Jump to content


Photo

Need Help - Cone 6 Oil Spot Glaze


  • Please log in to reply
11 replies to this topic

#1 sbesse

sbesse

    Newbie

  • Members
  • Pip
  • 6 posts

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!

Attached Files



#2 Marcia Selsor

Marcia Selsor

    Professor Emerita, Montana State University-Billings

  • Members
  • PipPipPip
  • 3,890 posts
  • Locationwhere Texas, Matamoros, Rio Grande and Gulf of Mexico come together.

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

#3 JBaymore

JBaymore

    Moderator

  • Moderators
  • 2,938 posts
  • LocationWilton, NH USA

Posted 16 July 2011 - 03:36 PM

Oil spot is heavily about the firing cycle.

best,

...............john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#4 Pres

Pres

    Retired Art Teacher

  • Moderators
  • 2,015 posts
  • LocationCentral, PA

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

bciskepottery

    Advanced Member

  • Members
  • PipPipPip
  • 1,444 posts
  • LocationNorthern Virginia

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

sbesse

    Newbie

  • Members
  • Pip
  • 6 posts

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

sbesse

    Newbie

  • Members
  • Pip
  • 6 posts

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

sbesse

    Newbie

  • Members
  • Pip
  • 6 posts

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

sbesse

    Newbie

  • Members
  • Pip
  • 6 posts

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

JBaymore

    Moderator

  • Moderators
  • 2,938 posts
  • LocationWilton, NH USA

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
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#11 sbesse

sbesse

    Newbie

  • Members
  • Pip
  • 6 posts

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

Marcia Selsor

    Professor Emerita, Montana State University-Billings

  • Members
  • PipPipPip
  • 3,890 posts
  • Locationwhere Texas, Matamoros, Rio Grande and Gulf of Mexico come together.

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




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users