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Cone 4 Oil-Spot Type Glaze

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I seem to have stumbled on a type of oil-spot glaze.

8% RIO on white stoneware

I'm a bit hesitant to call it an oil spot, for a couple of reasons. Firstly, it doesn't come close to traditional Chinese oil-spot glazes; the spots are more of a dull grey than silver, although some of the larger spots have shiny black areas in the centre.

Secondly, it seems the mechanism that creates these spots is different from that of a traditional oil-spot, at least according to the most common explanation, which is that when Fe2O3 thermally decomposes to Fe3O4 at about 1230 degrees C, the bubbles of oxygen that form drag the iron to the surface, forming the spots. My glaze firing didn't go higher than 1130 C (but I went slightly over cone 4 by firing slowly). So what's forming the spots in my case?
A slightly different explanation of how oil-spots form is given by Joseph Grebanier, in his book on Chinese Stoneware Glazes.

1) Bubbles rising through the molten glaze reach the surface and burst there, leaving pits or craters in the surface at those points.

(2) As the firing continues, the more soluble and therefore more fluid, extra iron-rich portions of the surrounding glaze pour into these pits and fill them before larger-particled, less soluble portions of the glaze can move to them.

(3) As a result, when the firing is concluded at just the right stage, the pits are more or less filled with the extra iron-rich glaze material that has crystallized into patterns which are more reflective of light than the rest of the glaze.

(I don't have a copy of the book, but I found the quote in Carleen Devine's write-up on Oil-spots). I don't understand what Grebanier means by the more soluble portions of the glaze. It sounds as if the glaze separates into two parts. Could something similar be happening with my glaze?

What's really interesting is that there are places where the glaze has cleared up. The spots are still there, but you can see the clay-body through the surrounding glaze. The test tile below is the best example:

8% RIO on red earthenware

This has 8% added RIO on red eathenware. I don't like the blotchy effect this gives, but I'd love to know why it happens.


I should add that there are a number of cone 6 oil-spot glazes out there, and unless you're firing fast, you won't go over 1230 degrees in a cone 6 firing:
There's also this paper about an oil-spot fired to 1160C.

The recipe I'm using is:

Refined Overberg earthenware  50
Neph Sy                                       4.8
Wollastonite                                 5.7
Talc                                            16.2
FSB 510 Frit                              23.4
+ Red Iron Oxide                          0 - 14% in 2% increments   (best results obtained with 8% RIO)

Overberg earthenware is a local clay that fires to an orange-brown colour at cone 4. By refined, I mean removed the larger particles, like you do when making terra sig, except that my slip still contained some medium-size particles. Unfortunately I don't have an analysis for this clay, and I'm not even sure there is one. To get a rough idea of what the UMF might look like, I assumed Overberg earthenware was a combination of 3 other local clays, and got the following bounds for the glaze without the added RIO:
SiO2      2.9 - 3.1
Al2O3    0.40 - 0.53
B2O3     0.30 - 0.33
MgO      0.43 - 0.45
CaO      0.32 - 0.34
Na2O    0.10 - 0.10
K2O      0.11 - 0.15
Fe2O3  0.01 - 0.10
TiO2      0.01 - 0.02

SiO2:Al2O3  6.0 - 7.4


Any thoughts on what's causing the spots?

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Jose from France has encountered this result in firing crystalline glaze. The results occurred when he used higher amounts of Fe, and slowed the cooling rate down: with ramp holds. We had this discussion a little over a year ago: he believes it is iron crystallizing. Not that I disagree, but I also believe that European materials (silica, for one) have much higher levels of iron, as well as magnesium. Looking at your limits: I see lots of magnesium in play, as well as iron. He also believes that reduction is playing a role, because it removes a percentage of oxygen molecules: which I tend to agree with that.


Jose website:



Click to the English translator at the top right tab. Scroll to the bottom of the home page to find: oil spots.

Feel free to send Jose a letter, he is a super nice guy; more than willing to share info.




I have seen those barren areas in crystalline glaze many times. I have thoughts about it, but nothing conclusive.

