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Transparent Glazes

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.

Key phrases linking here: transparent glazes - Learn more

Details

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 may show the color of the 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), agglomerations 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 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, glazes that are ball-milled well and applied as thinly as practical will also give best results.

The most brilliant transparents are high in Na2O and K2O (unfortunately these oxides contribute to a high thermal expansion and crazing). Glossy transparents also usually have a high SiO2:Al2O3 ratio (greater than 10).

Transparent glazes can also be coloured (as opposed to opaque colors). With transparent colours, variations in glaze thickness produce color highlighting (celadon glazes are a classic example). But many glazes can also change color with thickness, this happens because of the tendency to crystallize on cooling coupled with the fact that crystal growth often varies with glaze thickness (fluid melt high-iron glazes do this if cooled slowly enough).

Transparents can amplify the coloring effect of iron in an underlying body (because they are fluxing the body surface and making it more mature). For example, at cone 6, a porcelain or white stoneware may appear more yellowish under a transparent than it is without a glaze covering. One method to deal with this is add 0.05 to 0.1% blue stain (to the body). Transparents also affect the color of underlying brown and red bodies. While low-fire terra cottas may burn to a pleasant red color with no glaze, under a transparent they will often fire brown (middle-temperature red bodies suffer the same fate). For this reason, terra cotta bodies are usually fired well below the red-to-brown transformation point.

When a transparent is intended as a base one must consider the types of visual effects desired. Typical functional transparents may melt to a smooth glossy glass as-is, but when certain colorants or opacifiers are added the brilliance of the surface may be lost somewhat. For this reason, the melt fluidity must be taken into account (more fluid melts will stay fluid while hosting refractory colorants and opacifiers). However, they also present more of a danger of running onto the shelf, crazing and leaching.

Related Information

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.

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 more melt fluidity is needed, decrease the percentage of CaO. There is a positive: 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.

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.

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).

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.

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, I ended up getting it on pretty thick. Left: G1916S, a mix of Ferro frits, nepheline syenite and kaolin - one dip for 2 seconds and it was glazed. And it went on more evenly. Bubbles are of course generated by the body during firing. But also in the glaze. Raw kaolin loses 12% of its weight on firing, that produces gas. Low temperature glazes melt early, while gassing may still be happening. Keeping raw clay content in a glaze as low as possible is good, but at least 15% is normally needed for working properties. Improvements? Both of these could have been applied thinner. And I could have fired them using a drop-and-hold and a slow-cool schedule.

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. They are L3685U engobe (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 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? Likely not.


Clear glaze bubble clouding

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. At its worst pinholes can appear.

Orange-peel or pebbly glaze surface. Why?


An orange peel textured glaze

This is a cone 10 glossy glaze. It has the chemistry that suggests it should be crystal clear and smooth. But there are multiple issues with the materials supplying that chemistry: 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. A study to isolate which of these three materials is the problem might make it possible to adjust the firing to accommodate it. But probably not. The most obvious solution is to just use non-gassing sources of MgO, SrO and CaO (which will require some calculation). There is a good reason to do this: The glaze contains some boron frit, that is likely kick-starting melting much earlier than a standard raw-material-only cone 10 glaze. That fluid melt may not only be trapping gases from the body but creating a perfect environment to trap all the bubbles coming out of those carbonates and talc. The Aero chocolate bar of glazes!

The secret of the higher gloss glaze on the right? A lead frit addition.


Lead frit smooths a glaze

These cone 04 glazes have the same recipe (a version of Worthington Clear sourcing B2O3 from Ulexite instead of Gerstley borate). But the one on the right is more glassy, more transparent. Why? It has 10% added lead bisilicate frit. Lead bisilicate produces dazzling transparent glazes. no other method matches it. While potters gasp at the thought of using lead consider this: They thrive on unstable flux-deprived, glass-deprived and alumina-deprived base stoneware glazes with additions of large percentages of toxic colorants like chrome and manganese!

What material makes the tiny bubbles? The big bubbles?


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).

An 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 GBMF test 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. Out of this work came the G3806E and G3806F.

