iandol on sat 16 dec 00
The substitution game crops up periodically and a variety of =
alternatives are suggested to solve each problem. But the latest one to =
catch my eye was that of using Strontium Carbonate as a substitute for =
Lithium Carbonate.
I accept that Mary Lynch, who probably followed advice given on Clayart, =
knows it is the right thing to do because it worked for her. It removes =
the fear of the consequences of ingesting Lithium and her glazes retain =
their character.=20
But this is an incongruous exchange to suggest in the first place =
because these two substances have very different chemical and physical =
properties.
There are several questions which need to be addressed, for example, why =
is lithium being used in the glaze in the first place? Could Sodium or =
Potassium be used instead? What technical logic informed this =
substitution?
Without reference to the reasons why specific materials are used in =
glazes, that is, a sound knowledge of fact, function and behaviour of =
each substance, we have no idea if an adequate substitute product is =
being created.
In this case an Alkali Earth metal is being substituted for an Alkali =
metal. They have differing chemical reactivities because they have very =
different numbers of electron and differing electron configurations. =20
For example, Lithium Carbonate melts at 723 deg Celsius but does not =
decompose until it is heated to 1310 deg Celsius. This means it will =
assist the sintering process and act as a low temperature melting agent. =
I believe it would act like Sodium carbonate and react strongly with =
silica or silicate compounds once it melted in contact with them, =
hastening the dissolution of higher melting point compounds.=20
Strontium Carbonate does not melt at a low temperature. It decomposes, =
discharging carbon dioxide at 1340 deg Celsius. The Strontium oxide =
which forms has a melting point of 2420 deg Celsius. So it is not a =
melting flux.
I also suspect that when both have dissolved in the fluid which will =
become a glaze they have differing functions. Lithium, in a vitreous =
silicate solution changes the rate of thermal expansion, Strontium, =
because its ions have a large diameter, will lower the glass transition =
temperature possibly assisting in increasing glassiness. Glazes which =
contain it may also throw crystals if cooled slowly.
I would be interested to know how Robin Hopper treats such issues in the =
re-publication of the Daniel Rhodes Classics.
Ivor Lewis, Redhill, South Australia.
On a day when mercury is trying to boil
Khaimraj Seepersad on sun 17 dec 00
Hello to All ,
Ivor ,
I believe the answer may be in the muddle of information from the past .
[ 1 ] The idea being that strontium oxide if incorporated into a frit will
be
active from 650 deg.c or so .
[ 2 ] That the frit may need B203 for this to really show a difference .
[ 3 ] That there exist eutectics for various % compositions of Sr0 and B203.
some occurring nicely at 890 deg.c .
[ If you mix the Alkaline oxides with B203 and Si02 , the m.p can be between
650 deg.c and 750 deg,c , no Alkali's needed ] .
At the temperatures of over 1130 deg.c , Sr0 is supposed to also do
amazing things , but this is also , only what I have read and have yet to
test.
At the lower temperatures [ 900 deg.c ] I have used Sr0 in a high Si02 frit
[ 68% ] to recreate an almost water clear glass [ for use as a glaze ] .
The idea being to create a high Silicate / 0% Borate , with chemical
durability and hard wearing properties .
The Sr0 also replaces Zn0 , for greater chemical resistance .
I am not sure however , that Sr0 will replace Li02 , unless the replacement
is
taking place at a temperature [ over 1130 deg.c ] , which compensates for
the less active Sr0 [ in relation to Li20 in this case ] . Or if the glaze
was really
missing a glass former like Si02 [ or B203 ] , and that was really the cure
..
I go on the idea that all glazes - no matter what temperature - should have
at least 60 % Si02 and that the higher temperatures say 1200 +deg.c ,
greater amounts of Si02 should be included - 65 to 80 % .
This is my concept of gently allowing the glass to form as opposed to the
hammer technique where greater heat allows for low Si02 glass to form
say less than 50 %.
However I am working with Frits here and the sky is the limit , as opposed
to " raw glazes ".
Khaimraj
-----Original Message-----
From: iandol
To: CLAYART@LSV.CERAMICS.ORG
Date: 17 December 2000 1:01
Subject: Substitutions in Glazes
The substitution game crops up periodically and a variety of alternatives
are suggested to solve each problem. But the latest one to catch my eye was
that of using Strontium Carbonate as a substitute for Lithium Carbonate.
I accept that Mary Lynch, who probably followed advice given on Clayart,
knows it is the right thing to do because it worked for her. It removes the
fear of the consequences of ingesting Lithium and her glazes retain their
character.
But this is an incongruous exchange to suggest in the first place because
these two substances have very different chemical and physical properties.
There are several questions which need to be addressed, for example, why is
lithium being used in the glaze in the first place? Could Sodium or
Potassium be used instead? What technical logic informed this substitution?
Without reference to the reasons why specific materials are used in glazes,
that is, a sound knowledge of fact, function and behaviour of each
substance, we have no idea if an adequate substitute product is being
created.
In this case an Alkali Earth metal is being substituted for an Alkali metal.
They have differing chemical reactivities because they have very different
numbers of electron and differing electron configurations.
For example, Lithium Carbonate melts at 723 deg Celsius but does not
decompose until it is heated to 1310 deg Celsius. This means it will assist
the sintering process and act as a low temperature melting agent. I believe
it would act like Sodium carbonate and react strongly with silica or
silicate compounds once it melted in contact with them, hastening the
dissolution of higher melting point compounds.
Strontium Carbonate does not melt at a low temperature. It decomposes,
discharging carbon dioxide at 1340 deg Celsius. The Strontium oxide which
forms has a melting point of 2420 deg Celsius. So it is not a melting flux.
I also suspect that when both have dissolved in the fluid which will become
a glaze they have differing functions. Lithium, in a vitreous silicate
solution changes the rate of thermal expansion, Strontium, because its ions
have a large diameter, will lower the glass transition temperature possibly
assisting in increasing glassiness. Glazes which contain it may also throw
crystals if cooled slowly.
I would be interested to know how Robin Hopper treats such issues in the
re-publication of the Daniel Rhodes Classics.
Ivor Lewis, Redhill, South Australia.
On a day when mercury is trying to boil
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