iandol on sat 31 may 03
Dear Ron Roy,=20
Thanks for the quotation from Taylor and Bull. The physical properties =
of Lead fluxed glazes are well documented. But it is difficult to find =
information which will validate claims for the physical behaviour of =
Lead Compounds which might be incorporated into a batch recipe.
Lots of things are dependent on time and temperature. For example, the =
pace of many chemical reactions doubles with each 10 deg increment of =
temperature increase. Perhaps it is unfortunate that we cannot =
distinguish between fact and opinion without corroborating evidence when =
presented with descriptions and explanations for workplace observations. =
Until I trace the origins of the Taylor and Bull information I would =
prefer to keep an open mind.
I have touched on the CaO-Al2O3-SiO2 question in another post. I'm =
pleased you got those results with Wollastonite. I did that mixture as =
well but mine only sintered at Cone 8. Unfortunately, adding a K2O/Na2O =
ingredient would invalidate the conditions for testing for a Eutectic =
Melt. We might also add more silica to prevent crystallisation and =
ensure a vitreous sample. But that would invalidate the test as well.
Good points about using mixtures which are close to Eutectic Points. But =
they are very rare in our work. I think we employ them when Neph Syenite =
is employed in a recipe.
Remember that a Eutectic, as described in ACS and other literature, =
melts without change in temperature on the addition of heat and =
solidifies at the same temperature with the loss of heat as a =
crystalline substance. It is an invariant point just like the Triple =
Point of Water.
I must get that article about Eutectics away to the magazine.It clears =
up a lot of the misunderstandings which are created by what has been =
written in the past. Good to have these discussion. Keeps the old grey =
cells agile.
Best regards,
Ivor
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