A fully transparent glaze is simply one that does not have opacity. But there are degrees of transparency. For example, if a glaze is matte it will show the color of underlying body and decoration, but these will be muted (so it is actually translucent). Completely transparent glazes look like a glass container or a mirror, perfectly clear and glassy smooth. Glazes that you might have always taken to be transparent may appear much less so when compared side-by-side with a true brilliant glossy clear. It is actually quite difficult to achieve a true transparent. A variety of factors can cause light to scatter (reflect) from discontinuities or surfaces on or within the glass matrix. These include entrained bubbles (or surface disruptions caused when they break the surface), bodies containing impurities, phase separation, crystal growth during cooling (devitrification), unmelted or undissolved particles (e.g. silica) or simple lack of a smooth glassy surface. Changes in the chemistry, materials, application method and firing may all be needed to deal with these factors. Low LOI materials produce the least gases during firing. Frits melt better and phase change less. Low water content glazes have the most dense, bubble free laydown. Hold times on firing (either a the top-soak or drop-and-soak) help smooth out the surface. Certain chemistries are susceptible to crystallization, especially if cooling is too slow. Fine-particled bodies made from clean materials are best.
Transform the yellow-white of cone 6 to blue-white of cone 10R
Adding a little blue stain to a medium temperature transparent glaze can give it a more pleasant tone. Some iron is present in all stoneware bodies (and even porcelains), so transparent glazes never fire pure white. At cone 10 reduction they generally exhibit a bluish color (left), whereas at cone 6 they tend toward straw yellow (right). Notice the glaze on the inside of the center mug, it has a 0.1% Mason 6336 blue stain addition; this transforms the appearance to look like a cone 10 glaze (actually, you might consider using a little less, perhaps 0.05%). Blue stain is a better choice than cobalt oxide, the latter will produce fired speckle.
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).
What is the secret of the higher gloss glaze on the right? Yikes, it is lead!
These cone 04 glazes have the same recipe (a version of Worthington Clear sourcing B2O3 from Ulexite instead of Gerstley borate). While the one on the left is OK, the one on the right is great! Why? It has 10% added lead bisilicate frit. Of course, I would not recommend this, I am just demonstrating how well it melts. Still, we gasp at the thought of using lead while we thrive on unstable flux-deprived, glass-deprived and alumina-deprived base stoneware glazes with additions of toxic colorants like chrome and manganese!
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.
Boron blue in low fire transparent glazes
This high boron cone 04 glaze is generating calcium-borate crystals during cool down (called boron-blue). This is a common problem and a reason to control the boron levels in transparent glazes; use just enough to melt it well. If a more melt fluidity is needed, decrease the percentage of CaO. For opaque glazes, this effect can actually enable the use of less opacifier.
G1947U transparent glaze (left) vs. Ravenscrag Slip at cone 10R
Ravenscrag Slip is not ultra glossy but has a silky surface. It also contains some iron oxide and this colors the glaze somewhat. But the surface is much less sterile and pleasant to touch.
G1947U cone 10 transparent on Plainsman H550 and H570
This is a base recipe that was originally used for electrical insulators on a 25% porcelain recipe. Since most porcelains and whitewares used in high fire ceramics have this same type of formulation, this glaze recipe has proven to work well. It is not highly fluid, so if refractory colorants are added extra flux may be needed.
How do you turn a transparent glaze into a white?
Right: Ravenscrag GR6-A transparent base glaze. Left: It has been opacified (turned opaque) by adding 10% Zircopax. This opacification mechanism can be transplanted into almost any transparent glaze. It can also be employed in colored transparents, it will convert their coloration to a pastel shade, lightening it. Zircon works well in oxidation and reduction. Tin oxide is another opacifier, it is much more expensive and only works in oxidation firing.
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.
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.
When glazes are highly fluid they can...
An example of a highly fluid glaze melt that has pooled in the bottom of a bowl. The fluidity is partly a product of high KNaO, thus it is also crazed (because KNaO has a very high thermal expansion). While it may to decorative, this effect comes at a cost. The crazing weakens the piece, much more than you might think (200%+). Those cracks in that thick layer at the bottom are deep, they want to continue down into the body and will do so at the first opportunity (e.g. sudden temperature change, bump). Also, fluid glazes like these are more likely to leach.
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.
Orange-peel or pebbly glaze surface. Why?
This is a cone 10 glossy glaze. It should be crystal clear and smooth. But it contains strontium carbonate, talc and calcium carbonate. They produce gases as they decompose, if that gas needs to come out at the wrong time it turns the glaze into a Swiss cheeze of micro bubbles. One solution is to use non-gassing sources of MgO, SrO and CaO. Or, better, do a study to isolate which of these three materials is the problem and it might be possible to adjust the firing to accommodate it. Or, an adjustment could be make to the chemistry of the glaze such that the melting happened later and more vigorously (rather than earlier and more slowly). The latter is actually the likely cause, this glaze contains a small amount of boron frit. Boron melts very early so the glaze is likely already fluid while gases that normally escape before other cone 10 glazes even get started melting are being trapped by this one.
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.
