A mottled glaze created by double-layering glazes of different melt fluidity and color
Officially, the oil-spot variegation effect appears to be the product of bubble generation associated with the release of gases of decomposition associated with an iron oxide phase change near the climax of a high temperature (cone 8+) firing cycle. The effect appears to be accompanied by the precipitation and crystallization of iron during cooling. Some also call it "lizard skin". Peter Hessemer explainesit like this: "I believe traditional oil-spot glazes occur because the iron rich ... Albany slip glazes bubble naturally. The skin of the bubbles oxidizes and then lays down on the surface again creating the oi-spots."
However the oil-spot effect could theoretically be created at any temperature with any color if a decomposition process exists for one of the materials present (or an interaction). The bubbles could be employed to deliver a contrasting color from an underlying glaze layer. Another glaze double-layering method is to employ melts of different fluidity (and contrasting colors). Typically the base glaze is melt-fluid and the over glaze is a lighter non-fluid glaze (typically a matte). The under glaze breaks through and isolates islands of the upper glaze to create the effect. Vertical and horizontal surfaces display the effect differently. The relative thickness of the glazes can be tuned to optimize the aesthetic. More subtle effects can be created by using glazes of the same color but different surface character. With this mechanism it appears logical to tune the chemistry of the overglaze to have a higher surface tension that the lower one.
In our experimentation we have found that tin oxide, in higher percentages (e.g. 6%), bubbles a high-surface-tension base transparent (high SrO content). Thus, over a black, the surface activity created by the bubbling creates a variegated pattern and texture of black and white.
The lower black glaze is a base coat G3914A Alberta Slip base (with 4% Mason 6600 stain). The white overglaze is G3912A with tin oxide opacifier. Normally I would fire this combination at cone 6, but this time I tried cone 8. It is a little high for this clay body, but the glaze variegation is better.
Simulating a white-on-black oil-spot effect at cone 6 oxidation proved to be a matter of repeated testing (that got me past some misconceptions). Stopping to think about the results at each step and keeping a good audit trail with pictures, in my account at insight-live.com, really helped. I had three black glazes: G2934BL satin (G2934 with black stain), G2926BB super-gloss (G2926B with black stain) and G3914A Alberta Slip black. Going on a hunch, I mixed up a bucket of the G3914A first (with some gum to help it survive second-coating without lifting). Rather than just try any white, I created G3912A by substituting as much CaO and MgO as possible for SrO in the G2934Y base. I later learned this to be an error, SrO reduces the surface tension, I should have used MgO (the G2934Y is a high-MgO glaze so it would have been fine as-is)! As you can see on the far right, this white still worked (at cone 5, 6, 7, 8). Why? There is another factor even more important. The effect only works on the Alberta Slip black. But its LOI is not higher than the others. And it worked even after ball milling. So I need to continue to work on this to learn more about why this works.
This is Ravenscrag Slip Oatmeal over a 5% Mason 6666 stained glossy clear at cone 6. You have to be careful not to get the overglaze on too thick, I did a complete dip using dipping tongs, maybe 2 seconds. Have to get it thinner so a quick upside-down plunge glazing only the outside is the the best way I think. You may have to use a calcined:raw mix of Ravenscrag for this double layer effect to work without cracking on drying.
Making black using metal oxides normally involves cobalt oxide, manganese dioxide, copper oxide and iron oxide. Combined they typically comprise 10-15% of the recipe. Using the Alberta Slip GA6-B base recipe, only 4% Mason 6600 black stain is needed to get a jet black! These blacks were applied in varying thickness on a porcelain and buff stoneware before having the white second layer applied. The black glaze on the top-right tile only has 3% stain. The overglaze is a gloss white with increasing amounts of tin oxide added (4-7%). Beyond 5% there appears to be no advantage.
Oil Spot Glazes by John Brit
Creating a Cone 6 Oil-Spot Overglaze Effect
In this video I use my Insight-live account to do the chemistry to convert an existing MgO-matt base glaze into a tin-opacified, high-surface-tension melt for double-layering over a gloss black. I also use it to record my physical testing.