Paul Borian on thu 2 dec 10
i have heard different opinions on this topic so i thought to send the
question to clayart.
i bisque to cone 04 and have spoken to others who have said that, at that
temp, potash feldspar does not have a significant affect on free silica in
the clay body.
However, in the Hamer&Hamer book, it says that potassium oxide starts to
melt the silica as early as 1472 F and this is actually the point at which
the vitrification process first begins.
this is well within the range of my bisque firings.
the reason i ask is that certain forms seem to be dunting during the coolin=
g
phase of the bisque firings i do in electric kilns (skutt 1227). Some have
suggested that more feldspar will help and others have suggested that it
will not at this relatively low temp.
These are serious experts i am talking with and i have a feeling both side
are correct but i am not fully understanding the concepts here.
if anyone else feels to share opinions i would be very interested to read
them. If possible please forward to me off list since i am presently off
list.
thanks!
paul
ivor and olive lewis on sat 4 dec 10
Dear Paul Borian,
You say...... " i bisque to cone 04 and have spoken to others who have said
that, at that temp, potash feldspar does not have a significant affect on
free silica in the clay body.
However, in the Hamer& Hamer book, it says that potassium oxide starts to
melt the silica as early as 1472 F and this is actually the point at which
the vitrification process first begins. this is well within the range of my
bisque firings."
If that is what Hamer and Hamer say then they are proposing that Potash
Felspar decomposed or undergoes some other chemical reaction at 1472F
(800C). This is not evident in the phase diagrams associated with Potash
felspar. What can be found, and supported by a discussion in "Introduction
to Ceramics" (Kingery and others)is that at 1150 C +/- 20 2102F +/- 36
decomposition commences, in what is termed a Peritectic reaction causing
incongruous melting, creating a silica rich fluid and a new solid called
Leucite. (See p 289)
You will get a better picture of what happens during a ceramic firing if yo=
u
study Ch 10 (ISBN 0-471-47860-1) For a more up to date picture (1997)see C=
h
10, Michel Barsoum, "Fundamentals of Ceramics". (ISBN 0-07-005521-1)
Based on the teachings in these books I suggest you look for an alternative
reason for the dunting you are experiencing with some of your forms
Regards.
Ivor Lewis,
REDHILL,
South Australia
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