John Post on thu 13 oct 05
I read an article a while ago in CM about oxidation oilspot glazes. I
think it was by John Britt.
I looked at the unity numbers of some of his glazes and thought that Ron
and John's black glaze had some fluxes in common with the oilspot glazes
John Britt was firing.
I ran a Currie Grid using Ron and John's black as the "c" corner. In
the upper right of the grid where the glazes are stiffer, oilspot glazes
started to form at cone 6. I then tried the most promising candidate,
#8 from the grid. You can see the grid and a small bowl at this
page... http://www.wideopenwest.com/~jpost4400/
The glaze is fussy, it needs to go on really thick, and I am sure that
clay body choice will have a big effect on this glaze, as will the
firing cycle. The oilspots aren't as big as I would like either. I
think that if more whiting was worked into this glaze it might cause it
to bubble more, helping the formation of larger oilspots. It also needs
someone who has the passion to really work at it, investigating
different fluxes, clay bodies and firing schedules. It probably needs
to be approached the same way that mel worked out his Chinese iron glazes.
This glaze is not quite a slam dunk yet, it's just a place to start. It
does show that oil spot glazes can occur at cone 6.
The recipe that has worked the best so far is...
Oilspot #8 cone 6
20.1 Custer Spar
3.7 Whiting
4.6 Talc
23.8 Frit 3134
27 EPK
20.8 Flint
add
2 Bentonite
9 Spanish Red Iron ox
2 Cobalt Carbonate
Cheers,
John Post
Sterling Heights, Michigan
Taylor from Rockport on sat 15 oct 05
John:
Great work. I just might be one of those who does some follow on
testing. I'll need to get back to my copy of MC6 and see if spanish ox is
necessary or if my recently collected rusty-kiln-bottom iron oxide will do.
Do most people working with oil spot feel that it's the whiting's bubbling
action that causes oilspotting? Well I guess at cone 6 it just might be a
different animal. At any rate, good eye and thanks for sharing.
Taylor, in Rockport TX
On Thu, 13 Oct 2005 23:03:55 -0400, John Post
wrote:
>I read an article a while ago in CM about oxidation oilspot glazes. I
>think it was by John Britt.
>I looked at the unity numbers of some of his glazes and thought that Ron
>and John's black glaze had some fluxes in common with the oilspot glazes
>John Britt was firing.
>
>I ran a Currie Grid using Ron and John's black as the "c" corner. ...
Ivor and Olive Lewis on sun 16 oct 05
Dear Taylor from Rockport,
Gas evolution is a curious phenomenon. Since Calcium Carbonate =
decomposes totally with the evolution of Carbon dioxide at around 804=BA =
C it is improbable that any gas would remain to create bubbles in a =
molten glaze at 1200=BA C.
Was it last year of the year before that there was an article in CM =
about Oil Spot Glazes which made it clear that they happened in a kiln =
that was not being reduced at temperatures at or in excess of Orton Cone =
10 due to the decomposition of Red Iron Oxide. I am sure there will be a =
note in the archives about clear glazes which do not contain any =
deliberate colorants evolving a gas when fired to cone 8 in a non =
reducing kiln.
It is a great pity that institutions teaching of ceramic arts do not =
include some basic chemistry in the structure of their courses. Most of =
these problems can be solved by using Gibb's methods of Thermodynamic =
calculation for proposed chemical reactions. No knowledge of Quantum =
Mechanics required, just the abilities to apply some simple arithmetic =
processes and balance a chemical equation.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ron Roy on mon 17 oct 05
Good work John,
You might try subbing in strontium carb for the whiting - it disassociates
a lot later than calcium carb - it might be just the thing to produce more
bubbles.
RR
>I read an article a while ago in CM about oxidation oilspot glazes. I
>think it was by John Britt.
>I looked at the unity numbers of some of his glazes and thought that Ron
>and John's black glaze had some fluxes in common with the oilspot glazes
>John Britt was firing.
>
>I ran a Currie Grid using Ron and John's black as the "c" corner. In
>the upper right of the grid where the glazes are stiffer, oilspot glazes
>started to form at cone 6. I then tried the most promising candidate,
>#8 from the grid. You can see the grid and a small bowl at this
>page... http://www.wideopenwest.com/~jpost4400/
>
>The glaze is fussy, it needs to go on really thick, and I am sure that
>clay body choice will have a big effect on this glaze, as will the
>firing cycle. The oilspots aren't as big as I would like either. I
>think that if more whiting was worked into this glaze it might cause it
>to bubble more, helping the formation of larger oilspots. It also needs
>someone who has the passion to really work at it, investigating
>different fluxes, clay bodies and firing schedules. It probably needs
>to be approached the same way that mel worked out his Chinese iron glazes.
>
>This glaze is not quite a slam dunk yet, it's just a place to start. It
>does show that oil spot glazes can occur at cone 6.
>
>The recipe that has worked the best so far is...
>
>Oilspot #8 cone 6
>20.1 Custer Spar
>3.7 Whiting
>4.6 Talc
>23.8 Frit 3134
>27 EPK
>20.8 Flint
>add
>2 Bentonite
>9 Spanish Red Iron ox
>2 Cobalt Carbonate
>
>
>Cheers,
>John Post
>Sterling Heights, Michigan
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
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>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on thu 20 oct 05
Dear John ,
The best place to start solving this puzzle is to do the thermodynamic =
calculations.
