Lawrence Carter on sun 22 nov 98
Sorry for delay in answering your question. I obviously have no way of
knowing how much expertise you have producing what I call crystalline
glazes. See my website at pusey-footin.demon.co.uk, this may give you
some idea of my aims.
I would ideally like to produce glazes with larger crystalls about two
to three inches across, all on a contrasting background.I would like
these crystalls to be clearly differentiated from their background
rather than several small crystals swamping the surface of the pot.
Derek Clarkson from Lancashire UK produces glazes which I find very
attractive, I envy his skill and indeed he is aware of that.
Now my reasoning about the melt point. My experiment results with
crytalline glazes at high temperature in a computer controlled 2.6
cubic feet electric kiln suggests that there must be an optimum
temperature for actual crystal growth. I believe this reaction is a
result of a combination of Zinc Silicate and Lead Titanate ( reference
Hamer 'The Potters Dictionary') other glaze ingredients including a
variety of oxides.
My experience in practice seems to suggest the melt temperature for my
particular glaze recipes is between 1260 and 1285 Deg C. When the glaze
melt is reached ??? the glaze flows down the pot and in my case past a
pedestal into a saucer.Just imagine paint drying, there is a moment when
it is still fluid but not actually flowing, one can easily determine
this by touch, unfortrunately one cannot do the same with glaze.I reason
if it were possible for me to determine the actual flow point of my
glazes then I would be in a better position to encourage crytalline
growth and grow larger crystals.
In order to improve the finished quality of my glazes I ball mill them
for up to four hours, as a result this tends to reduce the final melt
temperature by 20 to 40 deg C, this is guess work and the root of my
problem. I cannot afford capital investment for sophisticated methods of
testing the method would need to be based on either calculation or
practical testing, always bearing in mind there are many variables.
Perhaps I am wasting my time and seeking the impossible !!
Crazy Crystalline Glazer
In message <001801be111b$b48d4960$1e896ac6@pavilion>, Craig Martell
>Good point about steel rods in an electric kiln. Not a good idea at all!
>There is something that you could use to extract draw trials but I don't
>have any suggestions. Fortunately for me, I fire gas and don't have to
>worry about this.
>Predicting the fusion point of your glaze with a +or- 20C margin would be
>quite a feat. You would have to look at the composition of the glaze and
>view all the materials in regard to when they begin fusion. You would also
>be facing the possibility of eutectic points which are hard to predict and
>sometimes cause oxides to melt at very unexpected temps. You might look
>around and try to find a lab that has a Thermal Gradient Furnace. This
>device subjects a glazed sample to temp gradients and would give you
>information on your glazes fusion point. This of course would cost a few
>quid. Ask some of the people that you purchase materials from and they
>might be able to steer you to a lab, or know a method to predict fusion
>points. I sure don't, sorry!
>What is your purpose in finding the fusion point, if you don't mind me
>regards, Craig Martell in Oregon
Craig Martell on fri 27 nov 98
Hello Again Lawrence:
My experience with crystalline glazes, is pretty much didley squat! I've
read about them, seen them, appreciated them, but that's as far as I go with
macro crystalline glazes. Micro crystals I can do. Well, usually.
I understand your interest in gaining some info on the melt and flow points
of your glaze, and I don't think you are crazy, and I don't think it's
impossible for you to attain your goal. My approach would be the empirical,
trial and error method. You already have a reasonable parameter to work in
and with some trial and hopefully not too much sacrifice, you should be able
to ascertain the correct "hold" temp to form the crystals that you want. I
think that attempting to calculate the melt point of your glaze would
require at least the same amount of work as the empirical method. What we
have on record as analysis for materials is often not exactly what comes out
of the ground. Then you have the milling factor to deal with as well as
possible eutectic points, ad. nauseum!
Anyway, I wish you the best with your work and sorry I don't have anything
positive to contribute.
Craig Martell in Oregon