Vince Pitelka on fri 17 oct 97
>> I am very interested in producing landscapes etc on majolica and would
>> like to have as accurate a drawing as possible. I intend to use cobalt
>> and possibly a mixture of cobalt and manganese on a tin glaze. I seem
>> to recall having heard somebody talking about sintering before painting,
>> or low firing the pieces to be decorated after they had been glazed with
>> the tin glaze which would produce a harder surface to decorate on as
>> opposed to the powdery surface you normally have.
>>
>> That way you could have better details and then fire again to maturity.
>> Is there anybody out there who has done this before? and if so what
>> temperature to you use for sintering, maturing etc, what glazes...
>> stains, any information would be appreciated.
From what Aurore Chabot and others say the secret here is to avoid firing
too high. I would be extremely careful, because some very undesireable
things can occasionally happen when a fully sintered but un-vitrified glaze
is cooled. It all depends on the temperature the glazed ware has reached,
whether or not the sintered glaze has developed a physical bond with the
body, and whether the cooling shrinkage of the partially-fired glaze is
radically different from that of the body. In a firing that proceeds
normally this is never a problem, because when the glaze and claybody begin
to shrink from sintering and vitrification they are both pyroplastic, and
barring other problems, the glaze will remain affixed to the surface of the
body until maturation point. But, if the glaze has been well sintered, but
without developing any strong physical bond to the clay surface, that's
where the trouble starts if the firing is ceased and the kiln is cooled. I
had never actually seen this happen until recently. A student of mine,
preparing for her BFA thesis show, blew up her green cone packs in a ^10
firing. Unfortunately, she did not discover this until well into red heat
late at night. Without consulting anyone she shut off the kiln, and by the
time I arrived the kiln was cool, and she had opened it and was placing new
cone packs. I could near a lot of "tinkling" and "pinging" from inside the
kiln. I cautioned her that this could be very bad, but there was little to
do at that point but go ahead and fire off the kiln. When she opened the
kiln the next day it was a real disaster, and most of the ware was trash.
Large areas of glaze had flaked off the outsides of the pots and had fused
to the kiln shelves. Understandably, she was completely devastated. But
she is a real trooper, and pulled herself together and cranked out some of
her best work for the show.
So, to clarify, in cooling the immature firing, the sintered glaze shrinkage
was significantly different from the sintered clay. Without a strong
physical bond between them, the glaze cracked and partially separated from
the clay. In re-firing, as the glaze became soft, it simply fell away in
patches from the outsides (and some of the insides) of the pots. There was
no saving the pots, so they became "twice thrown."
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
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