All 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.

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Details

Underglazes are heavily pigmented ceramic compounds, normally applied by brushwork, to leather hard or greenware and covered with a transparent glaze. Commercial underglazes are pigmented using ceramic stains (not metal oxides) and they are costly compared to self-prepared products.

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 hobby underglazes can paint very well, but more coats must be applied to get the degree of opacity needed.

Underglazes are usually opaque as their primary characteristic. They typically mix a high percentage of stain (e.g. multiple times the percentage used in a glaze). Blends predominate in fine-particled clay with feldspar and silica additions plus enough frit or feldspar to achieve enough maturity (but not too much to compromise opacity). 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 themselves or clouding the overglaze), 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. These needs imply recipes that are tuned to the target temperature. Yet commercial underglazes often target a wide temperature ranges! Stains themselves affect the degree to which the host underglaze recipe develops, different stain families each having its own effect.

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 onto 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 works. 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 spots 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.

Recipes

We did lots of work on a cone 6 fluid melt base transparent, partly for use over underglazes. Our most practical finds were G3806C and its derivatives. We have also done lots of work on engobe base recipes (also suitable for underglazes), examples are L3954B recipe for cone 6 stonewares, L3954N for cone 10 stonewares and L3685Z2 and L3685Z3 for low temperature (all of these have enough documentation to enable adjusting their firing shrinkage and thermal expansion fit).

Related Information

This pottery glaze is not flaking off - the underglaze is.

Pure ceramic stain powders don't melt at typical pottery temperatures so they cannot bond with clay body surfaces. They don't suspend in water and don't harden on drying. Pure stains contribute only one ceramic property: Color! Commercial underglazes sold in jars dilute stains into a recipe of materials, a 'base medium', designed to impart the missing properties. Unfortunately, consider a problem: Blue stains are fluxes and potent at low percentages. Orange stains (and others) are refractory and require much higher percentages. But for convenience underglaze manufacturers will want to use the same medium and percentage pigment to make an entire line of underglazes. That causes the problem seen here - flaking is happening at the interface between the body and orange underglaze. This underglaze recipe needs more frit (or a more potent one) so that enough melting occurs to create a better bond. For some colors it can thus be an advantage to make your own underglazes.

Underglazes, engobe, a good transparent glaze and cone 03. Life is good!

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.

Mural made using talc body and underglazes only

Anti-racism themed, this was a project by students in a school in Vancouver, BC. The clay is Plainsman L213, a talc-ball clay body. Spectrum underglazes. Pieces were fired at cone 04.

Reasons for low fire: Underglazing, color, transparency, fired stability

You can decorate the underside! The one on the right is the back side of the plate. This is Plainsman Snow clay, it can have 25% porosity. But when fired at cone 06 the porous body does not absorb any of the glaze. And the plates stay flat when fired on stilts. These are done by the team of Micah & Jeremiah Wassink of Creston, BC (at Pridham Studio). They make matching mugs, but fire those at cone 6 using underglaze decoration with a clear overglaze. But these plates are decorated using a combination of heavily pigmented viscous-melt low-fire glazes and a black underglaze and then finished with a thin layer of transparent glaze.

"Girls on the March" by Stephanie Osser - Underglaze decorated

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).

How can underglaze and engobe colors be this bright?

Top are V-326 and V-388 underglazes, painted on and bisque fired at cone 04. Although the layer is thin the coverage is very good and the brightness is stunning. How can these colors be so bright? Using very high, and expensive, percentages of stain. That explains why these commercial underglazes are double or triple the cost of a typical commercial glaze. The bottom mugs are clear-glazed and fired at cone 05, the one on the left with Amaco LG-10, The one on the right is Spectrum 700. The latter produces better results over the underglaze and is more transparent and less yellowish on the body.

Can you make bright-colored engobes and underglazes like this? Yes. Start with 50% stain and 50% stain medium (the percentage needed varies by color and type of stain).

Underglazes are more opaque than glazes of the same color

On the left is Spectrum 748 fire engine red cone 04 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.

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.

Underglaze brushwork decoration on a cone 6 porcelain plate

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.

Very low specific gravities on two commercial underglazes

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 gelling agent). When doing underglaze decorative brushwork it is important to get adequate thickness with each 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 down its drying and harden it well at the dried state).

A 2oz jar of underglaze dries down to 21g of powder. That is bad and good!

Only a part of that 21 grams is actual stain powder, it is mixed into a medium. A 16oz jar of another color yielded 190g, a slightly better percentage of powder-to-water. The high water content is actually a benefit, they condition it with a clay that causes it to be thixotropic (gels). In one scenario we calculated a 1500% saving in making your own underglaze vs. buying these jars, that is a more feasible idea than you might think. With out setup we can weigh and make a 1 pint jar of brushing glaze/underglaze in ten minutes.

