O Corno on sat 22 may 99
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This is the last time I'm going to post anything relative to this matter.
Seems silly saying the same thing over and over. Where's the FAQ?
The usual motives for combining glass and clay for decorative effect are
the depth of the glass or its color.
..... excuse me, I gotta go ditch the really obnoxious Organ music NPR
is torturing me with ......
Damn, that was nasty.
Really thick glaze is seriously underused as an effect in the decorative
vocabulary of modern ceramics. In Brasil a number of the (HUGE)
tilemakers in the South, together with their suppliers -- Ferro,
Esmalglas, Fritas Sul......) have developed what fall under the general
name of =22vidrados=22. These are low melting fluffy powdered frits with =
real
high surface tension that melt quite clear in a 45 minute cold to cold
firing on tile -- you'd never, of course, want to treat your personal
work to such a firing. These thick films are applied into wells and/or
over underglaze to spectacular effect -- my ambition is to introduce
colored =22vidrados=22. In all of this the main thing to note is that the
technology for making thick glaze is as off-the-shelf as a Sears suit.
Typical Studio ceramic bodies have thermal expansion qualities which
differ substantially from most glasses you might desire to fuse onto
them. The expansion of Studio Claware comes in at 60-75 x 10=5E7/=BAC and
readily found colored glasses (=22stained=22 glass, bottles, etc.) from
85-120 x 10=5E7/=BAC. Plenty enough to ensure that in a fusion between the
two, something will break on cooling.
Putting bits of these glasses in some kind of well on a pot, firing it
anything above cone 010, and expecting to retrieve much more than a
fried, cracked-up puddle of glass is not reasonable.
Powdered glass of the types above can and have been used as glaze frit
for a time so long that only your local deities could know it. It works,
but is soluble enough in water to make stable slip a delicate dance.
Those glasses most likely to give a successful result are =22Pyrex=22 baking
ware, like you'd find in a grocery store. This stuff is usually not made
of 7740 Pyrex=AE (32 expansion glass), but of other lower expansion
compositions which have expansions in the 50's. The composition of these
glasses actually owe quite a lot to ceramic glazes.
Baking dish glass should be applied in a second (or third) firing after
glazing and/or overglazing. It'll be pretty fluid at 1,600 =BAF. Oxides
might be added to the glass placed in the wells, or the baking dish
glass could be remelted in a crucible and poured into water to make
colored glass frit. Bearing in mind that baking dish glass would need to
be heated to 2,700 =BAF to melt well and flow from the crucible.
Cool the glass-fused-on-pot slowly to avoid stresses (temporary or
permanent) in the glass and between it and the pot -- there's an article
from the 1930's by J.T. Littleton in the ACer.S Journal which describes
why in clear concise terms. In short, it'd be wise to slow cooling to no
more than 2 =BAF/min.
KPP
--
=22It's not what we don't know that hurts, it's what we know that isn't so.
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