sbesse Posted July 16, 2011 Report Share Posted July 16, 2011 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! Link to comment Share on other sites More sharing options...
Marcia Selsor Posted July 16, 2011 Report Share Posted July 16, 2011 What was your firing schedule like? Oxidation or reduction? Clay body? There are many factors before knowing how to answer. Marcia Link to comment Share on other sites More sharing options...
Guest JBaymore Posted July 16, 2011 Report Share Posted July 16, 2011 Oil spot is heavily about the firing cycle. best, ...............john Link to comment Share on other sites More sharing options...
Pres Posted July 17, 2011 Report Share Posted July 17, 2011 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. Link to comment Share on other sites More sharing options...
bciskepottery Posted July 17, 2011 Report Share Posted July 17, 2011 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. Link to comment Share on other sites More sharing options...
sbesse Posted July 18, 2011 Author Report Share Posted July 18, 2011 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 Link to comment Share on other sites More sharing options...
sbesse Posted July 18, 2011 Author Report Share Posted July 18, 2011 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 Link to comment Share on other sites More sharing options...
sbesse Posted July 18, 2011 Author Report Share Posted July 18, 2011 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. Link to comment Share on other sites More sharing options...
sbesse Posted July 18, 2011 Author Report Share Posted July 18, 2011 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. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted July 18, 2011 Report Share Posted July 18, 2011 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 Link to comment Share on other sites More sharing options...
sbesse Posted July 19, 2011 Author Report Share Posted July 19, 2011 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 Link to comment Share on other sites More sharing options...
Marcia Selsor Posted July 19, 2011 Report Share Posted July 19, 2011 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 Link to comment Share on other sites More sharing options...
tkmog Posted December 21, 2016 Report Share Posted December 21, 2016 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. Link to comment Share on other sites More sharing options...
PeterH Posted December 21, 2016 Report Share Posted December 21, 2016 You may find this discussion -- and Norm's comments -- interesting: http://cone6pots.ning.com/forum/topics/6-oil-spot-mfa-thesis Link to comment Share on other sites More sharing options...
curt Posted January 4, 2017 Report Share Posted January 4, 2017 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. 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... Link to comment Share on other sites More sharing options...
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