All Glossary

Underglaze

A ceramic compound meant to be applied to green or bisque ware and covered using a transparent overglaze. There are good reasons to make your own underglazes if you are in production.

Details

Underglazes are ceramic stained ceramic compounds that are applied to bisque or green ware and over which a transparent glaze is normally applied. This method isolates the less durable colored glass from contact with food, drink and wear. Application is normally done by brush, but can also be done by spray, silk screening, dipping, etc. Different methods of application benefit when the underglaze has a medium tuned to that method (e.g. underglazes for use with dipping overglazes need to be less glassy to permit water passage to the body underneath). Underglazes are often flocculated and gummed to have the best application properties.

Commerical underglazes mix ceramic stains (a much better choice that raw metallic oxides) with a fritted stable glaze-like medium that has a chemistry compatible to the development of the color. This mix melts enough to adhere to the body well (very important), but not so much that it behaves like a glaze or seals the body surface. Potters often make their own underglazes using metal oxides mixed with a frit or transparent glaze recipe medium. For some colors (e.g. blue, black), as little as 10% stain is adequate. Others require much more (e.g. yellow). Ideally each class of stain has its own melting characteristics and each should therefore have its own melting medium! However, from a practical point-of-view companies producing underglazes commonly mix all their stains with one or a few melting mediums (good reason to be aware of what you are buying).

As noted, if the glass-producing vehicle hosting the stain fluxes too much, underglaze colors will feather at the edges of brush strokes. In practice, they can melt more than you might think (although commercial products often don't). Ideally, a finely balanced product melts enough to adhere well, but not so much that it feathers (yet many commercial underglazes claim that they work across a broad range of temperatures, however at the low end they develop almost no glass phase). In one development project we found that a 50:50 mix of glaze:porcelain body produced an engobe that worked this way. Of course, if an engobe experiences any degree of melting during bisque firing, it will produce a water-impervious surface that will shed off dipping glazes (brushing glazes will apply fine but will take much longer to dry). Many potters and smaller companies that use a fairly narrow range of colors have found that making their own underglazes enables them to tune the firing and working characteristics, greatly benefiting ware quality and ease of production.

Underglazes must contain some gum content to make them brushable (typically 1%). Enough gum is need to slow drying and aid spreadability, but not so much that they dry too slowly. It is possible to achieve a product that can dry enough to handle on leather hard ware in a few minutes. It is also possible to use more gum and much lower percentages of powder and still have a very workable product (we have seen products with specific gravities as low as 1.25). Of course, more coats are needed to cover (be aware of this when buying products, you might be getting more water and less glaze than you think).

Underglaze mediums must have plenty of clay in the recipe so they can shrink with the body if applied to leather hard ware (without it they will flake off as the body shrinks away from them underneath). This might seem counterintuitive since it suggests that the underglaze would shrink and crack if applied to bisque ware. But the significant gum percentage present is enough to counteract this and hold the them firmly on the bisque.

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 convex 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.

A glaze and underglaze are flaking off. Why?

At low temperatures glazes are not stuck on nearly as well as for stoneware and porcelain. One of the ways to compromise the bond is to put a layer of refractory material, for example certain underglazes, between the body and glaze. It is kind of like painting something that is dirty, the paint won’t stick. Certain ceramic colours are more refractory, that is, they melt at much higher temperatures. Because no melting is occurring they adhere poorly. Some underglaze manufacturers compensate by incorporating Frit in and underglaze composition, they treat each colour as an individual and adjust accordingly. Other manufacturers do not do this, they simply have one base medium recipe and add all the colours to that. In this case the bond between the Glaze and underglaze is better than between the underglaze and the body so that is where is flaking off.

Underglaze resisted coverage of dipping glaze yet heals during firing

The underglaze was made by mixing the Zero3 white engobe with Zero3 H clear glaze (50:50) and adding 20% black stain and gum to make it paintable. The piece was bisque fired at cone 06 and the engobe formed enough glass to block the porous body below from absorbing the glaze water during dipping. Notwithstanding this, the glaze has flowed out over the underglaze because of the slight 'wet' surface it develops during the glaze firing. For dipping of the cover glaze it would be better to adjust the underglaze to melt less. This underscores the need to tune underglazes to the exact purpose. For a brushing overglaze this one would work as is.

The specific gravities on two commercial underglazes might shock you

AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water)! The former is well below their recommended specific gravity of 1.4 (it still paints well but needs more coats, and more time to dry and apply them). Strangely, the Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (but thinning down as it dries). With underglazes it is important to get adequate thickness with one brush-stroke, so a high specific gravity is important. This may be reason enough to consider making your own (by adding stain powders to a base). Actually, this technique of adding-stains-to-a-base-transparent is even more practical for making your own glazes, it just takes the right amount of gum to make them paint well.

