|Monthly Tech-Tip |
There a many factors to deal with in your ceramic process to achieve transparent glazes that actually fire to a crystal-clear glass
It seems logical that a transparent glaze would fire transparent. Commercial ware often sports brilliant clear-as-crystal glazes, they make it look easy. But this is far from the truth. For certain bodies (e.g. low fire whites, bone china) transparent glazes seem to work almost every time. But on others, it can be very difficult to achieve a crystal clear (e.g. terra cotta). In the past, when lead-as-a-flux was common, clear glazes were actually easier. But today we mostly rely on fluxes like boron, sodium, lithium and zinc at low and middle temperatures - it is more difficult to create crystal-clear glasses. Even when we source these using frits. At higher temperatures, materials like feldspar, calcium carbonate, dolomite and talc supply fluxes, these do not melt to a brilliant clear glaze nearly as well as frit-based ones at lower temperatures. That is one reason why underglaze decoration just does not work well on high-temperature stoneware (by 'well' I mean bright colors coming through a perfectly transparent overglaze). Or even porcelain, unless it is bisque fired to vitrification. Even at middle temperatures, it can be tricky to develop body/underglaze/overglaze/firing combinations that enable transmitting the full vibrance of the color through a transparent glass. One of the most impressive accomplishments in ceramics is low fire terra cotta embellished with a brilliant thick transparent glass, showcasing the warm earthen-red color.
Clouding is most often, but not always caused by micro-bubbles. It can also be crystals growing in the glass, phase changes that can mar transparency and boron blue (which is also crystalline in nature).
For glazes to fire completely transparent everything has to be just right. Contributing factors to bubble clouding include firing schedule and firing temperature, glaze thickness, laydown density, application method, the amount of frit in the glaze recipe, the presence of carbonates (especially colorants) in body or glaze, the presence of particulates in the body (that generate gases of decomposition), the firing temperature of the bisque, the degree to which the body will be vitrified, the degree to which the glaze will melt, the viscosity and surface tension of the glaze melt, the chemistry of the glaze, the presence of unmelted particles in the melt (to act as a fining agent), the quality of the frit, the particle size of the glaze materials, and more. This means that you need to five attention to a range of factors before you succeed in getting a transparent glaze to fire clear.
It is important to understand the mechanisms under which your glaze becomes cloudy, this is the best way to avoid it happening. Hopefully, some of the pictures here will give you ideas on changes to your process to reduce this problem.
It was a clear glass out of the kiln, the 20x5 recipe. First, it would be helpful to look at the cloudy areas under a good microscope. Is it happening on the surface, under the surface or at the glaze:body interface. If the latter, the glaze chemistry could give clues about whether it is likely to be vulnerable to leaching (and thus a surface issue). In this case, the cloudy areas are occurring where the glaze is thicker (around the handle join). Perhaps the body is waterlogging and the water is migrating up into bubble networks and making them visible.
Two clear glazes fired in the same slow-cool kiln on the same body with the same thickness. Why is one suffering boron blue (1916Q) and the other is not? Chemistry and material sourcing. Boron blue crystals grow best when there is plenty of boron (and other power fluxes), alumina is low, adequate silica is available and cooling is slow enough to give them time to grow. In the glaze on the left B2O3 is higher, crystal-fighting Al2O3 and MgO levels are a lot lower, KNaO fluxing is significantly higher, it has more SiO2 and the cooling is slow. In addition, it is sourcing B2O3 from a frit making the boron even more available for crystal formation (the glaze on the right is G2931F, it sources its boron from Ulexite).
These are the inside uppers on two mugs made from the same clay with the same clear glaze. The one on the left was fired in a large electric kiln full of ware (thus it cooled relatively slowly). The one on the right was in a test kiln and was cooled rapidly. This glaze contains 40% Ferro Frit 3134 so there is plenty of boron and plenty of calica to grow the borosilicate crystals that cause the cloudiness in the glass. But in the faster cooling kiln they do not have time to grow.
This is crazing. It is bad on functional ware when glazes do not fit. There is no law that says that commercial glazes and clay bodies must be thermal expansion compatible, testing is required. Thus potters and artists invariably bump into this issue but science is needed to understand what it is and what the solution is. When some technical language is employed to describe it and methodical testing to prove a solution time is not wasted on "art solutions" that don't work.
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 G1916Q recipe uses common Ferro frits and fits most low fire bodies (except this with high talc). It is easier to tune its recipe to adjust thermal expansion adjustable than others we have published in the past. And it melts well down to cone 06. And we have a strategy to reduce clouding and micro-bubbling. These five test tiles were fired using the 04DSDH schedule (drop-and-hold) firing schedule. Results are flawless. All exited from the kiln without crazing. The L215, L213, L210 and L212 samples subsequently survived a 300F/Icewater test without crazing, but the Buffstone did not (it needs a higher thermal expansion glaze adjustment). The L213 would not likely survive a cold-to-hot test without shivering (it needs a lower thermal expansion adjustment).
These three mugs are made from L215 and fired to cone 04 (in a typical fast firing). The clear glazes are left: G3879E, middle: G2931K and right: G3879F1. The latter appears to be the most promising for creating the most transparent (although it is also applied a little thinner than the other two). That being said, the cloudy center glaze, G2931K, is aesthetically quite striking when applied thickly enough (as shown in the sample test tile).
