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Need Help - Cone 6 Oil Spot Glaze


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#1 sbesse

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Posted 16 July 2011 - 10:39 AM

I recently attemped to make and fire a Cone 6 Oil spot glaze (medium speed, cone 6). Clay is speckled buff. The glazed turned brown with no oil spots. Hydrometer reading was around 53%. I held the pot in the glaze for a 3 count. I am wondering if there is anything I can do with the glaze to get better results - I thought about adding an additional 2% copper carbonate. Does the thickness of the glaze matter? Would I get a different result if I fired to Cone 5? or on a slow speed firing? I have listed the recipe below. Does anyone have a good oil spot glaze recipe they would be willing to share?

Hal’s Oil Spot Glaze C5-6

Silica 19.5%
Bone Ash 9%

Iron Oxide 9.7%
Kaolin (EPK) 5.7%

Nepheline synite 44%

Talc 5.7%

Whiting 6.5%

Increased oil spots add:
Cobalt .15%
Copper Carbonate .5%
Rutile 5%
Tin 1%

thank you!

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#2 Marcia Selsor

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Posted 16 July 2011 - 01:52 PM

What was your firing schedule like?
Oxidation or reduction?
Clay body?
There are many factors before knowing how to answer.

Marcia
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#3 JBaymore

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Posted 16 July 2011 - 03:36 PM

Oil spot is heavily about the firing cycle.

best,

...............john

John Baymore
Adjunct Professor of Ceramics; New Hampshire Insitute of Art

Former Guest Professor, Wuxi Institute of Arts and Science, Yixing, China

Former President and Past President; Potters Council
 

http://www.JohnBaymore.com

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#4 Pres

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Posted 17 July 2011 - 09:11 AM

I recently attemped to make and fire a Cone 6 Oil spot glaze (medium speed, cone 6). Clay is speckled buff. The glazed turned brown with no oil spots. Hydrometer reading was around 53%. I held the pot in the glaze for a 3 count. I am wondering if there is anything I can do with the glaze to get better results - I thought about adding an additional 2% copper carbonate. Does the thickness of the glaze matter? Would I get a different result if I fired to Cone 5? or on a slow speed firing? I have listed the recipe below. Does anyone have a good oil spot glaze recipe they would be willing to share?

Hal’s Oil Spot Glaze C5-6

Silica

19.5%

Bone Ash

9%

Iron Oxide

9.7%

Kaolin (EPK)

5.7%

Nepheline synite

44%

Talc

5.7%

Whiting

6.5%

Increased oil spots add:

Cobalt

Copper Carbonate

Rutile

Tin

.15%

.50%

5.0%

1.0%


thank you!




Years ago, I was using an oil spot black that was a brown on all of my test tiles. I did a mixed firing with the kids, and so everything was the same. One day however, I mixed in too much water, and the glaze was too thin in the 5 gal. bucket. I told the kids to use a double dip, and not to leave for the full count(we had discussed double dipping in demonstrations). When the kiln was opened with 3 pots in it with the Oil Spot black-two had significant black/brown spotting, and a third was basically all brown. In the future I place pots on the shelf area where the first two were, and never in other areas-problems solved. You might try it.
Simply retired teacher, not dead, living the dream. on and on and. . . . on. . . . http://picworkspottery.blogspot.com/

#5 bciskepottery

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Posted 17 July 2011 - 10:38 AM

Here is a combination ^6 Oil Spot glaze from a John Britt workshop. Fires in oxidation.

Oil Spot Combo #1 (three coats), specific gravity 165
G-200 feldspar: 47.83%
Silica: 23.91%
Whiting: 17.39%
EPK: 10.87%
Red Iron Oxide: 9.78%

Oil Spot Combo #2 -- Cover glaze (apply over #1) (two coats)
Custar feldspar: 30%
Gerstley Borate: 30%
Silica: 25%
EPK: 5%
Zircopax: 10%

I've not mixed/used this one, so I cannot attest to the results.

#6 sbesse

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Posted 17 July 2011 - 08:43 PM

Hi Marcia, thanks for your reply/questions

I fired on medium speed (my Skuttt kiln is computer controlled so I use the medium speed setting - I don't know what the firing schedule is)
Oxidation Firing
Laguna Speckled Buff (Cone 6)

Today I added 2% additional copper carbonate to the glaze a did a couple of test tiles altering the thickness of the glaze - we will see what happens. Any insight is appreciated - thanks

Shelley


What was your firing schedule like?
Oxidation or reduction?
Clay body?
There are many factors before knowing how to answer.

