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.
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.
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?
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.
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.
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.
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.
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).
Underglaze decoration difficult to cover with clear overglaze
The underglaze was painted on to bisque ware and have 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.
The specific gravities on two commercial underglazes might shock you
AMACO and Crysanthos. 1.26 (67.5% water) and 1.22 (68% water). It is possible they might be thinking that you want to be able to apply these in a thinner layer to get some transparency. However, generally you likely want to be able to make a brush stroke and have it cover well without having to go over it a second or third time. Strangely, the Crysanthos, although having a lower specific gravity is more viscous and goes on thicker. But really, these products are mostly water. If you make your own underglaze colors (by adding stain powders to a base) you would be able to mix them to have double the amount of powder that these have (per liquid volume unit) and with the right amount of gum they would paint on much better. Plus it would need less gum and would dry more permeable and the clear overglaze would cover without pinholes and bare spots.
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.
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