Thickly applied and multi-layered slips

This is why they need to fit the body and each other

Fit? It has to stick well. And stay stuck during drying (and shrinking). And the bond has to survive shrinkage that happens during firing. This potter is doing thick applications of each slip (actually that makes them engobes). She uses stains, that's wise, metal oxides bring baggage when used to color slips (e.g. their decomposition can affect the bond, they can gel the slurry, flux the fired product thereby increasing the firing shrinkage of the slip). Stains are better because they affect slurry and fired properties less. But there are still enough issues that each colored slip deserves testing. This potter first slaked B-mix as a slip (it is highly plastic), using it at a runny yogurt consistency. But it bubbled when fired hotter than a cool cone 6. A switch to porcelain slip (which is non-plastic) is shown here. It flaked off as it dried (even in a damp box for 24 hours), also after bisquing to cone 08, and sometimes even after firing to cone 6. This signalled a drying mismatch between body and slip, the bond that managed to survive drying was weakened enough to fail on firing.

The solution was an engobe recipe that is super plastic and sticky. The popular Fish Sauce recipe is an example, it contains 10% bentonite and is unbelievably sticky. We formulated L3954B with this in mind. It adheres well to leather-hard clay and doesn't flake off (misfit is instead evident by surface cracking if its shrinks more than the body). And it is not highly vitreous, keeping its fired shrinkage low enough to match stoneware bodies. Mixing your own recipe also enables compensating the amount of feldspar if the stain affects the slip's degree of vitrification, and therefore fired shrinkage (e.g. blues, oranges, yellows).

Related Pictures

This pottery glaze is not flaking off - the underglaze is.

This picture has its own page with more detail, click here to see it.

Pure ceramic stain powders don't melt at typical pottery temperatures so they cannot bond with clay body surfaces. They don't suspend in water and don't harden on drying. Pure stains contribute only one ceramic property: Color! Commercial underglazes sold in jars dilute stains into a recipe of materials, a 'base medium', designed to impart the missing properties. Unfortunately, consider a problem: Blue stains are fluxes and potent at low percentages. Orange stains (and others) are refractory and require much higher percentages. But for convenience underglaze manufacturers will want to use the same medium and percentage pigment to make an entire line of underglazes. That causes the problem seen here - flaking is happening at the interface between the body and orange underglaze. This underglaze recipe needs more frit (or a more potent one) so that enough melting occurs to create a better bond. For some colors it can thus be an advantage to make your own underglazes.

Videos

Links

Recipes

L3954B - Cone 6 Engobe (for M340)
Dry and firing shrinkage fitted to Plainsman M390, M340

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