|Monthly Tech-Tip |
A intensely pigmented and highly opaque brushing compound meant to be applied to leather hard pottery and covered with a transparent overglaze.
Underglazes are heavily pigmented ceramic compounds, normally applied by brushwork, to leather hard or green ware and covered with a transparent glaze. Commercial underglazes are pigmented using ceramic stains (not metal oxides) and they are very expensive compared to prepared glazes.
To have good brushing properties underglazes are heavily gummed and gelled (e.g. CMC gum, Veegum CER) and thus dry slowly and hard. With sufficient gelling, very high water contents are possible. High-water-content underglazes can paint very well, but more coats must be applied to get the degree of opacity needed.
Underglazes are opaque, that is their primary characteristic. They typically mix a high percentage of stain with a fritted medium and fine particled clay. The mix must melt enough to adhere to the body but not so much that it seals the surface, making it difficult to adhere an overglaze. Engobes need to have a low LOI (to avoid bubbling), so stains are a far better choice than raw metal oxide or carbonate colors. Engobes also must not shrink too much during firing or feather too much at the edges of brushstrokes. Most importantly, they must not melt to the point that opacity is lost. These needs imply recipes that are tuned to the target temperature. Yet commercial underglazes often target a wide temperature ranges! Stains themselves react differently to temperature so the recipe they are put into must adapt them to the firing temperature needed, it is not clear that commercial stain manufacturers always do this.
An underglaze is not the same as a ceramic slip or engobe. Potters and hobbyists seldom make their own underglazes, they thus provide a large market for bottled products. That being said, making your own underglazes enables tuning characteristics and performance (and saving money).
Underglaze mediums must have plenty of fine particled clay in the recipe so they can shrink with the body when applied to leather hard ware (without it they will flake off). Commercial underglazes are gummed enough that they can be painted on to bisque or dried ware. But this is not as good as application to leather hard surfaces, early contact enables better physical adherence. And firing them together, from dry, furthers the bond.
Potters and hobbyists encounter a key problem in the use of underglazes: Finding an overglaze that will fire transparent. Transparent glazes often have micro-bubble clouds that obscure the underglaze decoration. Generally, it is best to use a fluid-melt transparent and apply it as thinly as possible while still getting good coverage with a glassy surface. It can take lots of experimenting to find a glaze and application technique that consistently work. At low temperatures there is much less problem with this, clear glazes typically fire to a far more transparent glass on white-burning bodies.
Another common issue happens with overglaze application: Underglaze surfaces do not allow passage of water to nearly the degree of the surrounding bisque (especially if applied to the bisque and thus are in the dried state). Dipping glazes often will not build up a thick enough layer during immersion time. And pinholes and bare spot often develop during the drying. The most practical solution is to apply a transparent brushing glaze, usually made by the same manufacturer as the underglaze. Even then, drying over the underglaze is slower and multiple coats are required.
We did lots of work on a cone 6 fluid melt base transparent, partly for use over underglazes. Our most practical finds were G3806E and G3806F.
Anti-racism themed. The clay is Plainsman L213. Spectrum underglazes.
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.
On the left is Spectrum 748 fire engine red cone 04 clear glaze. With three coats. On the right is Amaco V-388 underglaze, also with three coats. It is covered with Spectrum 700 transparent. Both were glaze fired to cone 05 and bisque fired to cone 04. The color intensity of the glaze on the left varies with thickness, the ridges of the throwing lines are clearly highlighted by this. But underglaze on the right is completely opaque, there is no variation on any contours. Of course, the underglaze method is more costly, the bottles are three times the price of those of the glaze of the same color.
Stephanie decorated this porcelain plate using Amaco Velvet underglazes on both unfired porcelain and touched up on bisque (left image). She over-painted Amaco HF-9 Zinc-Free Clear (at least 3 coats in the center to make it pool into the recessed parts of the image, so it is flat to the touch like émail ombrants technique). The plate rim is a shallow bas-relief so two coats of clear were sufficient there. She fired it to cone 6 (right).
Top are V-326 and V-388 underglazes, painted on and 04 bisque fired. Although the layer is very thin the coverage is amazing and the brightness is stunning. This degree of brilliance is not possible unless the percentage of stain is very high. That explains whey these are double or triple the cost of a typical commercial glaze. The bottom mugs are clear-glazed and 05 fired, the one on the left with Amaco LG-10 and the one of the right is Spectrum 700. The latter produces better results over the underglaze and is more transparent and less yellowish on the body.
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.
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.
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).
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). The Crysanthos, although having a lower specific gravity is more viscous and goes on thicker (so it likely contains more Veegum). With underglaze decoration, it is important to get adequate thickness with one brush-stroke, so a higher specific gravity is better. This may be reason enough to consider making your own (by adding stain powders to a base and using Veegum CER to gel the slurry, slow drying and harden well at the dried state).
True, these underglazes have high percentages of stain, but even if this was a pure encapsulated stain there is still only 21 grams of it. The high water content is actually a benefit as it produces better painting properties. If you only use small amounts these commercial underglazes are likely OK, but if you use a lot then making these might be feasible (and you can add all the water you want if you condition the slurry with Veegum CER). In one scenario we calculated a 1500% saving.
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.
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.
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.
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.
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.
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.
Underglaze brushstrokes were applied to this cup at the leather hard stage (lower left). It was then bisque fired. On the lower right a ball of the pure underglaze emerged from the same bisque firing, notice that although not melting as much as a glaze, it is certainly fusing enough to seal the surface of the bisque where applied. Notice what happens on the upper right: The bisque piece was immersed in a dipping glaze for a few seconds - the underglaze is not covering. On the upper right a transparent brushing glaze has been applied over the underglaze brushstrokes. Notice that it has covered. But three coats were needed with plenty of drying time between them, especially over the brushstrokes.
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.
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.
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.
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.
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?
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.