Edouard Bastarache on tue 21 jan 03
----- Original Message -----
To: Edouard Bastarache
Sent: Tuesday, January 21, 2003 4:41 PM
Subject: Re: Zinc Crystal Fundamentals
Salut ED, voici pour Clayart si tu as le temps de traduire :
Hello Ivor, hello all,
My SiO2/ZnO ratios go from 2.7 to 3, but it is not fundamental. It is
necessary also that the amount of Alumina be lower or equal to 0.05. If not,
beyond, if it is > to 0.150, the viscosity of the glaze will be too high to
allow the fast growth of crystals during the stage.
Then an alcaline mixture (K + Na + Li) sufficiently reactive should be
carried out to favour low viscosity and to establish a vitreous phase rather
quickly. The presence of the three alkaline oxides, well proportioned,
allows early eutectics which facilitate the later development of crystals.
Little or no boron is necessary.
Calcium, strontium and the barium are elements which play a part especially
in the shape of the crystals but little in their development. Finally, one
needs crystal nuclei in sufficient number and not too numerous; it is
necessary to control well the kiln and firing ( I recommend a
programmer-regulator allowing up to 30 or 40 segments per program for
managing well the effects of temperature variations on crystal growth, this
is with an electric kiln obviously) and to take care to use a well adapted
clay composition l (purity of the clay, porcelain recommended).
It is necessary to bring the glaze to a stage of viscosity allowing good
crystalline growth with a reasonable number of « germs » to obtain a
harmonious ratio crystals/surface of the pieces. For that, it should
particularly be prevented that the highly alkaline glaze does not digest too
much the shard (rich in silica and alumina), which would prevent it from
maintaining its low viscosity and would slow down considerably the
development of crystals.
The whole is a question of speed and perfect control of heating (and
cooling). For that, the kiln must be built consequently and dedicated to
this type of firing, it is a key point for succeeding in crystal making.
The recipe, the firing curve, the clay, the kiln and especially the guy who
makes everything work are fundamental criteria to succeed in great quest of
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