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 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.

A. Insight-live

You can also watch this at Screencast-o-matic.com

Please read the caption on the linked picture below, I had misconceptions when making the white glaze. However the method used here to achieve the intended goal is still valid.

Filter recipe manager to "g3912|g3911"
Show the lizard-skin (oil-spot) effect being sought (G3911)
Questions: Is tin needed? Mechanism is white-on-black, but what kind of white? Black?
Plan: Choose a good black. High surface tension. Try different opacifiers.
Show digitalfire.com/glossary/surface+tension - SrO is highest (we'll exchange it for as much of the MgO/CaO as we can)

Open G2934 base and assess suitability of:
-It's recipe: raw/calcine kaolin ideal for control of shrinkage
-It's formula (trade CaO/MgO for SrO, add more B₂O₃ for good melting and to dissolve more SiO₂)
(Frit 3124 sourcing CaO, using 3249 as a more concentrated B₂O₃, 3110 for KNaO)

Duplicate G2934, name it G3911D (calculation on)
-Add strontium, Frit 3249, Frit 3110
-Match B₂O₃, increase SrO, add Na₂O
-Thermal expansion is up, but should be ok (could reduce with MagCarg+B₂O₃+SiO₂)

Which opacifier? How much tin is needed?

Links

Glossary	Oil-spot glaze A mottled glaze created by double-layering glazes of different melt fluidity and color

Cone 6 oil-spot glaze effect, what works and does not work?

Cone 6 glazed tiles of white-on-black oil spot glazes

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.

Click here for case-studies of Insight-Live fixing problems

You will see examples of replacing unavailable materials (especially frits), fixing various issues (e.g. running, crazing, settling), making them melt more, adjusting matteness, etc. Insight-Live has an extensive help system (the round blue icon on the left) that also deals with fixing real-world problems and understanding glazes and clay bodies.

By Tony Hansen
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