When making your own 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.

Blending an engobe and a glaze to produce an underglaze

These fired samples demostrarte how you could develop your own Stroke & Coat glazes. I made these blends by dewatering an engobe (L3954B) and glaze (G2926B) and then hand-mixing line blending them. The left column is the fired blends (cone 6), the right one the unfired ones (showing the increase in soluble salts as the percentage of glaze increases). The right one shows the increase in melt fluidity. The top ones are 30:70 glaze:engobe and the bottom ones are 90:10 glaze:engobe. Notice the big change in melt fluidity between 60 and 70% glaze (third and fourth from bottom). And that the 70% one is quite glossy even though it has very low melt fluidity. Somewhere between 60 and 70 we could isolate a glaze-like product having underglaze stability. Stains could be incorporated and the proportion of glaze:underglaze adjusted according to its effect on melt fluidity. Doing this yourself enables targeting an exact melt fluidity for each color at the target temperature. The commercial product does not do that. You can also use more stain then they do to get more vibrant color for thinner applications.

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

These porcelain mugs were decorated with the same Amaco Velvet underglazes (applied at leather hard), then bisque fired, dipped in clear glaze and fired to cone 6. While the G2926B clear glaze (left) is normally a great super glossy transparent for general use, its melt fluidity is not enough to clear the micro-bubbles (a product of the LOI of body, glaze and underglaze). However the G3806C recipe (right) has a more fluid melt and thus transmits the underglaze colors better. But using a glaze like this can be tricky and it has downsides. Its melt fluidity requires care not to get it too thick (or it will run). Its high flux content means it is not as durable. And, its high KNaO content raises the thermal expansion (COE) considerably (and thus the danger of crazing). This porcelain has a high enough COE to fit most glazes so it is OK. But still, to be on the safe side, I always use the G2926B on the insides. Is this glaze perfect? No. There is room for more improvement (e.g. slow cooling the firing, drop-and-hold firing, even thinner layer).

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.

Choose the right transparent glaze to cover your underglaze decoration

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 glaze firing reveals that the specific gravity of this V-303 terra cotta under glaze is too low

Amaco V-303 Terra Cotta underglaze was applied in one coat, at leather hard stage (on these Plainsman Snow mugs). It appeared to be thick enough on application and after bisque (left) but the clear overglaze on the right has difused into it to melt it enough to lose the opacity. A higher specific gravity (lower water content) would make it possible to apply it thickly enough in one coat, this is especially important because each layer rewets the piece and adds a lengthy drying period. An appropriate specific gravity is thus the difference beween practical and impractical.

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.

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.

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.

Bleeding underglaze. Why?

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.

Why dipping transparent glazes may not cover underglazes well

Underglaze brushstrokes were applied to this test cup at the leather hard stage (lower left) and then bisque fired on. On the lower right a ball of the pure underglaze emerged from the same bisque firing, notice that it is certainly fusing enough to seal the surface of the bisque. Notice what happened on the upper left: Although the bisque piece was immersed in a dipping glaze the underglaze is not covering. Yet on the upper right a just-applied transparent brushing glaze has covered well (although three coats were needed with plenty of drying time between). Why is this? Brushing underglazes contain a lot of gum, it makes them spread well like paint - and dry hard, impervious and non-absorbent. And the frit they contain makes them fire even denser. Manufacturers of brushing underglazes assume that gummed brushing glazes will also be used over them and thus no coverage issues will occur. This being said, a thixotropic dipping glaze may cover. Or a first coat dipping glaze.

Zero3 Porcelain with underglaze decoration

Fired at cone 03 (in three hours). Clear glaze is G2931K. This porcelain is vitreous and appears very similar to a cone 10 reduction product. By Tony Hansen.

How to make a clear dipping glaze work over solid underglazes

This is what typically happens when applying a dipping glaze over an underglazed bisque-fired piece - it does not stick. Commercial underglazes impede the absorptive powder of the bisque but it still might be possible to make it work. Here are some options:
- Bisque the under-glazed ware to a low temperature (e.g. cone 010 or lower) and apply dipping glaze to that.
- Make your own underglaze for dipping, leaving out the CMC gum.
- Make your glaze thixotropic - because it is gelled it may hang on in an even layer over the underglaze.
- Make a brushing glaze and paint that over the underglaze. 1%-1.5% CMC gum will make it paintable. 1% additional Veegum will gel it enough that it can tolerate more water and go on thinner, this enables applying multiple coats and gives good control of the final thickness.

Inbound Photo Links

Underglaze color mayhem at cone 5!

Here is motivation to make your own underglazes!

Make AMACO velvet underglazes work better at cone 04

Is it impossible to get a good clear over-glaze for cone 6 brush decorated stoneware?

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