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A light bulb moment in solving bubble clouding:
The same black engobe with two transparent glazes.
This is a buff stoneware body, Plainsman M340. A L3954F black engobe was applied inside and upper-outside at leather hard. The piece was fired at cone 6 using the PLC6DS schedule. The inside, totally clouded glaze, is G2926B. Outside is GA6-B Alberta Slip amber transparent. This inside glaze is crystal-clear on other bodies (and on this one without the black engobe). The black stain in the engobe appears to be the issue. How?
Underglazes (or engobes) become a semi-dense layer and impede LOI by slowing gas diffusion. If the glaze then melts early and lacks viscosity, remaining channels of escape are sealed (increasing bubbling dramatically). Double-melt interfaces can form between vitreous engobes and glaze when the former softens, the clear glaze begins melting. Gases get trapped at the boundary, being generated at the exact wrong time during the firing.
Look at the outside amber transparent glaze, GA6-B. Although also early melting and on the same engobe, it has very little micro-bubble clouding! Why? It contains a lot of Alberta Slip, a material that is not finely ground like others. Particles across the range from 60-200 mesh are present; these are likely acting as a fining agent that enables bubble merging. The larger bubbles break at the surface because of sufficient melt mobility and lower melt surface tension.
Related Pictures
Thick application clouds a transparent glaze on a terra cotta clay
This picture has its own page with more detail, click here to see it.
Glaze clouding is a universal issue in ceramics. Terra cotta bodies often demonstrate this well. Pretty well all transparent glazes, even commercially available ones, can cloud, especially when applied thickly. This example is G2931K, it can be beautifully crystal clear when thin. But, as thickness increases, this happens. We ball-milled it to see if that would help, but as you can see, that has not impacted the problem. This is a dipping version, so that is part of the reason why it is easy to get it on too thick.
One of the advantages of brushing glazes is the ability to carefully control thickness. Many are formulated with a low specific gravity, requiring multiple layers for adequate thickness. Two coats rather than three are often better for this type of body.
2% Copper carbonate in two cone 6 transparents:
One does not bubble and orange-peel. Why?
This picture has its own page with more detail, click here to see it.
The top base glaze, G2926B, has enough melt fluidity to produce a brilliant functional gloss when used as a transparent. However, for this 2% copper carbonate addition, it has too little melt fluidity and/or too much surface tension to merge, pass and heal the entrained micro-bubbles (generated by the decomposition of the carbonate).
The lower glaze, G3806B, diversifies the fluxes (half the B2O3 in exchange for more Na2O and introduction of SrO and ZnO) and increases their total compared to Al2O3 and SiO2. The result is a more fluid cone 6 melt having lower surface tension. The mixed-oxide effect is also a factor here; the diversity itself improves the melt.
The above factors are enough to solve the problem here. But more can be done. More zinc (in exchange for KNaO) could produce later melting, especially in combination with sourcing some or all of the latter from a feldspar instead of the low-melting frit. The benefit would enable more gas escape until melt-sealing (and reduce the COE).
Glaze bubbles behaving badly!
We see it in a melt fluidity test.
This picture has its own page with more detail, click here to see it.
These melted-down-ten-gram GBMF test balls of glaze demonstrate the different ways in which tiny bubbles disrupt transparent glazes. These bubbles are generated during firing as particles in the body and glaze decompose. This test is a good way to compare bubble sizes and populations, they are a product of melt viscosity and surface tension. The glaze on the top left is the clearest but has the largest bubbles, these are the type that are most likely to leave surface defects (you can see dimples). At the same time its lack of micro-bubbles will make it the most transparent in thinner layers. The one on the bottom right has so many tiny bubbles that it has turned white. Even though it is not flowing as much it will have less surface defects. The one on the top right has both large bubbles and tinier ones but no clouds of micro-bubbles.
Zircopax as a fining agent to de-bubble a stained glaze
This picture has its own page with more detail, click here to see it.
The cone 03 porcelain cup on the left has 10% Cerdec encapsulated stain 239416 in the G2931K clear base (this is not an underglaze with a transparent over it). The surface is badly orange-peeled because the glass is full of micro-bubbles that developed during the firing. Notice that the insides of the cups are crystal-clear, no bubbles. So here, they are a direct product of the presence of the stain. The glaze on the right has even more stain, 15%. Why isn't it even worse? It also has a 3% addition of Zircopax (zircon), which acts as a fining agent. As you can see, this makes a dramatic difference. Suppliers of encapsulated stains even recommend a zircon addition, but are often unclear about why. This is the reason.
The yellow on the right could be improved further with a slow-cool or drop-and-hold firing. And if the glaze was reformulated to use a later melting frit (so fewer bubbles even get trapped).
Videos
Links
| Glossary |
Fining Agent
Individual tiny bubbles in a glaze melt can coalesce around the undissolved particles of a fining agent, the growing bubble swallowing others around it and finally exiting at the surface. |
| Glossary |
Glaze Bubbles
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them. |
| Troubles |
Clouding in Ceramic Glazes
There a many factors to deal with in your ceramic process to achieve transparent glazes that actually fire to a crystal-clear glass |
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