As glazes melt, gases from decomposition of organics, carbonates, sulphates and hydrates are generated (if the body was glazed green, or unbisqued, many more of these gases will be present). If glazes are already melting while the gases are being generated, bubbles form and suspend in the glass melt. Ideally, ware should be bisqued fired at the highest possible temperature (yet still be absorbent enough to glaze). If ware is being fired green, kilns should be soaked at the highest point possible before the glaze melts (on the way up).
Example of how bubbles dissipate in a glaze with increasing temperature
This is a Gerstley Borate based recipe (45%) melted in crucibles at increasing temperatures. Although the recipe is well melted at cone 2, it is still not fluid enough to enable their migration in the time available. By contrast, the melt at the upper temperature is much less viscous, enabling all bubbles to completely clear on the thinner sections. If this glaze were applied to ware it would be in a thin layer and the bubbles would likely clear at cone 6. Not to be ignored is the degree to which the thousands of bubbles passing upward through the melt have helped to mix the melt and remove discontinuities in the cone 7 and 8 specimens.
Close-up of cone 10R celadon bubbles suspended in the glass
This is happening because this glaze lacks flux and is not fluid enough to enable their migration. In the upper half they are more evident (double thickness).
What material makes the tiny bubbles? The big bubbles?
These are two 10 gram balls of Worthington Clear glaze fired at cone 03 on terra cotta tiles (55 Gerstley Borate, 30 kaolin, 20 silica). On the left it contains raw kaolin, on the right calcined kaolin. The clouds of finer bubbles (on the left) are gone from the glaze on the right. That means the kaolin is generating them and the Gerstley Borate the larger bubbles. These are a bane of the terra cotta process. One secret of getting more transparent glazes is to fire to temperature and soak only long enough to even out the temperature, then drop 100F and soak there (I hold it half an hour).
Calcium carbonate and glaze bubbles
The two cone 04 glazes on the right have the same chemistry but the center one sources it's CaO from 12% calcium carbonate and ulexite (the other from Gerstley Borate). The glaze on the far left? It is almost bubble free yet it has 27% calcium carbonate. Why? It is fired to cone 6. At lower temperatures carbonates and hydrates (in body and glaze) are more likely to form gas bubbles because that is where they are decomposing (into the oxides that stay around and build the glass and the ones that are escaping as a gas). By cone 6 the bubbles have had lots of time to clear.
Comparing the brands of calcium carbonate
A cone 6 melt flow test to compare two calcium carbonates (they make up 27% of this glaze recipe that was designed to maximize their percentage). Notice the amount of bubbles (due to the high loss on ignition of the material). Different brand-names of the material obviously have slightly different chemistries so they exhibit different flow properties during firing.
Why is that transparent glaze firing cloudy? The balls test us.
G1916Q and J low fire ultra-clear glazes (contain Ferro Frit 3195, 3110 and EPK) fired across the range of 1650 to 2000F (these were 10 gram balls that melted and flattened as they fired). Notice how they soften over a wide range, starting below cone 010 (1700F)! At the early stages carbon material is still visible (even though the glaze has lost 2% of its weight to this point), it is likely the source of the micro-bubbles that completely opacify the matrix even at 1950F (cone 04). This is an 85% fritted glaze, yet it still has carbon; think of what a raw glaze might have! Of course, these specimens test a very thick layer, so the bubbles are expected. But they still can be an issue, even in a thin glaze layer on a piece of ware. So to get the most transparent possible result it is wise to fire tests to find the point where the glaze starts to soften (in this case 1450F), then soak the kiln just below that (on the way up) to fire away as much of the carbon as possible. Of course, the glaze must have a low enough surface tension to release the bubbles, that is a separate issue.
A matte that is matte because it is not melting completely
Left: This specimen of VC71 cone 6 matte glaze was felt-marked and cleaned with acetone. A closeup of the ink specks reveals they are held in micro-bubbles breaking at the surface. This specimen has also been thermally stressed in a 300F/icewater test (causing the crazing pattern, which curiously, only shows up on part of the surface). Right: An adjustment to VC71 that adds more boron and Al2O3/SiO2 (while preserving the Si:Al ratio). It is much glossier, confirming that, even though the VC71 matte surface feels functional to the touch, it is a product of improper melting.
