|Monthly Tech-Tip |
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.
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 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.
It can be difficult to find an overglaze that will fire transparent. Often they have micro-bubble clouds that obscure the underglaze decoration. Raw colors generate gases that produce even more bubbles, so use stains. Commercial transparents don't necessarily work any better than ones you mix yourself. Generally, it is best to use a fluid-melt transparent and apply it as thinly as possible. It will take lots of experimenting to find a glaze and technique that will work.
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.
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?
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.
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.
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).
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.
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.
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.
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 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.
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.
At low temperatures glazes and slips/engobes are not stuck on nearly as well as with stoneware and porcelain. So the glaze fit has to be better (poor fit will be evidenced by flaking at the lip). But that is not what is happening here. In this case a pigmented slip, or underglaze, was applied first, at leather-hard stage (thus it is being used as an engobe). The integrity of two bonds must now be considered: Slip-with-body and glaze-with-slip. Slip-to-body bonding is never as good as glaze-to-body or glaze-to-slip. When an engobe, or underglaze, is refractory then the bond-with-body is especially poor. Ceramic stains are highly refractory in comparison with low-fire bodies, simply adding them to an underglaze base recipe will make it refractory also. In addition, stains vary widely in their refractory character and the percentage of stain needed varies greatly with color. Some underglaze manufacturers compensate by incorporating a compensatory percentage of frit in each underglaze recipe. Other manufacturers simply have one base and add all the colours to that. Claims that underglazes work well across wide temperature ranges do not get tested when they are brushed on as decoration, but when they are applied like this, as an engobe, disaster strikes! In this case we can see that the failure is occurring at the underglaze-body interface and the glaze/underglaze "sandwich" is releasing in large flakes.
You might be impressed by the underglaze decoration, but I am more impressed by the transparency of the clear over glaze. This type of decoration is quite easy to achieve at low temperatures, like cone 04, but much more difficult at medium and higher temperatures. That is why many people shy away from this type of decoration, they have bad experiences with clouding in the glaze that obscures the design. Because a lot of work goes into the design, one wants assurance it will not be ruined in the glazing and firing process. Reliable transparency is a combination of glaze application thickness, glaze recipe, glaze materials, firing temperature and firing schedule.
When clear-glazing terra cotta ware (Plainsman L215 here) an important issue is glaze thickness. The mug on the left was double-dipped (so suspended bubbles are present in the handle recess, thumb-hold and along its edges). The glaze needs to be thick enough so that it feels glassy smooth but thin enough to avoid the bubbles. Normally, if applied the thickness of the one on the left, it would be completely milky, filled with micro-bubble clouds. Why has it not done so here? Because it is fired at cone 03 (using G2931K glaze and the C03DRH firing schedule). An added benefit is that the body is so much stronger than it would be if fired at cone 06 or 04. And the underglazes work fine.
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The white engobe was applied by pouring at leather hard stage. The underglazes were also painted on at leather hard. The mugs were then dried, cleaned, bisque fired, dipping in clear glaze and final fired to cone 03. The clay and engobe have frit additions to make them vitrify at low temperatures.
This cobalt underglaze is bleeding into the transparent glaze that covers it. This is happening either because the underglaze is too highly fluxed, the over glaze has too high of a melt fluidity or the firing is being soaked too long. Engobes used under the glaze (underglazes) need to be formulated for the specific temperature and colorant they will host, cobalt is known for this problem so it needs to be hosted in a less vitreous engobe medium. When medium-colorant compounds melt too much they bleed, if too little they do not bond to the body well enough. Vigilance is needed to made sure the formulation is right.
The background plate was decorated using Amaco Velvets and overglazed with Amaco Glaze HF-9 Zinc-Free clear. The front one was overglazed using Amaco Celadon clear. The hazing of the latter is most evident in the center area where it has been applied in a thicker layer. However this made no difference when using the HF-9 transparent. For complex designs like this it is often better to paint on the clear rather than dip, since the highly gummed Velvet underglazes impede the absorbency of the underlying body, and thus its ability to build up a layer during dip.
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.