There will be an equilibrium temperature at which the change from RIO to =
Black "FeO" (This is not a stoichiometric compound !) proceeds both ways =
at equal velocity. This will confirm if you can achieve Oil Spot Tenmoku =
at cone 6.
Running the ruler along the lines of the Ellingham diagram for a rapid =
answer seems to suggest that at the Mp of "FeO", Heats of Formation =
values favour the creation of that compound from RIO. Below that =
temperature it becomes necessary to reduce Oxygen Partial Pressure to =
encourage the decomposition, At 1200=BA C, Aka Cone 6, an Oxygen partial =
pressure of 1*10 exp minus 7 would encourage the decomposition of RIO. =
(At 1350=BA C it would seem this is likely to happen at atmospheric =
pressure.) You should be able to relate this to an Oxyprobe reading. =
From the table in my possession I would suggest a value greater than 3 =
at cone 6. There is no need to introduce Carbonaceous stuff into an =
electric kiln. Flushing with Nitrogen or Argon once maturing temperature =
is attained would do the trick and be relatively safe.
Best regards,
Ivor
mtigges@NOSPAM.SHAW.CA on fri 21 oct 05
I fired John Posts recipe yesterday. I made three tests. All small
little bowls. My results were identical to Johns. Little small
polygonal specks rather than spots of iron on the bottom of the bowl.
Virtually none on the walls. But I think that is because the glaze
was not as thick there.
Which leads me to elaborate on application. John mentioned that this
glaze needs to be thick. So, my three tests were different
thicknesses. One, two and three coats. The one is horrible. An ugly
featureless black which isn't glossy enough nor matte enough to
interest anyone in the pot. It's just murky. The three crawled
severely, too thick, and notably, no oil-spots, maybe they didn't get
to the surface? The two dips is the good one. I have no idea what
the SG of my glazes are, I've always just eye-balled it.
Ron Roy has kindly reformulated the recipe to use strontium instead of
whiting. His thinking is that strontium bubbles later than any other
carbonate and he is hoping that these bubbles carry the iron to the
surface. As I understand it this is a different mechanism than
traditional oil spots, but if there is another mechanism that can
carry the iron up to be deposited on the surface it doesn't matter.
On Fri, Oct 21, 2005 at 11:22:10AM -0500, Ron Roy wrote:
>
> So here is the recipe with strontium instead of whiting.
>
> Custer - 19.0
> Strontium - 5.5
> Talc - 4.5
> Frit 3134 - 23.5
> EPK - 27.0
> Silica - 20.5
> Total - 100.0 + Bentonite, iron and cobalt.
>
> Be sure to let us know if it does work or not - please.
>
I am not sure when I will be able to mix and test this. But it
shouldn't be too long. Xmas craft fairs are fast approaching and need
to get cracking!
Best regards,
Mark.
NB: It should be mentioned that my supplier does not carry spanish red
iron oxide as John Post used. My tests were with regular red iron
oxide. It would seem that the spanish is not cardinal.
Hank Murrow on fri 21 oct 05
On Oct 21, 2005, at 8:57 AM, mtigges@NOSPAM.SHAW.CA wrote:
> I fired John Posts recipe yesterday. I made three tests. All small
> little bowls. My results were identical to Johns. Little small
> polygonal specks rather than spots of iron on the bottom of the bowl.
> Virtually none on the walls. But I think that is because the glaze
> was not as thick there.
Dear Mark and John;
Britt's article clearly suggests a way to get the required thickness
right (up to 5/16"!) for these small bowls. He dips once the whole pot,
then again thirds down from the rim, and a third time one third of the
way down. This gives the glaze 'room to run' a bit.
> Which leads me to elaborate on application. John mentioned that this
> glaze needs to be thick. So, my three tests were different
> thicknesses. One, two and three coats. The one is horrible. An ugly
> featureless black which isn't glossy enough nor matte enough to
> interest anyone in the pot. It's just murky. The three crawled
> severely, too thick, and notably, no oil-spots, maybe they didn't get
> to the surface? The two dips is the good one. I have no idea what
> the SG of my glazes are, I've always just eye-balled it.
Maybe you need to calcine a portion of your clay to stop the crawling,
and perhaps stiffen the glaze a bit to slow down the runs.
>
> Ron Roy has kindly reformulated the recipe to use strontium instead of
> whiting. His thinking is that strontium bubbles later than any other
> carbonate and he is hoping that these bubbles carry the iron to the
> surface. As I understand it this is a different mechanism than
> traditional oil spots, but if there is another mechanism that can
> carry the iron up to be deposited on the surface it doesn't matter.
Actually, if you follow Britt's thinking, it is the iron itself which
reduces around cone 9 in an otherwise oxidizing atmosphere. It works
best in a gas kiln which can be fired in true oxidation, because most
electrics fire in neutral unless they have an air supply coursing
through the wares during the firing. I have gotten very beautiful
results from following John Britt's recommendations, as found here:
http://www.ceramicsmonthly.org/mustreads/oilspot.asp
Cheers and Good Luck!
Hank in Eugene
www.murrow.biz/hank
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