Red burning clays bubble glazes more
A closeup of the rim and a transparent-glazed cup made from a high-iron clay (Plainsman Redstone) and fired at cone 6. Iron-bearing clays tend to gas more on firing and can generate many more bubbles in glazes for this reason (a buff stoneware would be almost free of bubbles with this glaze, Plainsman M340 transparent). Thus, it is best to use opacified glazes with this type of body.
Reduction and oxidation porcelains
Left: Cone 10R (reduction) Plainsman P700 porcelain (made using Grolleg and G200 Feldspar). Right: Plainsman Cone 6 Plainsman Polar Ice porcelain (made using New Zealand kaolin and Nepheline Syenite). Both are zero porosity. The Polar Ice is very translucent, the P700 much less. The blue coloration of the P700 is mostly a product of the suspended micro-bubbles in the feldspar clear glaze (G1947U). The cone 6 glaze is fritted and much more transparent, but it could be stained to match the blue. These are high quality combinations of glaze and body.
Cone 03 stoneware. Red and white body and slips. Clear glaze.
Cone 03 white stoneware with red terra cotta ball-milled slip and transparent overglaze. These are eye-popping stunning. They are test L3685U (Ferro frit 3110, #6 tile kaolin, Silica), near the final mix for a white low fire stoneware. The G1916J glaze is super clear. Why? Two reasons. These were fired in a schedule designed to burn off the gases from the bentonite in the body before the glaze fuses (it soaks the kiln for 2 hours at 1400F). Terra cotta clays generate alot of gases at cone cone 03 (producing glaze micro-bubbles), but here the terra cotta is only a thin slip over the much cleaner burning white body.
Bubbles in Terra Cotta transparent glazes. What to do?
Two transparent glazes applied thickly and fired to cone 03 on a terra cotta body. Right: A commercial bottled clear, I had to paint it on in layers. Left: G1916S almost-zero-raw-clay glaze, a mix of Ferro frit 3195, 3110, calcined kaolin and a small amount of VeeGum T. The bubbles you see on the left are from the gas generated by the body. The ones on the right are from body and glaze. How can so many more bubbles be generated within a glaze? Raw kaolin. Kaolin loses 12% of its weight on firing, that turns to gas. Low temperature glazes melt early, while gassing may still be happening. So to get a crystal clear the raw clay content has to be as low as possible. Obviously, a white burning body made from refined materials would be even better. A good compromise: A red slip (or engobe) over a white burning body, it would generate far less gases because of being much thinner and still exhibit the nice red color.
Microbubbles in low fire glaze. Why?
Left: G1916Q transparent fired at cone 03 over a black engobe (L3685T plus stain) and a kaolin-based low fire stoneware (L3685T). The micro-bubbles are proliferating when the glaze is too thick. Right: A commercial low fire transparent (two coats lower and 3 coats upper). A crystal clear glaze result is needed and it appears that the body is generating gases that cause this problem. Likely the kaolin is the guilty material, the recipe contains almost 50%. Kaolin has a 12% LOI. To cut this LOI it will be necessary to replace some or all of the kaolin with a low carbon ball clay. This will mean a loss in whiteness. Another solution would be diluting the kaolin with feldspar and adding more bentonite to make up for lost plasticity.
Glaze bubbles on buff and red burning bodies at cone 6
These Plainsman Midstone and Redstone cups are fired to cone 6 with M340 Transparent glaze liner (these are raw materials that body manufacturers incorporate into their products in fairly high percentages). Notice how many more glaze bubbles there are with the red cup. This is typical using other transparent glazes also. To get a bubble-free clear on this red burning body a glaze having a higher melt fluidity is needed.
Why is this transparent so full of bubbles?
An example of how a micro-bubble population in the matrix of a transparent glaze can partially opacify it. If this glaze was completely transparent, the red clay body would show much better. However this is not the fault of the glaze. On a white body it would be more transparent. The problem is the terra cotta body. This is fired at cone 02. As the body approaches vitrification the decomposition of particles within it generate gases that bubble up in to the glaze. A positive aspect of this phenomena that this glaze could be opacified using a lower percentage of zircon. This type of glaze responds better to opacifier additions.
LOI is not important? Think again!
This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters. The LOI (loss on ignition) of these materials can affect your glazes (e.g. bubbles, blisters, pinholes, crawling). Notice that talc is not finished until after 1650F (many glazes have already begin melting by then).
What does it take to get a crystal-clear low fire transparent? A lot!