In pursuit of a reactive cone 6 base that I can live with
These melt-flow and ball-melt tests compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are: G3814 - Moderate zinc, no boron G2938 - High-soda+lithia+strontium G3808 - High boron+soda (Gerstley Borate based) G3808A - 3808 chemistry sourced from frits G3813 - Boron+zinc+lithia G3806B - Soda+zinc+strontium+boron (mixed oxide effect) This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, excessive running, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually chose G3806B and further modified it to reduce the thermal expansion.
One example where a highly melt fluid clear glaze is better
On dark burning medium temperature stoneware bodies, clear glazes often do not look good. These bodies contain more raw clays that contain larger particles that generate gases on decomposition during firing. These often cloud up typical clear glazes with micro bubbles, marring their appearance. There are solutions. Although more fluid-melt clear glazes have risks (e.g. running, blistering) they do clear bubbles better. If applied thinly (so they do not run during firing) they can work very well in this circumstance. Of course they do darken the body color (this body, Plainsman M390, fires red without glaze). This outside glaze is G3806C.
Checking new glazes using a melt fluidity test
This is an example of how useful a flow tester can be to check new glaze recipes before putting them on ware and into your kiln. This was fired to only cone 4, yet that fritted glaze on the left is completely over-melted. The other one is not doing anything at all. These balls are easy to make, you only need weigh out a 50 gram batch of glaze, screen it, then pour it on a plaster bat until it is dewatered enough to be plastic enough to roll these 10 gram balls.
Comparing the melt fluidity of four copper blue cone 6 glazes
The first glaze is a control, a standard non-fluid clear with copper. The other three are the short-listed ones in my project to find a good copper blue recipe starting recipe and fix its problems (which they all have). The flow testers at the back and the melt-down-balls in front of them contain 1% copper carbonate. The glazed samples in the front row have 2% copper carbonate. L3806B, an improvement on the Panama Blue recipe, has the best color and the best compromise of flow and bubble clearing ability.
Same glaze, same clay, same cone 6 electric firing. Why is one speckled?
These are jiggered lids made from Plainsman M340 middle temperature stoneware. The one on the right was sponged in the dry stage to smooth issues that occurred during jiggering. That has exposed speck producing particles that were under the surface. This body is made from quarried materials that are ground to 42 mesh.
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.
Two bases, 2% copper additions. Which is the better transparent?
Wrong. It is the one on the right. While the copper looks so much better in that fluid one on the left, that melt mobility comes at a cost: blisters. As a clear glaze it is no glossier than the other one, but it runs into thicker zones at the bottom and they blister. This is because the high mobility coupled with the surface tension blows bubbles as gases of decomposition travel through (in a normal cooling kiln they break low enough that mobility is insufficient to heal them). The fired glass in the one on the left is also not as hard, it will be more leachable, it will also craze more easily and be more susceptible to boron-blue devritrification. But as a green? Yes it is better.
Two transparent glazes on the same dark burning clay. Why different?
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: You need two base transparents in which to put your colors, opacifiers and variegators. Reactive glazes need melt fluidity to develop those interesting surfaces. But they are more tricky to use and do not fire as durable.
Surface tension is a big deal in transparent glazes at cone 04
Low fire glazes must be able to pass the bubbles their bodies generate (or clouds of micro-bubbles will turn them white). This cone 04 flow tester makes it clear that although 3825B has a higher melt fluidity (it has flowed off onto the tile, A has not). And it has a much higher surface tension. How do I know that? The flow meets the runway at a perpendicular angle (even less), it is long and narrow and it is white (full of entrained micro-bubbles). Notice that A 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 similar to SG13, although flowing less). However they all dry very slowly. Watch for a post on G2931J, a Ulexite/Frit-based recipe that works like A but dries on dipped ware in seconds (rather than minutes).
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 G2931F, middle G3806C, right G1947U).
Can you cone-03-clear-glaze a cone 10R bisque-fired stoneware mug? Yes.
But it is better not to. Left is an unglazed (but bisque fired) mug. Right is the same thing with with G2931F clear glaze fired on at cone 03 (G2931G would fit without crazing). Since the clay is not porous the glaze must be gelled to hang on and the ware needs to be heated before dipping (or it takes too long to dry). It also needs to be applied thicker. It is very difficult. It is better to use a fritted low fire porcelain and put the G2931F glaze on it.
G2931K Zero3 transparent glaze on Zero3 Fritware Porcelain
This is an all-fritted version of G2931F Zero3 transparent glaze. I formulated this glaze by calculating what mix of frits must be employed to supply the same chemistry of the G2931F recipe. The mug is made from the Zero3 porcelain body (fired at cone 03) with this glaze. This glaze fits both the porcelain and the Zero3 terra cotta stoneware. The clarity, gloss, fit and durability of this glaze are outstanding.
A secret to an ultra clear at low fire. Magnesia-alkali, low Si:Al ratio, more boron.
On the left is G2931J, a zinc alkali fluxed and high Si:Al ratio glaze. Those look like micro-bubbles but they are much more likely to be micro-crystals. High-zinc and high-silica is the mechanism for crystalline glazes, so it appears that is what they are. G2931K on the right has much more boron, double the Al2O3, less SiO2 and is magnesia-alkali instead of zinc-alkali. It is the product of dozens of tests to find an ultra-clear having a glassy smooth surface. This particular chemistry, although having only a 6:1 SiO2:Al2O3 ratio is ultra-gloss. In addition is has low expansion, will fast fire and the boron is not high enough to compromise the hardness.
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