The center mug is clear-glazed, at cone 6, with G2926B (notice it is saturated with bubble clouds). This dark body, M390, is exposed inside and out (the other two mugs have the L3954B white engobe inside and midway down the outside). G2926B is an early-melter (starting at cone 03) so it is bubble-susceptible to dark-burning bodies that generate more gases of decomposition. It appears to perform well on the inside engobed surface of the mug on the left but actually, the bubbles are just less visible against the white background. The honey colored glaze on the outside mugs is GA6-B, it fires with very few bubbles and is a good choice for use on dark clay bodies.
This is a buff stoneware body. A 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 clear. Normally that inside glaze is crystal-clear on other bodies, but this black engobe is generating tiny gas bubbles at the exact wrong time during the firing and the melt is unable to pass them. The black stain seems implicated as the gas generator or catalyst. The outside glaze, although amber rather than completely transparent, is demonstrating its ability at melting to a transparent. If the GA6-B was also inside, it would be transparent there also.
Both pieces are the same clay body, Plansman L215. Both are fired to cone 03. Both are glazed using G1916Q recipe. The glaze on the piece on the left has 2% added iron oxide (and sieved to 80 mesh). Each particle or agglomerate of iron (which is refractory in this situation) acts to congregate the micro-bubbles so they can better exit the glaze layer. Notice also how much richer the color is as a result. The piece on the right, without the added iron oxide, is neither as red nor as transparent. Both of these mugs, by the way, are glazed on the bottom and were fired on stilts.
These are fired in cone 6 oxidation. They are all the same clay body (Plainsman M390). The center mug is clear-glazed with G2926B (and is full of bubble clouds). This dark body is exposed inside and out (the other two mugs have a white engobe inside and midway down the outside). G2926B clear glaze is an early-melter (starting around cone 02) so it is susceptible to dark-burning bodies that generate more gases of decomposition - they produce the micro-bubble clouding. That being said, the other two glazes here are also early melters, yet they did not bubble. Left: G2926B plus 4% iron oxide. That turns it into an amber color but the iron particles vacuum up the bubbles! Right: Alberta Slip GA6-A using Ferro Frit 3195 as the melter. It also fires as an amber-coloured glass, but on a dark body, this is an asset.
G1916Q and J low fire ultra-clear glazes (contain Ferro Frit 3195, 3110 and clay) fired across the range of 1650 to 2000F (these were 10 gram GBMF test 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.
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.
These two glazes are both brilliant glass-like super-transparents. But on this high-iron stoneware only one is working. Why? G3806C (on the outside of the piece on the left) melts more, it is fluid and much more runny. This melt fluidity gives it the capacity to pass the micro-bubbles generated by the body during firing. G2926B (right) works great on porcelain but it cannot clear the clouds of micro-bubbles coming out of this body. Even the glassy smooth surface has been affected. The moral: Two base transparents are needed, each being able to host colors, opacifiers and variegators. But there is a caveat: Although reactive glazes leverage melt fluidity to develop interesting surfaces they are more tricky to use and do not fire as durable.
Low-fire glazes must be able to pass the bubbles they and the underlying bodies generate (or clouds of micro-bubbles will turn them white). This cone 04 flow tester makes it evident that 3825B has a higher melt fluidity (A has not even dripped onto the tile). And its higher surface tension is demonstrated by how the flow meets the runway at a perpendicular angle (it is also full of entrained micro-bubbles). Notice that A, by contrast, meanders down the runway, a broad, flat and relatively clear river. Low-fire glazes must pass many more bubbles than their high-temperature counterparts, the low surface tension of A aids that. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). These two represent very different chemistry approaches to making a clear glaze. Which is better? Both have advantages and disadvantages.
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 two mugs are the same dark burning stoneware (Plainsman M390). They have the same clear glaze, G2926B. They are fired to the same temperature in the same C6DHSC firing schedule. But the glaze on the left has 4% added iron oxide. On a light-burning body the iron changes the otherwise transparent glass to honey colored (with light speckle because of agglomerates). But on this dark burning clay it appears transparent. And, amazingly, the bubble clouds are gone! We have not tested further to find the minimum amount of iron needed for this effect but with other glazes 2% is working.
Left two: Plainsman M390 stoneware. Lower right: M370 porcelain. The bottom two samples are a popular cone 6 ultra clear commercial bottled glaze that costs about $20/pint. On the porcelain, it is crazing. On the red clay it is saturating with micro-bubbles and going totally cloudy and even a satin surface (it is likely very high in boron and melting too early). Whose fault is this? No ones. This glaze is simply not compatible with these two bodies. The one on the upper left has almost no bubbles and no crazing. It is the GA6-B recipe and is well documented and easy to adjust.
These are two 10 gram GBMF test 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).
These are the same glaze, same thickness, Ulexite-based G2931B glaze, fired to cone 03 on a terra cotta body. The one on the right was fired from 1850F to 1950F at 100F/hr, then soaked 15 minutes and shut off. The problem is surface tension. Like soapy water, when this glaze reaches cone 03 the melt is quite fluid. Since there is decomposition happening within the body, gases being generated vent out through surface pores and blow bubbles. I could soak at cone 03 as long as I wanted and the bubbles would just sit there. The one on the left was fired to 100F below cone 03, soaked half an hour (to clear micro-bubble clouds), then at 108F/hr to cone 03 and soaked 30 minutes, then control-cooled at 108F/hr to 1500F. During this cool, at some point well below cone 03, the increasing viscosity of the melt becomes sufficient to overcome the surface tension and break the bubbles. If that point is not traversed too quickly, the glaze has a chance to smooth out (using whatever remaining fluidity the melt has). Ideally I should identify exactly where that is and soak there for a while.
This is supposed to be transparent. We ball milled it. See G2931K1.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Ceramic Glaze Defects
|By Tony Hansen|
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