attached is a Masters thesis on cone 6 oil spot glazes which is very thorough and produces wonderful results. I corresponded with the author for a while and he repeatedly emphasized to put the glazes on very very thick. One problem I had with this was taking his advice to seriously. I put them on so thick that on vertical surfaces that they slid all the way down to my kiln shelf and then some.

Since thick is a relative term I never really know what people mean by thick, thin etc in terms of actual measurements. I wish there were some clear standards of what these terms meant..

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The auto-reduction that happens in an oil-spot glaze is a temperature related event.  When the glaze hits that temp, the RIO auto-reduces and bubbles up to the surface dragging some shiny iridescence up with it.  I'd have to look up exactly what that temp is.  It's in several articles.....jhp

Reduction of red iron oxide to black iron oxide occurs at oxidation temperatures reached at Cone 7 and above - 2,250 F according to Jon Britt - well above Cone 6 firings.

This is the reason Cone 6 red iron oxide glazes look best when using nearly 100% pure synthetically produced red iron oxide - because the red iron oxide you mix into the glaze is exactly the same red iron oxide your end up with on display in the fired glaze.

The mixing mechanism in Cone 6 "oil spot glazes" is something other than what Jon Britt postulates for cone 9/10 oil spot glazes - since Jon Britt agrees red iron oxide doesn't reduce at Cone 5/6.

Perhaps Jon Britt meant for his Cone 6 Oil Spot glaze recipe to be fired in reduction in a non-electric kiln.

Jeff Poulter said:

The auto-reduction that happens in an oil-spot glaze is a temperature related event.  When the glaze hits that temp, the RIO auto-reduces and bubbles up to the surface dragging some shiny iridescence up with it.  I'd have to look up exactly what that temp is.  It's in several articles.....jhp

The oil spot glaze requires something which will off-gas while the glazes are molten.  As an example, Laguna Clay's Gerstley Borate has a LOI of 29.5% - about 30% of the weight of the Gerstley Borate becomes lost as gas.

The gas created by Jon Britt's Cone 6 base iron glaze is Calcium Carbonate (whiting) with 43.9% LOI and Kaolin with 13.2%.  Calcium Carbonate calcines (releases the carbon dioxide gas) above 848 C which is roughly Cone 013

If you create a top coat oil spot glaze which is not liquid close to this temperature, the lost gas will not create any mixing of the two glazes.  Ferro Frit 3134 melts around Cone 015 so, as Alisa Clausen has found, Ferro Frit 3134 makes a good ingredient for an Oil Spot covering glaze.  The frit in the top coat will be molten at the time the calcium carbonate gasses-off its carbon dioxide.

One of the captions in Jon Britt's article is the following:

This test tile shows the bubbling of an immature oil spot glaze. As CO2 bubbles rise through the glaze, they deposit iron on the surface after they seal over.

So clearly the mixing reaction in that particular "oil spot" glaze is the off-gassing of carbon dioxide from a carbonate glaze ingredient, calcium carbonate, rather than the loss of oxygen from red iron oxide at Cone 7 and above.

So use Black Iron Oxide if you want a black colored Oil spot, and use Red Iron Oxide if you want a red colored Oil spot.

Lyn Rapley said:

What happens if you put some RIO and some BIO in the glaze, Norm.  Would that give you the reducing and fluid portion of the glaze from the BIO and the crystalizing portion from the RIO? 

I'm not knowledgeable about glaze formulation, I'm just working on what I've read about what to use different chemicals for in cone 6 glazes.

Thank you.  That is very informative.  I'm going to play . . .

This has been a very informative discussion.  If I remember correctly Vickery theorized in his masters thesis that the kiln actually reached cone 7.  The CO2 release makes much more sense.  I'm going to sub another base glaze with more "chalk" in it and see what happens.

Just saw the newsletter. Thought I would share this.

I have been tinkering with an oil spot at cone 6. I have had some pretty good results. The thicker you put it on the bigger the spots. I haven't tried a really really thick application because I personally like the spots on the middle tile the best. So I quit pushing it further. The oil spot is just one I found online, although I guess the author abandoned it. I assume because their surface was not satisfactory.. I don't have any problem with surface as you can see from the pictures. Oilspots are really a beautiful thing. I plan on coating entire bowls in it and selling them soon. It looks so deep and galaxy like. 

Here is a bowl with a thin coat: 

Recipe and source:

I have attached the tiles:

Left to right: Thin to thick.


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