Janet Kaiser on fri 25 mar 05
Yikes! Sounds to me "insulator porcelain" is what you need to look for and=
source Mike. That is an industrial clay mix/product not usually available=
from a studio potter's ceramic supplier. Have you tried getting disused or=
scrapped electric fires from charity shops or attic sales? Would recycling=
those element bars be a solution?
I do not know which country you are in, but in the USA and UK i.e.=
countries which do not historically have a hard porcelain tradition, there=
are specialist producers who have developed their own electro-technical=
porcelain. In countries like Japan, Central/Western Europe and Russia on=
the other hand, it is just another branch of their porcelain production...=
They may produce tea sets at one end and insulators at the other end of=
the factory floor. Since they were accustomed to producing white ware with=
extremely hard glaze, totally resistant to acids and scratching by steel=
cutlery, it was obvious they could extend their production. USA & UK had=
to develop this production from scratch, so methods were quite different.=
You can always tell were anything in this branch of ceramics is made...=
The UK/US are a cream or ivory in colour, all the others are white. At=
least that is how it used to be up to 20-30 years ago! What follows is=
also probably out of date by now, but was what I learned way back when.=
Whatever...
However techno-porcelain is made, it must possess three main=
characteristics:
1. impervious to moisture
2. unattackable by atmospheric conditions
3. smooth and easily cleanable surface/s
I am afraid my brain tends to turn off when being told about highly=
technical aspects of ceramic production and this is buried very deep too!=
But I do know that appearance is of absolutely no importance in this=
field, for example there is no need for bright white, so kaolin can be=
replaced by more plastic ball clay or similar. Translucency is equally not=
a requisite, so feldspar can be reduced and silica increased, etc. etc.=
These alterations improve the MECHANICAL STRENGTH, reduce the brittleness=
associated with table ware and increase the toughness and resistance to=
temperature change.
Of course the ELECTRICAL PROPERTIES are even more important... They depend=
on many factors such as body composition, method of shaping, method of=
firing and glaze characteristics. They must be achieved without reducing=
the mechanical strength. We are talking "an exact science" here where=
extremely accurate measurements are necessary from beginning to end with=
no leeway allowed. I believe (do not know for sure) than it is almost all=
either slip moulded or dry pressed, with little plastic clay used=
nowadays. Whichever main method is employed, plastic clay is first dried,=
then passed through a "disintegrator" and sieved through rotating cylinder=
to separate different sized "grains". Forming or shaping is by jollying,=
casting, turning, extruding, pressing... Naturally all mechanised, not a=
hand in sight!
Drying is a bugger even for this highly developed and controlled=
industry... You are looking at uneven thicknesses and rather plastic=
bodies which are programmed to crack. Ultra slow drying is the answer, but=
this industry also speeds up the process by drying in a hot, humid=
atmosphere! They use either chamber or tunnel humidity driers. At least=
the firing appears simple! Just once.
I cannot understand much more from my notes... Other technical stuff means=
little to me personally and does not look at all like it would apply to a=
potter! For example what is this supposed to mean...
"1. Volume Resistivity and Surface Resistivity
2. Breakdown strength (dielectric strength)
3. Power factor*
4. Dielectric constant (permittivity)*
*particular importance in condenser design and high frequency work=
generally."
I think I need go lie down...!! Anyway, I have to say that this is a long=
way around advising you to approach a manufacturer of elements similar to=
what you are trying to make. They may be willing to help you further. If=
not, you are back to looking at a "hard paste" porcelain which fires up to=
1300-1400 =B0C oxidation and has an extremely hard, very smooth glaze...=
Yet, those ceramic supports to elements on electric fires were not glazed,=
so you can probably forget all I have told you! I do not pretend to=
understand either your requirements or the viable alternatives open to=
you. But I wish you good luck and would be interested in hearing what you=
achieve.
Good luck!
Janet Kaiser
*********** REPLY SEPARATOR ***********
On 24/03/2005 at 12:05 Mike Flaherty wrote:
>i'm making structures to hold electric heating elements. they will be
>heating up very unevenly and quickly, and so far all the pieces that i
>have made cracked. i'm hoping to find a clay that won't crack as it is
>being heated.
>
>mike
>
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