I.Lewis on fri 21 jan 00
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Accepting the admonitions and information which has been contributed over =
the
past couple of weeks as being valid, may I make the point that I am looking =
for
first or basic principles. I am not overly concerned with specifics. In
addition, my intention is to try to keep things simple. I accept that there =
is a
variety of plastic clays and casting slips which will fire at some =
temperature
of another, in some atmosphere or other, in one form of kiln or another, to =
a
white vitrified ceramic which has some degree of translucency when viewed by
transmitted light or when thin, when illuminated by back lighting. My =
interest
is in eventually explaining that translucency
For the purpose of this discussion I make an assumption that the stuff which=
is
being used contains a quantity of Kaolin, has sufficient fluxes to cause =
melting
and enough silica to make the material glassy.
My opinion is that the phenomenon of translucency will be better understood =
if I
have a clear picture of what happens in the fire. It would seem that the
transformation during firing has five phases.
1 Under 600Celsius (1112F) Kaolin looses it's combined hydrogen oxygen =
units. It
forms a material called Metakaolin.
2 900Celsius.Metakaolin undergoes chemical changes The crystal structure
changes. The result is an aluminium silicate with a spinel structure and
structureless (amorphous) silica. Elements or compounds with low temperature
fluxing potential become active.
3 1050-1100Celsius. Another chemical change occurs. The Spinel structure =
change.
Mullite forms and more amorphous silica is created.
4 1100-1200Celsiius. Felspars become active and start to melt. Glass phase
starts to form. Free silica from all sources starts to dissolve in glass.
5 1200Celsius plus. Solution proceeds. Mullite crystals grow. If all the =
silica
does not dissolve that which was quartz changes into cristobalite. This =
depends
on the proportion of glass and the particle size of the quartz..
Three factors appear to determine the rate at which these changes occur and =
the
time which is needed for them to be completed.
1 Particle size. The smaller the particle size the faster the reactions and =
the
sooner they will be completed.
2 Temperature. Higher temperatures make chemical reactions, including the
dissolving of soluble materials, go faster and finish sooner.
3 Time. Longer firing times, especially at the upper temperatures promote =
the
completion of reactions.
Reference. Ceramic Science for the Potter, Second Edition. W. G. Lawrence =
and R.
R. West. Ch 2, 31-37. Differential thermal analysis. Ch8, Whiteware Bodies.
120-122. Reactions in Clay-flint-Felspar Bodies. Chilton Book company, =
Radnor,
Penn. ISBN 0-8019-7155-1
In the next part Iwill discuss consequences of firing to maturity.
Best wishes to all who are contributing thoughts, comments, criticism and
sharing their experiences. One thought for the historically minded. Those
porcelain monuments to the Drinkers of the World may have had Royal Approval=
and
made By Appointment to an HRH of yesteryear. Look for a three feathers mark.
Ivor Lewis.
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