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I've got a cone 6 glaze that's very similar to that. The original was a cone 10 reduction glaze called St. John black, which we used to use way back in undergrad. It's an Albany slip glaze that fires to a really beautiful metallic black. Recipe below. I reformulated it down to cone 6, which took way more testing than one might think. For some reason it was really difficult to get the melt and color right, probably because I wasn't using Albany any more. Anyway, I settled on the formula below, which has the metallic surface of the original, but also gives the iron spotting that Pieter shows above. The thicker the glaze, the better the spots, and you can go really thick without worrying about running. This glaze also looks best on porcelain.

St John Black 10.pdf

St John Black 6.pdf

St John Black 10.pdf

St John Black 6.pdf

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Nerd, thanks for the link to Jose's website. His pots are amazing. My firing schedule is quite different to his though. I do a controlled cool, but it's at over 3 degrees C per minute. I see he also fires above 1230 C.


PeterH, I fire in an electric kiln, but I turned the vent off during the last 100 degrees of the firing, which took about 3 hours. I had the bungs in, so probably wasn't firing in a totally oxidizing environment.


Thanks for the recipes, Neil. I've also found that thicker application gives bigger spots, but it's also more likely to crawl (not surprising given there's 50% clay in the recipe).

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with the correct mix of oxides?

Been reading many technical papers about fluxes in general: and the additional fluxing ability of iron in combination with magnesium. The magnesium levels in the glaze is not a minor amount either. I would be looking hard at those combinations.




U of I study quote:


Although the amounts of CaO, MgO and FeO,3 are small in relation to the other constituents(KnaO),their effects on the thermal behavior are quite pronounced.

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What's interesting to me is that this glaze has the same ingredients as a lot of other glazes I've tested with iron, and no others have done this. So there's something special about this particular mix. What I've noticed is that this one has a lower Si:Al ratio than most of the others, but not a lot. It's not super glossy. This one also has very little boron (almost none), which is unusual for a cone 6 glaze recipe. And the alumina and silica and magnesium are near the top of their limits. Beyond that I have no idea.

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Thanks Neil. I'm still curious about what's causing the spots, if it isn't the red iron oxide decomposing. Or is it possible that the decomposition happens at a lower temperature with the correct mix of oxides?


Hi Pieter,


Think you may be on the right path regarding a lower temperature decomposition with the right mix of oxides.   It is possible that your clay body and/or glaze materials are producing a localized reduction environment in the glaze, reducing the Fe2O3 to FeO.  This can happen when carbon or sulphur or other materials burn out from the clay body.


Phase diagrams for FeO-SiO2-Al2O3 show a eutectic as low as 1088C.  This occurs with (approximately) 12 Alumina, 40 Silica and 48 Iron, an entirely possible combination in an iron rich glaze one would think. 


And even less iron (and proportionately more silica and alumina) still works if the temperature is a bit higher.  For reference, at 1300C (around cone 11) you only need 27 FeO, 52 Silica and 21 Alumina.  Again, a very feasible combination in a normal stoneware body going through the alumina rich transformation involving metakaolin.   Just a thought. 


It also occurred to me that since this glaze is so flux-rich, perhaps the oil spots themselves are not iron?  Could they in fact be some other glaze oxide bubbling its way up through the molten glaze, which is so iron rich that it LOOKS LIKE oil spotting but isn't.  Hmmmm.....

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Thanks for the replies, everyone. The thing about this glaze is that even with no added RIO, there are still spots forming.

0% added RIO on white stoneware


You might not be able to see them in the photo, but they're matt on a shiny background (The lighter spots are unpopped bubbles). Could the bubbles be bringing something to the surface that makes the glaze devitrify, even though I cooled fast?


I posted this recipe on the Ceramic Recipes facebook group, and one of the suggestions was that there might be manganese in the Overberg earthenware that's catalysing the thermal decomposition of iron. Any thoughts?

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I think that this is not necessarily related to the clay. At least not to any one specific clay body, and definitely not related to manganese. We regularly use 4 different clay bodies in my studio, and my glaze will do the spots on all of them. It seems like it does it the most on the porcelain, Standard 365, although I will say that my students are more likely to apply it thicker on the porcelain, which contributes to the spots.

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I'm not suggesting that it's the clay-body I use that gives the spots; I've gotten spots on both red earthenware and white stoneware. I was wondering if there's something special about the 50% Overberg earthenware in the recipe. A variation of the recipe with a screened commercial clay-body in place of the Overberg EW produced spots as well, but I suspect that the commercial body contains quite a bit of Overberg EW, so that's neither here nor there.

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