In pursuit of a reactive cone 6 base that I can live with


These GLFL tests and GBMF tests for melt-flow 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, low run volatility, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually evolved the G3806B, after many iterations settled on G3806E or G3806F as best for now.

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.

Highly melt fluid glazes that run are in style but do they produce quality ware?


A melt fluid pottery glaze

This is an example of a highly fluid glaze melt that has pooled in the bottom of a bowl. While it may be decorative, this effect comes at a cost. The melt fluidity is often a product of high KNaO - so not surprisingly it is crazing like mad! This crazing weakens the piece, much, much more than you might think. when glaze are thick like this the crazing becomes cracks, they go right down through that thick layer at the bottom of this bowl. And they want to continue right down into the body - and will do so at the first opportunity (e.g. sudden temperature change, a bump). Also, fluid glazes like this are much more likely to leach (because they lack SiO2 and Al2O3). That is crystallization in the middle, who knows how chemically stable that is? Yes, these are commercial glazes having a seal that suggests they are somehow immune to leaching, but multiple different layers are interacting here, there is no possible way to predict the resistance to leaching.

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 GLFL testers for melt flow at the back and the GBMF test melt-down-balls in front 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.

Our G2926B glaze may not work on dark burning clays


These two glazes, applied to the outsides of these mugs, both fire as brilliant glass-like super-transparents. But on this high-iron stoneware, from which both pieces are made, only one is working well. 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 as the body gases during firing. G2926B (right) works great on porcelain and buff stoneware but it cannot clear the clouds of bubbles coming out of this body (the bubbles are actually partially opacifying it). Even the normal glassy smooth surface has been affected. The moral: Potters need more than one base transparent recipe. Being able to host colors, opacifiers and variegators is nice, but sometimes just a transparent that works well is needed. An interesting trade-off of reactive melt-fluid glazes is that, while they develop more interesting surfaces, their lower SiO2 and Al2O3 contents make them susceptible to crazing, settling of the slurry and cutlery marking.

Two transparents having opposite melt fluidity/surface tension balances


Melt flow test demonstrates surface tension

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.

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


A fluid melt glaze over underglaze

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).

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.

One way for 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). 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.

Three low fire bodies need three different clear glazes. Why?


Glaze fit. The left-most clay mug contains no talc (Plainsman Buffstone), the centre one about 25% talc (L212) and the right one is about 45% talc (L213). Talc raises thermal expansion. The centre glaze is G2931K, it is middle-of-the-road thermal expansion (Insight-live reports it as 7.4) and fits the low-talc bodies (and Zero3 porcelain and stoneware). But it crazes on Buffstone and shivers on L213. The lesson is: Forget about expecting one clear or base glaze to fit all low fire bodies. But there is a solution. I adjusted it to reduce its expansion to work on zero-talc porous bodies and raise it to work on high talc bodies. How? By decreasing and increasing the KNaO (in relation to other fluxes). The three fire crystal clear and work the best in a drop-and-hold firing.

Transparent glazes often work poorly on dark stoneware bodies


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 L3954B 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 act as a fining agent (vacuuming up the bubbles)! Right: Alberta Slip GA6-B. It also fires as an amber-coloured glass, but on a dark body, this is an asset.

Vitreous red body color with a clear glaze at cone 6. Impossible.


Terra cotta bowls behind stoneware ones emulating their appearance

The two back pieces are traditional Mexican terra cotta ware. Lead glazed, porous, not durable at all. The front two pieces are stoneware, produced by two Mexican manufacturers to emulate the look of traditional ware. Both tried. Both failed. It is pretty well impossible to clear-glaze a mid temperature red burning stoneware and get a red color. The glaze fluxes the clay surface and turns the color to the more vitrified version of the body, which is always brown. The terra cotta is fired so far below the temperature at which the clay vitrifies to brown that it maintains the red body color. To get the color in the stoneware plate on the left they brushed on a red-stained engobe (you can see the brush strokes). This worked. And the glaze has fired crystal-clear (likely because they bisque fired it very high). But there is a problem. Although not crazed when purchased, it crazed badly after several months. The bowl on the right also has a beautiful transparent glaze and it has maintained crisp edges on the decoration (and with no micro-bubble clouding). But this is as dark as they could make the clay without having it turn brown under that clear glaze.

