iandol on sun 16 feb 03
Dear Ababi=20
You say...<<"There is Val Cushing porcelain for raku. Ivor Lewis wrote =
that it was not a true porcelain, He might be right, but it fires white =
in raku as well as ^6 and ..">>...
I cannot recall making any comment about Val Cushing or his work.
Perhaps you misunderstood me if I did say, that until a clay body has =
been fired so that it is Translucent it could not be classed as =
Porcelain. Whiteness is accepted as another hallmark, but total =
reflection or transmission of the complete visible spectrum is possibly =
impossible, hence a variety of weak hues as well as lowering of the =
value of the light we perceive from some examples and samples.
One thing which does confuse the issue is to believe that a vitrified =
clay is not translucent in thick sections. To think that the effect is =
only present in "Eggshell" thin sections. Thick sections of Porcelain =
which have been fired to maturity exhibit a radiance from their surface =
because light which has entered the clay is reflected internally and =
returns to the viewer. This would not happen if the surface were opaque. =
This quality distinguishes porcelain from other high fired white =
stoneware bodies and from white raku.
Another thing to consider are the optical properties of the materials we =
use.=20
Quartz (silica), Corundum (Alumina), Felspar (Orthoclase-Potassium =
Alumino-Silicate) and Kaolinite (Alumino-Silicilic acid) are all =
transparent materials with respect to visible light, as is Mullite, a =
product of the thermal degradation of Kaolinite. So, ideally, all of the =
clay bodies we make and use should respond given sufficient time and an =
adequate temperature to become porcelain.
That they do not is due to three circumstances. The first is the =
impurities which discolour the clays and minerals, in the main due to =
Iron Oxides or minerals containing iron. The second is our failure to =
eliminate air inclusions once the flux in the clay is activated. The =
third is the discrepancy between the refractive indices of the resulting =
components in the fabric of the fired clay, that is, the way light =
behaves as it passes across interfaces between crystals or crystals and =
voids
Our solutions are: For the first, to select materials which are as pure =
as possible. For the second, to fire high enough and long enough to =
ensure good vitrification and the elimination of as much porosity as =
possible. For the third, well, that is something which we have not =
really thought about yet, but the solution is pretty obvious.
Hoping things are well with you in the Desert.
Best regards,
Ivor Lewis. Redhill, South Australia.
Lois Ruben Aronow on sun 16 feb 03
I am confused.....
does this mean that when i throw super thin, or carve, that the clay I
use is porcelain, as it is translucent? And when I put colored engobe
on the same claybody, thrown to the same thinness, that it is NOT
porcelain, as it is no longer translucent? Makes no sense to me.
>Perhaps you misunderstood me if I did say, that until a clay body has =
been fired so that it is=20
>Translucent it could not be classed as Porcelain. Whiteness is accepted =
as another hallmark,=20
>but total reflection or transmission of the complete visible spectrum is=
possibly impossible,=20
>hence a variety of weak hues as well as lowering of the value
> of the light we perceive from some examples and samples.
>
--------------------------------------------
=46ine Craft Porcelain - New and Updated for 2003!!
http://www.loisaronow.com=20
Ababi on sun 16 feb 03
I think I understand you but your English is too good for me.. : I will try to explain this
way:
Limoge porcelain gives me beautiful crystals in my ^6 or ^7 crystal glaze recipe.
So is the English porcelain of Laguna. In this temperature it is not "fully porcerained"
It is like driving a race car in town where you cannot have the highest speed. It is good
but defiantly it can do better and show itself better in the real race speed!
I am not using the 25/4 because it does not give me nice crystals. In the next crystal
firing I belive in the end of this week I will test two new kinds of Kaolain.
We in the desert fine and sad!
Thank you Ababi
---------- Original Message ----------
>Dear Ababi
>You say...<<"There is Val Cushing porcelain for raku. Ivor Lewis wrote that it was not
a true
>porcelain, He might be right, but it fires white in raku as well as ^6 and ..">>...
>I cannot recall making any comment about Val Cushing or his work.
>Perhaps you misunderstood me if I did say, that until a clay body has been fired so
that it is
>Translucent it could not be classed as Porcelain. Whiteness is accepted as another
hallmark,
>but total reflection or transmission of the complete visible spectrum is possibly
impossible,
>hence a variety of weak hues as well as lowering of the value of the light we perceive
from some
>examples and samples.
>One thing which does confuse the issue is to believe that a vitrified clay is not
translucent in thick
>sections. To think that the effect is only present in "Eggshell" thin sections. Thick
sections of
>Porcelain which have been fired to maturity exhibit a radiance from their surface
because light
>which has entered the clay is reflected internally and returns to the viewer. This
would not
>happen if the surface were opaque. This quality distinguishes porcelain from other
high fired
>white stoneware bodies and from white raku.
>Another thing to consider are the optical properties of the materials we use.
>Quartz (silica), Corundum (Alumina), Felspar (Orthoclase-Potassium
Alumino-Silicate) and
>Kaolinite (Alumino-Silicilic acid) are all transparent materials with respect to visible
light, as is
>Mullite, a product of the thermal degradation of Kaolinite. So, ideally, all of the clay
bodies we
>make and use should respond given sufficient time and an adequate temperature to
become
>porcelain.
>That they do not is due to three circumstances. The first is the impurities which
discolour the
>clays and minerals, in the main due to Iron Oxides or minerals containing iron. The
second is our
>failure to eliminate air inclusions once the flux in the clay is activated. The third is the
>discrepancy between the refractive indices of the resulting components in the fabric
of the fired
>clay, that is, the way light behaves as it passes across interfaces between crystals
or crystals
>and voids
>Our solutions are: For the first, to select materials which are as pure as possible.
For the
>second, to fire high enough and long enough to ensure good vitrification and the
elimination of
>as much porosity as possible. For the third, well, that is something which we have
not really
>thought about yet, but the solution is pretty obvious.
>Hoping things are well with you in the Desert.
>Best regards,
>Ivor Lewis. Redhill, South Australia.
iandol on thu 20 feb 03
Dear Lois Ruben Aronow,
What is narrow about the definition I use? Please explain yourself.
You could throw your clay, if it is compounded to be a porcelain, a yard =
thick and if it fired to maturity and has vitrified is it is still =
Porcelain. It will look opaque. The fact that it is so thick that all =
the light which enters is scattered so that none seems to pass through =
is irrelevant.
I made a heavy porcelain Mortar and a Pestle. Couldn't care less about =
translucency, just wanted the hardness and the purity. but it rings like =
a bell~!
Just remember, scattered light is reflected back towards you from within =
the surface of porcelain. This can increase the radiance of the glazes =
put on the surface of Porcelain, especially those glazes which are =
transparent. If you wish to exploit this fact of nature you might =
reconsider the idea of using an opaque Engobe.
It is said that all clays which are fired to vitrification should be =
transparent. Scattering and adsorption prevent this from happening.
I accept the standard definition. Vitreous, Mechanically Hard, =
Translucent, Sonorous and white or nearly so. No ambiguities, no =
problems. Just made sure I was fully aware of why this should be so.
Best regards,
Ivor Lewis.
Ps. Read Kingery and Vandiver on Ancient Ceramics. Sorry, can't recall =
the precise title.
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