Here is motivation to make your own underglazes!

Commercial hobby underglazes are high in stain and very expensive. But does expensive mean suitable? To help answer we have over-fired these commercial products in a melt fluidity tester (to cone 8). They are recommended for use from cone 06-6 (some can go higher e.g. the green). Underglazes need to melt enough to bond with the underlying body, but not so much that opacity is lost (any melting loses opacity). Excessive melt can also cause design edges to bleed. To work well at greater thicknesses, underglazes need to have a firing shrinkage similar to the body (an ill-fitted underglaze and body forced into marriage are eventually going to divorce, in the form of flaking or cracking at their interface). Thus, while a regular glaze would melt enough to go well down the runway on this tester, an underglaze should not flow at all. At this temperature, none of these have achieved the right degree of maturity (the green is too refractory, the others over-melt to varying degrees). The only one that has a chance of suitability at cone 6, two cones lower than this, is the blue. Clearly, the base recipe and stain percentage in each underglaze recipe color needs attention, if that can be achieved all of these would mature to the same degree.

Related Pictures

Is your underglaze forming a bond with the body?

The red underglaze on this low-fired bowl is not properly fluxed (melted), it does not adhere to the body (this is a commerial product). The bottom-most contour of this bowl is concave and the transparent overglaze, which is under some compression, has popped right off! This is a serious hazard on the inside of functional ware. Each stain has it own melting temperature, and the underglaze formulation using that stain must employ a mix that supplies sufficient fluxes. So test your underglazes (by firing without an overglaze), even if they are a commercial product.

When a DIY black underglaze makes sense

Black brushwork needs to go on thick enough in one brushstroke. Commercial products we have don't do that - thus my motivation to work on this. Another issue is that they try to cover too wide a firing range (thus they melt too much at the high end and not enough at the low end). I am experimenting on cone 6 Polar Ice porcelain jiggered bowls using G2926B dipping glaze. The base underglaze recipe here is a 90:10 MNP:nepheline syenite mix (you could use your own porcelain instead of MNP and feldspar instead of nepheline). To that, I add 15% black stain, 1.5% CMC gum and 5% bentonite. With the CMC gum and bentonite, and blender mixing, a brushable consistency that stays put can be achieved at a fairly low water content compared to commercial products (enabling it to go on thicker in a single brush stroke). Assuming application at leather hard state, the drying and firing shrinkage can be matched to the body by varying the plasticity of the porcelain used (e.g. the percentage of bentonite it contains). And the percentage of stain can be tuned for enough color but no bleeding, bubble clouding or crystallization. And we can adjust the degree of maturity by varying the proportions of MNP and Nepheline (commercial underglazes often melt too much by cone 6 and fade and diffuse as a result), this one stays opaque black.

Underglazes suitable for making silk screen transfers are another special case. The ideal one needs to cover well like this one. But it also needs to gel and harden enough to hang onto the paper but not so hard that it does not separate and transfer to the ware. Commercial products for brushing are unlikely to be optimal so it makes sense to mix your own and experiment with different amounts of gum.

Here is why you don't want an engobe to fire too vitreous

Too much frit in an engobe and it will lose opacity and whiteness. The white slip on the left is an adjustment to the popular "Fish Sauce" slip recipe (L3685A: 8% Frit 3110 replaces 8% Pyrax to make it harder and fire-bond to the body better). The one on the right, L3685C, has 15% frit. Although applied at the same thickness, it is becoming translucent, moving it into glaze territory. That means it will have a far higher firing shrinkage than the body (a common cause of shivering at lips and contour changes). This slip is basically a very plastic white body. Since white burning slips are made from refractory materials they are not nearly as vitreous as red ones, at low fire they need help to mature and a frit is the natural answer. With the right amount of frit the fired shrinkage of body and slip can be matched and the slip will be opaque. This underscores the need to tune the maturity of an engobe to the body and temperature. Although zircon could be added to the one on the right to opacify and whiten it, that would not fix the mismatch in fired shrinkage between it and the body. And it would increase the price.

Links

Glossary	Underglaze An intensely pigmented highly opaque non-melting ceramic material mix meant to adhere best to leather hard pottery and fire-fit the body. Often transparently overglazed. Starter recipes.
Glossary	Silk screen printing Silk screen printing is one of the best options for hobbyists and potters to reproduce crisp and detailed decoration. But there are many details to know.
Troubles	Bleeding colors In ceramics, the edges of overglaze and underglaze color decoration often bleeds into the over or under glaze. How can this be avoided.

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