Marcia



#7 sbesse

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Posted 17 July 2011 - 08:45 PM

Would you recommend a slower firing? I use a Skutt Kiln (I just use the preset computer setting - usually medium speed)


Oil spot is heavily about the firing cycle.

best,

...............john



#8 sbesse

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Posted 17 July 2011 - 08:46 PM

Thank you! - if I try it I will post my results

Shelley

Here is a combination ^6 Oil Spot glaze from a John Britt workshop. Fires in oxidation.

Oil Spot Combo #1 (three coats), specific gravity 165
G-200 feldspar: 47.83%
Silica: 23.91%
Whiting: 17.39%
EPK: 10.87%
Red Iron Oxide: 9.78%

Oil Spot Combo #2 -- Cover glaze (apply over #1) (two coats)
Custar feldspar: 30%
Gerstley Borate: 30%
Silica: 25%
EPK: 5%
Zircopax: 10%

I've not mixed/used this one, so I cannot attest to the results.



#9 sbesse

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Posted 17 July 2011 - 08:49 PM

Thank you - I will try a couple of few different options when I fire next time

Shelley


I recently attemped to make and fire a Cone 6 Oil spot glaze (medium speed, cone 6). Clay is speckled buff. The glazed turned brown with no oil spots. Hydrometer reading was around 53%. I held the pot in the glaze for a 3 count. I am wondering if there is anything I can do with the glaze to get better results - I thought about adding an additional 2% copper carbonate. Does the thickness of the glaze matter? Would I get a different result if I fired to Cone 5? or on a slow speed firing? I have listed the recipe below. Does anyone have a good oil spot glaze recipe they would be willing to share?

Hal’s Oil Spot Glaze C5-6

Silica

19.5%


Bone Ash

9%


Iron Oxide

9.7%


Kaolin (EPK)

5.7%


Nepheline synite

44%


Talc

5.7%


Whiting

6.5%


Increased oil spots add:

Cobalt

Copper Carbonate

Rutile

Tin


.15%

.50%

5.0%

1.0%



thank you!




Years ago, I was using an oil spot black that was a brown on all of my test tiles. I did a mixed firing with the kids, and so everything was the same. One day however, I mixed in too much water, and the glaze was too thin in the 5 gal. bucket. I told the kids to use a double dip, and not to leave for the full count(we had discussed double dipping in demonstrations). When the kiln was opened with 3 pots in it with the Oil Spot black-two had significant black/brown spotting, and a third was basically all brown. In the future I place pots on the shelf area where the first two were, and never in other areas-problems solved. You might try it.



#10 JBaymore

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Posted 17 July 2011 - 10:18 PM

Would you recommend a slower firing? I use a Skutt Kiln (I just use the preset computer setting - usually medium speed)



Oil spot is heavily about the firing cycle.

best,

...............john



The mechanism of traditional oil spot (which is high fire, not Cone 6 range) is based on the instability of the red iron oxide molecule above about 2250 F. Orton Cone 6, if you are using the large cones, is going to barely reach that temperature. The slower you fire the LOWER the end point temperature will be at any given cone. (Remember, cones measure heat work, not tmperature.) At 270F per hour rise, Cone 6 end-point is 2266-ish F. So it will start to break the bonds.... but it is "close".

I'd suggest that you fire the last about 200 F of the up cycle at a fast rate.........like 270 to 300 F per hour (if your kiln will do it). That will make sure that you end at a "hot" remperature for the Cone 6 dropping. Then play with the soak period at the end to allow time for the evolution of the oxygen gas to bubble out of the underlying glaze layer and bring the "spots" to the surface. It will require some testing for you to see how long a soak gives you what type/size of spots.

best,

.....................john

John Baymore
Adjunct Professor of Ceramics; New Hampshire Insitute of Art

Former Guest Professor, Wuxi Institute of Arts and Science, Yixing, China

Former President and Past President; Potters Council
 

http://www.JohnBaymore.com

http://www.nhia.edu/...ty/john-baymore


#11 sbesse

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Posted 18 July 2011 - 09:39 PM

Thank you John


Would you recommend a slower firing? I use a Skutt Kiln (I just use the preset computer setting - usually medium speed)



Oil spot is heavily about the firing cycle.

best,

...............john



The mechanism of traditional oil spot (which is high fire, not Cone 6 range) is based on the instability of the red iron oxide molecule above about 2250 F. Orton Cone 6, if you are using the large cones, is going to barely reach that temperature. The slower you fire the LOWER the end point temperature will be at any given cone. (Remember, cones measure heat work, not tmperature.) At 270F per hour rise, Cone 6 end-point is 2266-ish F. So it will start to break the bonds.... but it is "close".

