iandol on fri 19 apr 02
Dear Brandon Phillips,=20
This is an interesting observation, but a predictable effect. <thought that since it was such a low temp. that he could just stack =
them. They all stuck together, at CONE 08!!>>
One piece of information which seems not to be commented on in popular =
pottery texts is a factor called "Glass Transition Temperature" which I =
have mentioned before in conversations with Ron Roy. (Ron has the =
apparatus which can be used to determine this property for a glaze)
Your comment is an illustration of that factor coming into play. Many =
people assume that because a glaze need to be taken to ^6, ^8 or higher =
that it needs to be returned to it's initial maturing temperature to =
cause remelting. This is not so. Glazes which are fluxed with Soda, =
Boron and Lead may become soft and sticky as low as 500 Celsius. Most of =
the stoneware glazes we use reach that state by the time they have been =
returned to 800 Celsius. This is why it is possible to do post firing =
treatments such as "On Glaze Enamelling", "Metallic Lustre" and "Smoked =
Lustre" and for the surface to be permanent. The glaze becomes "tacky" =
and acts as an adhesive.
In connection with smoked lustre, if Silver is the active ingredient and =
the temperature is taken too high, the vaporous silver compound diffuses =
into the glaze to give a muddied yellow stained colouration instead of =
the metallic blue and pink reflective sheen which is highly desirable.
Which, of course, has nothing to do with trapped Carbon!! But I would =
imagine it has some bearing on the reheat temperature for developing =
"Shino" oranges. And that is a whole new field of chemical unknowns.
Best regards,
Ivor. =20
iandol on wed 24 apr 02
Dear Gail Dapogny,
What a lot of question you do ask!!
Regardless of the temperature you are firing up too, the things which =
start to occur in a glaze have more or less nothing to do with melting. =
Sintering is the first step in the process. This may start to happen =
lower than 500 Celsius. Your Alkali Earth Carbonates start to loose =
their Carbon dioxide at just over 800 Celsius, Say 1450 F. I know that =
Calcium oxide then undergoes a solid state reaction with any free silica =
you have put in the glaze, or which is created when you clay fraction =
decomposes. This creates a series of Calcium silicates, some of which =
are unstable and continue to react with more silica. The same may be =
true for Magnesium oxide. But these materials have very high melting =
points (They make good refractory cements.)Talc looses its hydroxyl at a =
fairly low temperature as well, changing to Eka-Enstatite, another =
refractory material.
Any sodium salts, including sodium carbonate will melt just over 800C as =
well. They contribute to a function called Fluid Phase Sintering which =
drives the sintering process until the lowest melting point flux kicks =
in to form a real liquid. Now you have a solvent in which everything =
else in the recipe has a chance to dissolve by the time you reach your =
chosen maturity point. In all probability the colouring compounds have =
no effect. Some are readily soluble such as Iron, Manganese and Copper, =
others almost insoluble, like Tin, Chromium and Nickel. But they will =
not affect the temperature You have chosen as your maturing point.
Have a good discussion in the Guild.
Best regards,
Ivor Lewis. Redhill. South Australia
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