These three cups are glazed with G1916S at cone 03. The glaze is the most crystal clear achieved so far because it contains almost no gas producing materials (not even raw kaolin). It contains Ferro frits 3195 and 3110 plus 11 calcined kaolin and 3 VeeGum. Left is a low fire stoneware (L3685T), center is Plainsman L212 and right a vitreous terra cotta (L3724F). It is almost crystal clear, it has few bubbles compared to the kaolin-suspended version. These all survived a 300F/icewater test without crazing!
Is clear glazing a dark burning oxidation stoneware a good idea? No.
Dark bodies tend to have more carbon impurities and the burnout of these can generate gases that create bubbles in the glaze. Because of the dark background, the bubbles impart a muddy look. The body on the left is a finer particle size, so the lower thinner glazed section is a partial success, but the upper section is bubbling. The body on the right, although a more pleasant red color, is bubbling worse. Notice also that the warm color of the body is at least largely lost under the glaze.
A body containing manganese bubbles the glaze
Laguna Barnard Slip substitute fired at cone 03 with a Ferro Frit 3195 clear glaze. The very high bubble content is likely because they are adding manganese dioxide to match the MnO in the chemistry of Barnard (it gases alot during firing).
Why does this glaze variegate like this?
This is a cone 10R copper red. First, it is thick. "Thick" brings it own issues (like running, blisters, crazing). But look what is under the surface. Bubbles. They are coming out of that body (it is not vitreous, still maturing and generating them in the process). The bubbles are bringing patches of the yellow glass below into the red above. Normally bubbles are a problem, but in this decorative glaze, as long as everything goes well, they are a friend.
G2931F Ulexite-based transparent bubbles, G2931K frit-based version does not
I melted these two 9 gram balls on tiles to compare their melting (the chemistry of these is identical, the recipes are different). The Ulexite in the G2931F (left) drives the LOI to more than 14%. That means the while the ulexite is decomposing during melting it is creating gases that are creating bubbles in the glass. Notice the size of the F is greater (because it is full of bubbles). While this seems like a serious problem, in practice the F fires crystal-clear on most ware.
Adding iron to a clear glaze has cleared the micro-bubbles!
The glaze on the right is a transparent, G2926B, on a dark burning cone 6 body (Plainsman M390). On the left is the same glaze, but with 4% red iron oxide added. The entrained micro bubbles are gone and the color is deep and much richer. It is not clear how this happens, but it is certainly beneficial.
A super glassy ultra-clear brilliantly glossy cone 6 clear base glaze? Yes!
I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. In the end I will pick one or two, fix the issues and provide instructions.
Glaze bubbles behaving badly! We see it in a melt fluidity test.
These melted-down-ten-gram 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.
Frits melt so much more evenly and trouble free
These two specimens are the same terra cotta clay fired at the same temperature (cone 03) in the same kiln. The chemistry of the glazes is similar but the materials that supply that chemistry are different. The one on the left mixes 30% frit with five other materials, the one on the right mixes 90%+ frit with one other material. Ulexite is the main source of boron (the melter) in #1, it decomposes during firing expelling 30% of its weight as gases (mostly CO2). These create the bubbles. Each of its six materials has its own melting characteristics. While they interact during melting they do not mix to create a homogeneous glass, it contains phases (discontinuities) that mar the fired surface. In the fritted glaze all the particles soften and melt in unison and produce no gas. Notice that it has also interacted with the body, fluxing and darkening it and forming a better interface. And it has passed (and healed) most of the bubbles from the body.
Glaze melt fluidity comparison between G2931F and fritted G2931K show the effect of LOI
These two glazes have the same chemistry but different recipes. The F gets its boron from Ulexite, and Ulexite has a high LOI (it generates gases during firing, notice that these gases have affected the downward flow during melting). The frit-based version on the right flows cleanly and contains almost no bubbles. At high and medium temperatures potters seldom have bubble issues with glazes. This is not because they do not occur, it is because the appearance of typical glaze types are not affected by bubbles (and infact are often enhanced by them). But at low temperatures potters usually want to achieve good clarity in transparents and brilliance in a colors, so they find themselves in the same territory as the ceramic industry. An important way to do this is by using more frits (and the right firing schedules).
Cone 6 glazes can seal the surface surprisingly early - melt flow balls reveal it
These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.
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.
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.
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