Underglaze brushwork decoration on a cone 6 porcelain plate


A cone 6 plate with underglaze fish design

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.

Plate by Stephanie Osser - Color highlighting by thickness variation


"Émail ombrant" (French for “enamel shadow”) is a pottery-decorating technique developed in France in the 1840s (at the Rubelles factory by Baron A. du Tremblay). Designs were etched or stamped into the pottery and a transparent colored glaze, in this case green, was applied thickly enough to re-level the surface. The varying depths produce colour highlighting. The design appears shadowy, hence the name. Stephanie calls these plates “Girls on the March”, they were inspired by parents who supported their girls with dynamic signage at the Boston of “Women on the March” rally in 2017.

Two low fire transparent highly fritted glaze recipes for pottery


Melt fluidity comparison of two clear glazes

These work well from cone 04 up, they are better than any commercial transparent brushing glaze we have used. And far better than glazes made using raw sources of boron (like ulexite, colemanite, Gerstley Borate). These glazes have lower thermal expansion and do not craze on any body we have tried (yet are ultra gloss and ultra clear). They are G1916QL1 and G3879C. We developed them for use on the dolomite-based (rather than talc-based) L4410L low temperature art clay body. These are a good demonstration of the technical and economic sense it makes to use highly fritted glazes at low temperatures. Having a good base glaze is the key to adopting low temperatures for your production. You would likely agree that no stoneware glaze has melting patterns like these shown in this melt fluidity test! These recipes and all details about their development and adjustment are openly available.

Inbound Photo Links



Melting glaze balls at various temperatures to see when all carbon has been expelled

One glaze is transparent, the other milky because it is filled with micro bubbles.
Iron oxide as a fining agent to debubble a low fire transparent

Side by side melt flow tests of the original glaze and the fritted variation
Using a frit instead of feldspar in a cone 10R glaze. Why do that?

Two transparent liner glazed mugs
Could a difference of only 0.1% iron affect the clarity of a transparent glaze?

Links

Glossary Colorant
In ceramics and pottery, colorants are added to glazes as metal oxides, metal-oxide-containing raw materials or as manufactured stains.
Glossary Opacifier
Glaze opacity refers to the degree to which it is opaque. Opacifiers are powders added to transparent ceramic glazes to make them opaque.
Glossary Boron Blue
Boron blue is a glaze fault involving the crystallization of calcium, boron and silicate compounds. It can be solved using ceramic chemistry.
Glossary Crystallization
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
Glossary Glaze Durability
Ceramic glazes vary widely in their resistance to wear and leaching by acids and bases. The principle factors that determine durability are the glaze chemistry and firing temperature.
Glossary Liner Glaze
Liner-glazing is a very good way to assure that your ware has a durable and leach resistant surface. It also signals customers that you care about this.
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.
Glossary Translucency
A highly sought-after property in porcelain, fired close enough to melting to take on the glass-like property of passing light. Translucency implies tendency to warp during firing.
Glossary Ceramic Glaze
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
Articles Concentrate on One Good Glaze
It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
Articles High Gloss Glazes
A transcript of a presentation at the 3rd Whitewares conference at Alfred University in the spring of 2000 by Richard Eppler.
Properties Glaze Color
Recipes G1947U - Cone 10 Glossy transparent glaze
Reliable widely used glaze for cone 10 porcelains and whitewares. The original recipe was developed from a glaze used for porcelain insulators.
Recipes G2926B - Cone 6 Whiteware/Porcelain transparent glaze
A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
Recipes G1916Q - Low Fire Highly-Expansion-Adjustable Transparent
An expansion-adjustable cone 04 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.
Recipes G2931K - Low Fire Fritted Zero3 Transparent Glaze
A cone 03-02 clear medium-expansio glaze developed from Worthington Clear.
Media Remove Gerstley Borate and Improve a Popular Cone 6 Clear Glaze
How I found a ceramic glaze recipe on Facebook, substituted a frit for the Gerstley Borate, added the extra SiO2 it needed and got a fabulous more durable cone 6 clear.
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
By Tony Hansen
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