Dave Finkelnburg on sun 28 apr 02
Gail,
As you can see, I'm behind again. :-)
Lead, boron, sodium and potassium all melt at relatively low
temperatures, say as low as cone 012. Think of the fluxes in American Raku
glazes. I suspect lithium is in this group also.
Calcium and zinc start melting around cone 04 to 02 in most glazes.
Magnesium and barium and likely strontium begin melting around cone 2 to
4.
Lead boils around cone 5, but all the other fluxes, in oxidation, stay
in the glaze as high as you want to fire it.
The number and kind of fluxes and especially the size of the glaze
particles also affect when they melt.
Iron is the principle metal oxide affecting glaze melt temperature, IF
one is firing in reduction. Then the iron is also a flux.
Other flux oxides affect glaze viscosity--how runny the glaze is, but
not so much melt temperature.
In general, these things are so. In particular you need to do draw
trials, actually pull glaze samples out of the kiln during the firing. I
got that lecture most recently from Ian Currie. He is right, of course.
The off-gas question is excellent! Any carbonate is problematic. But
so is offgas from iron. The clay body can be as big an issue. I have
thought the carbonate evolution of CO and CO2 would occur well before
2,000-degrees F. If the glaze melts at a very low temperature, the CO or
CO2 will be trying to get out through a liquid glaze, much more difficult
than from a glaze that melts at mid-range.
Dave Finkelnburg in Idaho, planning to pull draw trials from the next
firing
From: Gail Dapogny
>Have a question.....What is the range of glaze melting? What are the
>earliest and latest temps (ballpark), and are these temperatures affected
>by the type and amount of metal oxides in the mix? Also, since whiting and
>talc and ...--I forget-- dolomite (?) produce more gases than other
>substances, are these gases affected by when the melt occurs and how fast
>you are firing at that time?
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