Wes Rolley on mon 25 apr 05
versus porcelain
Rod wrote:
> I was under the impression from literature I have read (Parmlee, Hamer,=
etc)
> that one of the defining factors which separates stoneware/porcelains f=
rom
> earthenware is "mullite spinel" formation which has been gathered from
> recent scientific observation and cold hard data.
I do not know, but for some reason I have been questioning terminology=20
of our craft, partly because improper use of terminology can hinder=20
understanding...though there are many examples of practical knowledge=20
that works, even though it is described by imprecise terminology. In=20
fact, the statement of Rod's above is "good enough for government work"=20
if not exactly precise. Those who are not interested in the science need=20
read no further.
In this case, I looked for the scientific observations that Rod was=20
talking about and found that "mullite" and "spinel" are two different=20
things, not that mullite is a type of spinel. I also found out that:
- formation of mullite and / or spinels is greatly affected by the Al2O3=20
/ SiO2 ratio.
- it is also affected by temperature and is the range between 1000 C and=20
1250 C the increase in temperature reduces mulliite formation while=20
increasing spinel formation.
Note that the term spinel refers to a specific crystal lattice structure=20
with a chemical forumla X2YO4. In fact Fe3O4 is a spinel where X=3DY. In=
=20
high temperature ceramics, Al2FEO4 is a spinel with Aluminum and Iron.=20
The gemstone, Spinel, is Al2MgO4.
I quote from the Journal of Materials Science extracts:
Both metastable tetragonal mullite and spinel were observed to form at=20
temperatures <1000 =C2=B1C in the gels with Al:Si ratios < 8:1 and mullit=
e=20
transformed to the orthorhombic structure at ~1250 =C2=B1C. However, at=20
higher Al:Si ratios, spinel was the only crystalline phase detected at=20
<1000 =C2=B1C and orthorhombic mullite formed directly at temperatures ~1=
250=20
=C2=B1C. As the Al:Si ratio increases, both the tetragonal mullite and sp=
inel=20
formation temperatures decrease while the orthorhombic mullite formation=20
temperature increases. Based on the Al:Si composition where the=20
formation extents of tetragonal mullite and spinel were
maximum, their compositions are estimated to be (2Al2O3) (SiO2) and=20
(6Al2O3) (SiO2), respectively.
http://www.mrs.org/publications/jmr/jmra/1995/apr/P00912.PDF
--=20
"I find I have a great lot to learn =E2=80=93 or unlearn. I seem to know =
far too=20
much and this knowledge obscures the really significant facts, but I am=20
getting on." -- Charles Rennie Mackintosh
Wesley C. Rolley
17211 Quail Court
Morgan Hill, CA 95037
(408)778-3024
http://www.refpub.com/
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