Janet Kaiser on thu 29 apr 04
You probably mean Cornwall or Cornish Stone (aka China Stone or
Pegmatite)? I know there is a US equivalent, but the name escapes
me...
Sorry you may be learning all about it, only to be bitterly
disappointed in the near future. It sounds very like it is going
to go the same way as the ghastly borate. We are holding our
collective breath... At least those who have heard are. The
others will be in for a very big shock indeed... If and when it
happens. End of an era and all that. English potters may well end
up sourcing from South America, China, Egypt or wherever sooner
than they thought... Whether they will have the same properties
or not remains to be seen...
Cornwall... Yes, it is the English county in the South West and
it was the primary source for over two centuries. Very bottom
left hand corner... See "The Eden Project" for what is done to
one-time clay quarries down there. Yes, they can be recycled!!
Sincerely
Janet Kaiser
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The Chapel of Art : Capel Celfyddyd
8 Marine Crescent : Criccieth : Wales : UK
Home of The International Potters' Path
Tel: ++44 (01766) 523570 http://www.the-coa.org.uk
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mailtoandrew@FSMAIL.NET on fri 30 apr 04
As a number of myths and misinformation have surrounded China stone for
some time I hope the following clarifies
What is it?
A feldspar
It is the only ceramic flux that currently is commercially extracted in
the UK, and is found in the county of Cornwall in the SW of the country.
More specifically it is a medium grained, feldspar-rich partially
decomposed granite. In composition it is a heterogeneous mix of quartz,
feldspar and mica, with minor traces of kaolin and fluorspar, and a low
content of iron-bearing minerals
Traditionally China stone was sold as a variety of grades dependent on the
fluorine content and extent of kaolinisation, which was evident by both
colour and texture of the parent rock, These characteristics were
reflected in their names: Hard purple, Mild purple and Hard White. However
the appearance of the processed materials were identical.
Other names
Include China stone and Cornwall stone, and historically Moorstone or
Growan.
It is not
A pegmatite
A feldspathoid
Petunse, which is a relatively similar material, used historically by the
Chinese.
Colour
The misnomer regarding the colour of the dried milled material has
sometimes been repeated in print. It has been claimed that the slight
green or blue colour that can be detected is evidence of mineralogy or
even the fluorine content. However the colour is simply and identification
stain added during processing. This practice is common during ball milling
to help identify the different materials produced, without which all the
nonplastics look identical white suspensions. Although used for
identification when in aqueous suspension a faded hint can remain when the
material is subsequently dried. The dyes are vegetable based, used at less
than 0.5 litre per tonne, and burn out without effect.
Consistency
China stone has suffered from the undeserved reputation of being variable
in composition. The source of the perception is likely that from its first
use over 250 years ago until quite recently there were a number of
companies worked separate deposits that were all being marketed by similar
names. Therefore whilst material from one supplier may have remained
consistent it would not necessarily have been identical to that produced
by another.
For some years only the grade known as 80:20 has been sold. Production
controls, including regular analysis, result in a product that has been
consistent for many years. Typical compositions of this are
Chemically
SiO2 72.8
Al2O3 15.5
Fe2O3 0.14
TiO2 0.05
CaO 1.5
MgO 0.10
K2O 4.3
Na2O 3.1
F 1.0
LOI 1.7
Mineralogically
Quartz 29
Orthoclase 11
Albite 27
Muscovite 22
Kaolinite 2
Fluorite 2
It is perhaps worth noting that all commercial feldspars contain other
minerals. Although treatment processes are often used to enrich the
feldspar content it is erroneous to think of commercial grades as being
pure. The most common ancillary mineral is quartz and can represent
between 5 and 35% of most of the feldspar raw material used in ceramics.
Fluorine
To alleviate the problem associated with the fluorine emissions froth
flotation treatment was introduced in the 1960s. Marked in as DF Stone, or
Defluorinated Stone, it had a substantially reduced fluorine content. Due
to the reduced market demand production of DF stone ceased in the late
1990s.
Sustainability
China stone is now mined by a single company, it continues to be processed
and sold for ceramic applications. As geological reserves are large it is
economics that govern its life and whilst demand is sufficient it will
most likely continue. However the biggest users are UK industrial
manufacturers have largely moved to Scandinavian feldspars.
Hope that helps,
Andrew
Edouard Bastarache Inc. on fri 30 apr 04
Andrew,
very interesting and very informative post.
"Ils sont fous ces quebecois"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://perso.wanadoo.fr/smart2000/index.htm
http://www.digitalfire.com/education/toxicity/
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