Edouard Bastarache on sat 23 aug 03
Hello ED,
In response to your preceding request and the interrogation by Ivor, here is
a short text (Heheheh!) giving some explanation: If you have too many
problems to translate this text, do not hesitate to ask me for
advice(Hehehehe!)
Short text :
Ivor is right, my article gives information without examples nor documents
in proof apart from my own word. I put forward explanations which do not
seem credible to him because he does not have like me experience with
glazing significant numbers of pieces. In 25 years of career I have observed
the
results of firing more than 30 million pieces.
Here, we are specialized in the production of fine feldspathic earthenware
in bi-firing (bisque firing, glaze firing), this product is particular
because
the bisqued pieces contain alkaline silicates which are hydrated easily and
with
very strong bonds with H2O During the glaze firing, the bisqued pieces
release water bond at very high temperature (between 750 and 950°C).
If at this time the glaze is already sufficiently molten on the surface,
released vaporized water produces a high pressure and there are
bubbles of significant size which disfigure the products and the production
is lost.
This phenomenon is related to the feldspathic nature of the clay, all those
who produce this type of clay know this problem well. In addition, this clay
brings to us many interesting properties which makes it that we refuse to
change some.
All the art of making a success of this type of production thus consists
in regulating the curve of firing and the properties of the glaze.The
majority
of glazes for feldspathic earthenware have points of softening located
near 610 to 660 °C, which does not make things easy. It is thus
necessary to generally slow down the speed of the rise of temperature "to
outgas"
carefully before the point of "closing" of the glaze (moment at which the
molten
glazed surface can lock up gases coming from the shard).
Thus when you are facing this type of problem, you end up noticing quickly
that batches function well and that others do not.
That quickly brings you after the control of the conditions of manufacture
to blame the methods of application used for glazing.
A simple test makes it possible to show the influence of the application
type
on the precocity of glaze vitrifivation :
One glazes two batches of identical bisqued wares made up of feldspathic
earthenware
clay with the same batch of glaze, one by dipping and the other by spraying
while making sure to deposit the same weight of dry glaze on each bisqued
ware (small calculation to make...).
The amount of glaze deposited must allow a suitable thickness on the piece
after firing (use the standard of manufacture).
One bakes both batches of pieces together in the kiln. If nothing has
occurred and that both batches do not have bubbles, one
starts again, but firing a little more quickly. One repeats the
experiment until defects appear. At this time, you will note each
time it is the batch glazed by dipping which has bubbles.
Same batches of bisqued wares, same batch of glaze, same weight of glaze
per piece, same kiln, same firing, but a different result according to the
mode of application.
From there, you can manage your manufacturing by selecting the batches
to be fired according to the mode of application of the glaze and by
adapting the firing curve.This will facilitate the optimization of firing
times
without harming quality.
My article on the effects of wet glaze surface tension at the time
of application and according to the mode used, conveys this
experience.
This principle also applies to ceramic clays according to their mode of use
(casting, atomization, plastic clay). The consequences are different but one
finds very similar differences in behaviour specific to the arrangement and
the distribution of the particles according to the mode of use concerned.
Cordialement, Denis
Smart.Conseil
Le site Français dédié aux passionnés de céramique
El sitio Francés dedicó a amantes de cerámica
The French site dedicated to ceramics lovers
smart2000@wanadoo.fr
http://perso.wanadoo.fr/smart2000/
(Langue Française par défaut : Tutoiement à la "Québécoise")
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