Lily Krakowski on sat 1 feb 03
A few days ago Ron wrote about what happens when a highly ferruginous glaze
is refired at lower temperatures. I KNOW that what happens is that it can
go from brown to quite lovely reds. Ron had a name for the phenomenon which
I failed to write down. Ron, would you tell it again, and could you explain
a bit more? You said that the crystal formation involved would affect the
silica in the glaze...would it destabilize it? Make it less
functional/resistant? Or would it simply move the silica around?
Then: Cardew mentions on p144 of his book that in an iron glaze in which the
iron cannot disolve at all, it will turn bright red even in reduction. A
special formula type is called for, and he cites Parmelee thus: .8 K20, .2
Na20, 1.17 Al203, .05 Fe203, 5.35 Si02 and then goes on a bit more in
detail. Does this red relate in any way to the change mentioned above? Or
is it just a similarity?
Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
Hank Murrow on sat 1 feb 03
On Friday, January 31, 2003, at 11:35 PM, Lily Krakowski wrote:
> A few days ago Ron wrote about what happens when a highly ferruginous
> glaze
> is refired at lower temperatures. I KNOW that what happens is that it
> can
> go from brown to quite lovely reds. Ron had a name for the phenomenon
> which
> I failed to write down. Ron, would you tell it again, and could you
> explain
> a bit more? You said that the crystal formation involved would affect
> the
> silica in the glaze...would it destabilize it? Make it less
> functional/resistant? Or would it simply move the silica around?
>
> Then: Cardew mentions on p144 of his book that in an iron glaze in
> which the
> iron cannot disolve at all, it will turn bright red even in reduction.
> A
> special formula type is called for, and he cites Parmelee thus: .8
> K20, .2
> Na20, 1.17 Al203, .05 Fe203, 5.35 Si02 and then goes on a bit more in
> detail. Does this red relate in any way to the change mentioned
> above? Or
> is it just a similarity?
>
Dear Lily; Yes, this is the same phenomenon. It is also the reason for
those 'japanese' (read no soda ash) Shinos. The iron forms crystals
at the surface of the glaze during the cooling phase, and this indian
red color can be enhanced by a period of oxidation around 1800F during
the cooling. I regularly do this for 4-6 hours for my Shinos, and have
found similar enhancement with my cone 10 oxidation firings. The Cardew
indian red has 1.17 alumina, and a silica/alumina ratio of 4.57/1,
while mine has 1.593 and a silica/alumina ratio of 3.31/1. As you can
see by visiting my Shino page at
http://www.murrow.biz/hank/shino-pots.htm
I get some pretty strong indian red color. It is also the same
phenomenon in the woodfire, where the high alumina of the body creates
good iron flashing conditions where the ash buildup is not too
strong..........calcium really kills the color.
Long winded, but helpful I'm hoping,
Hank in Eugene, looking forward to putting faces to the 'voices' in the
Clayart room.
Ron Roy on sun 2 feb 03
Hi Lily.
When crystals form in a glaze - my understanding is - most materials in a
molten state want to recrystallize during cooling - and will if given
enough time before they turn solid. Shiny glazes are called super cooled
for this reason.
As crystals form they gather silica - from where ever they can get it -
Tayor and Bull - when describing stability of glazes say - when crystalline
glazes are acid tested - you can sometimes actually see the crystals -
which have not been etched by the acid - raised above the surface of the
surrounding glaze - The crystals have robbed the surrounding glaze of
silica - making the crystals silica rich - and we know that more silica
means a glaze is more likely to be stable - the surrounding glaze - which
has had silica removed - is less stable and has been "eaten" away by the
acid.
I have tried to duplicate the Cardew quoted glaze below using the spars we
have available and it is impossible - if you read on in his description you
will see how difficult it would be to make this glaze without fritting
material.
It is not a Shino type - the iron crystals are imbedded in the glaze and
never get melted into the glaze - no CaO, MgO - any alkaline earths are
allowed - that lets out just about anything I know of in terms of materials
available. If any potter could make this glaze - well good luck. Cardew
explains how to make the frit necessary on page 147
Shinos get the red colours from iron moving to the surface during cooling
and reoxidizing there. It is because - shinos are short of silica and high
in alumina - very difficult for iron to remain in solution under those
circumstances. Putting a Shino in a bisque firing gives the process more
time proceed so you get more iron red on the surface. If you want your
shinos to be more red - hold the cooling up around 1000C - the longer you
hold them there the more "red" you are likely to get - remember - if you
are relying on the iron from the clay body to provide the colour - the iron
has to travel all the way through the white glaze.
As I understand it - when a glaze is over loaded with an oxide - like iron
- the glaze simply cannot hold the excess in solution - so during cooling
the excess tends to "float" to the surface - this is different from crystal
formation involving silica - simply too much iron, cobalt, copper barium,
MgO etc. which forms a coating on the surface - this becomes obvious
because when such glazes are acid tested the leaching of those excess
oxides is high - so in this way it is easily seen to be different.
My head hurts - RR
>> A few days ago Ron wrote about what happens when a highly ferruginous
>> glaze
>> is refired at lower temperatures. I KNOW that what happens is that it
>> can
>> go from brown to quite lovely reds. Ron had a name for the phenomenon
>> which
>> I failed to write down. Ron, would you tell it again, and could you
>> explain
>> a bit more? You said that the crystal formation involved would affect
>> the
>> silica in the glaze...would it destabilize it? Make it less
>> functional/resistant? Or would it simply move the silica around?
>>
>> Then: Cardew mentions on p144 of his book that in an iron glaze in
>> which the
>> iron cannot disolve at all, it will turn bright red even in reduction.
>> A
>> special formula type is called for, and he cites Parmelee thus: .8
>> K20, .2
>> Na20, 1.17 Al203, .05 Fe203, 5.35 Si02 and then goes on a bit more in
>> detail. Does this red relate in any way to the change mentioned
>> above? Or
>> is it just a similarity?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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