Underglaze decoration difficult to cover with clear overglaze

The underglaze was painted on to bisque ware (has not be fired on). This is a problem. It has a high gum content and has sealed the surface so the porous body underneath is unable to pull water out to dry it quickly. During the slow dry the little absorption that is taking place is generating air bubbles from below and these are producing bare spots. The solution is to either make your own underglaze having a lower gum content or decorate ware in the dry or leather hard stage so the bisque fire will neutralize the gum.

Underglaze decoration at low, medium and high temperature reduction

Left is Plainsman Zero3 stoneware fired at cone 03. Middle is Polar Ice fired at cone 6d. Right is Plainsman P600 fired at cone 10R. The same black and blue underglazes are used on all three, but each has its own transparent glaze (left G2931K, middle G3806C, right G1947U).

Underglazes at low fire are brighter than at medium temperature

Medium temperature transparents do not shed micro bubbles as well, clouds of these can dull the underlying colors. Cone 6 transparents must be applied thicker. The stains used to make the underglazes may be incompatible with the chemistry of the clear glaze (less likely at low fire, reactions are less active and firings are much faster so there is less time for hostile chemistry to affect the color). However underglazes can be made to work well at higher temperatures with more fluid melt transparents and soak-and-rise or drop-and-soak firing schedules.

Does it matter which transparent glaze you use over underglazes? Yes.

These porcelain mugs were decorated with the same underglazes (applied at leather hard), then bisque fired, dipped in clear glaze and fired to cone 6. While the G2926B clear glaze (left) is a durable and a great super glossy transparent for general use, its melt fluidity is not enough to clear the micro-bubbles generated by the underglazes. G3806C (right) has a more fluid melt and is a much better choice to transmit the underglaze colors. But I still applied G2926B on the inside of the mug on the right, it has a lower thermal expansion and is less likely to craze.

The green underglaze is failing on impact

This is a low fire fritted stoneware fired to cone 03. But it still has about 4% porosity. The green underglaze is not developing enough glass to bond well with the body surface. Repeated blows to the surface by a hammer are chipping off chunks of glaze/underglaze at the bond with the body. This is not happening with the other underglazes. The green underglaze is obviously more refractory than the others and should be reformulated.

4 good reasons to consider making your own underglazes

Commercial underglaze colors fired at cone 8 in a flow tester (this is another good example of how valuable flow testers are). Underglazes need to melt enough to bond with the underlying body, but not so much that edges of designs bleed excessively into the overlying glaze. A regular glaze would melt enough to run well down the runway on this tester, but an underglaze should flow much less. The green one here is clearly not sufficiently developed. The black is too melted (and contains volatiles that are gasing). The pink is much further along than the blue. And cone 5, these samples all melt significantly less. Clearly, underglazes need to be targeted to melt at specific temperatures and each color needs specific formulation attention. Silk screening and inkjet printing are increasingly popular and these processes need ink that will fuse to the surface of the body.

Underglaze color mayhem!

Commercial underglaze colors fired at cone 5 in a flow tester. Underglazes blend stains with a host recipe that should fuse them enough to adhere well to the body (two of these have not even begun to do that). The blue, green and red are from one manufacturer. Stain powders have different melting temperatures, so underglaze formulators must treat each stain individually, customizing the underglaze recipe to its melting behavior. As you can see, they have failed to do that here, the pink one has shrunk to half its size and is about to melt (it needs less flux). The green one is only sintered (it needs more flux). The black underglaze (D) (from a second manufacturer) contains gassing materials, it has become an Aero chocolate bar and is about to race down the runway. The E black (a third manufacturer) has not even started to melt or even sinter. The blacks were plastic, the colored ones were not. I am confused. How could the glaze possibly stick well to the body with the green or unmelted black under it?

Underglazes can be incompatible with the clear overglaze

These are porcelain tiles that we bisque fired, one-coat decorated with underglazes (Crysanthos), glazed with G3806PS fluid-melt glossy clear glaze and fired to cone 6. Fluid melt clear glazes cover colors much better (without crawling or clouding). Some colors are bleeding, if needed this glaze can be adjusted (by adding kaolin) to make it melt a little less. The rose color on the upper right, #093, is not working? Why? It likely employs a chrome-tin stain, these have requirements: A clear glaze having a minimum amount of CaO, no ZnO and not too much B₂O₃. This glaze does not qualify. But no transparent glaze works with all underglazes. You could find others that work with #093 but they could cloud, craze, crawl and not be glossy enough. The other orange/pink colors here are working. Why? Because they likely employ inclusion stains. A key factor is that the black is working well, even when applied over the white underglaze.

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By Tony Hansen