in his article on
soda firing from the early seventies, '...pots located near the salt [soda]
port showed a yellowish, glassy buildup...when unvolatized soda ash strikes
the ware, the yellowish buildup occurs.' in my case, the grey areas are
generally most affected by this yellow buildup, while the areas retaining
red claybody color or yellow slip color seem to have a lighter soda coating.
the solution i spray into the kiln's two ports over the fireboxes is 1.5 kg
soda ash dissolved in 4.25 L water (brought to a boil). i sprayed five
times, each time using about a fifth of the hot solution, over the course of
ninety minutes, from cone 9 bending to cone 11 bending. i spray over the
fireboxes, not directly onto the pots. the reduction seemed heavy, tho i
don't have an oxygen meter to really know. the damper was closed during the
spraying, then opened partway between sprayings. kiln is 37 cubic ft, with
stacking space of 15 cubic ft.
any idea how to avoid the wash-out? do i use too much soda for the kiln
size? even tho i spray over the firebox, perhaps some unvolatized soda
solution drifts onto the pots, creating both the yellow buildup and the grey
clay color? that suggests i might do better to use less soda per spraying...
or is the reduction too heavy? should i begin spraying at a lower cone,
maybe 8?
by the way, included were some pots sprinkled with wood ash, inspired by a
slide of ian currie's, showing a japanese wood-firer 'cheating' that way.
the colors are not bright like the red, yellow, and cobalt blue, but they
are richly varied; its something i'll keep doing.
brian
iandol on thu 11 oct 01
Dear Brian=20
One of the things which can be done to alleviate bleaching is to =
reoxidise strongly during the first part of your cooling cycle.
Unfortunately, the chemistry between clay bodies and sodium carbonate is =
not clearly defined in a scientific sense so we do not know what is =
happening. My current understanding is that it is a an acid/base =
reaction with free silica, discharging carbon dioxide. This is very =
different form the Sodium chloride reaction where the salt does not =
initially react with free silica but with potassium bearing compounds =
such as felspar and mica to create potassium chloride, and with iron =
oxides to become volatile ferric chloride. This is known to react with =
water to make hydrogen chloride and explains why iron bearing clays =
bleach.
Good to hear you are having some good firings.
Best regards,
Ivor Lewis.
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