I'd suggest that you fire the last about 200 F of the up cycle at a fast rate.........like 270 to 300 F per hour (if your kiln will do it). That will make sure that you end at a "hot" remperature for the Cone 6 dropping. Then play with the soak period at the end to allow time for the evolution of the oxygen gas to bubble out of the underlying glaze layer and bring the "spots" to the surface. It will require some testing for you to see how long a soak gives you what type/size of spots.

best,

.....................john



#12 Marcia Selsor

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Posted 19 July 2011 - 09:49 AM

Why are you adding copper carbonate to this recipe?
I would focus of the oil spot recipe and work it out with the firing schedule. John gave good suggestions on slowing the firing. I would add that you might slow cool as well. I have seen vast improvements in iron glazes in oxidation with slow cool. When you have too many variables, you won't know/understand what works exactly when you achieve what you want.
Marcia
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#13 tkmog

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Posted 21 December 2016 - 11:32 AM

Yu may be reducing the firing which may destroy the effect. Are you firing in a gas kiln? If you are - do not reduce. If you are firing in an electric kiln the problem is probably something else.



#14 PeterH

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Posted 21 December 2016 - 02:43 PM

You may find this discussion -- and Norm's comments -- interesting:

http://cone6pots.nin...spot-mfa-thesis



#15 curt

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Posted 04 January 2017 - 08:00 AM

Hey all you oil-spotters....

 

I just posted these pics over at the currie tile thread (because they are of a currie tile), but given the interest in this thread thought I would put them up here as well, with a few comments about what they MAY show.  If nothing else it is a good example of how the currie tile approach can facilitate glaze experiments.  

 

These are pics of a basalt glaze fired in Cone 10 oxidation.    I did not expect oilspots, was not hunting them, but there they are, they just appeared.  This firing was in my small electric test kiln, which is not particularly well insulated and normally cools very fast.  However, I have programmed it to fire down slowly to 1100C (I think), so I guess it may be a slow cool after all.

 

A few observations:

 

First let me say, I am only showing you the oxidation tiles here, but I did all this on identical claybodies in reduction and there was no evidence whatsoever of any oilspotting.

 

The first tile is a white (so-called) porcellanous stoneware body, the second is an identical standard currie set of glazes fired on an irony buff speckly body.

 

Cone 10 Currie Tile with Oilspots On White Body - Oxidation
Cone 10 Curie Tile with Oilspots On Iron body in Oxidation

 

The oilspotting phenomenon seems to be fairly dependent on having just the right amount of alumina and silica.  The size and number of oilspots changed noticeably as alumina and silica changed.  Oilspots ONLY appeared in cells 18, 19, 22, 23, 27 and 28.  They stopped abruptly

 

Focusing on cell 23, which seems to have the biggest boldest oilspot blossoms, although the piece of kiln element is melted into the cell on the white body, when I look around the edges I can see that it has the same large oilspots to the identical cell on the irony body.  Cell 23 has the following chemistry:

This is RO Unity (UMF) in Insight

 

CaO: .473

MgO: .368

K2O: .014

Na2O: .135

SrO: trace

TiO2: .074

Al2O3: .563

SiO2: 4.420

Fe2O3: .213

MnO: trace

 

Silica Alumina ratio 7.85

 

Feeling the surface of cell 23, it seems to sit rather more proud of the clay surface than surrounding cells somehow, almost as if it was a bit puffed up.  It is smooth to touch, but mildly bumpy, as if the little oilspot craters had bubbled up but then mostly melted down and smoothed over.   My best guess is that this has something fundamentally to do with the viscosity of the melt.  The surrounding cells with oilspots are much smoother and flatter - more glassy - than cell 23.  I have done a lot of these currie tiles now, so my glaze application does not vary much from one cell to the next.

 

The oilspotting phenomenon was generally more successful on the white stoneware body than the iron one. In particular cell 18 on the irony body is rather mean, rough and cratered with very few actual oilspots, whereas the same cell on the white body (an identical glaze remember) is smooth and glassy with easily visible and consistently dispersed oilspots. Similarly, cell 28 on the irony body shows few if any oilspots, while cell 28 on the white body shows good coverage of consistent oilspots.

 

Too much silica (4th column) definitely kills the oilspots.   And it seems kaolin has to be in the correct zone as well (2nd and 3rd rows up from the bottom.

 

Finally, let me say that what happens on a flat tile may not happen on a vertical one.  Just something to keep in mind... 






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