If you are doing low temperature earthen ware, especially for functional pottery, then start with a good transparent base clear recipe. Read about all of these and choose.
Low Fire Clear Glaze Comparison
A good base clear for low temperature ceramics is vital for success. It can become the base of all of your coloured glazes. And a kiln firing situation where you can do lots of testing is important. After all, if you can do a firing in 3 hours, it is much more practical to do testing.
At low fire, boron is the king of fluxes. And frits are the key sources (not a single commercial glaze manufacturer uses Gerstley Borate to source boron, yet many potters still do). What is the perfect combination of Ferro frits to create a super clear glossy glaze for low fire? There are an infinite number. But that number is drastically reduced when we eliminate those of too-high and too-low thermal-expansion-adjustable. And those that are not durable and resistant to leaching. And those that are not compatible with stains we need. And those that do not heal blisters and pinholes well.
Potters coming to low fire from higher temperatures expect fired strength, that is just not there at cone 06-04. But terra cotta bodies fired at cone 03-02 are dramatically better. G2931K is intended for exactly this (at cone 04 and lower it loses clarity and the surface is not as smooth). G3879, on the other hand, melts well at lower temperatures.
Good low fire glaze can be applied in a much thinner layer than at higher temperatures and yet still look full bodied. A thinner layer will also produce a more crystal-clear product. One way to get it thinner is to bisque higher so ware is less absorbent or thin the glaze with water (to make it too watery) and then flocculate it to a slight gel (in this state it will apply in an even yet thinner layer and hang on well with no drips). If faster drying is needed or ware is very thin and easily waterlogged by the glaze, heat it to 300F before immersing it into the glaze.
But low fire can bring alot of problems. What is needed in a glaze glaze is one that does not go cloudy on firing, does not bubble over underglazes, does not crystallize or devitrify, produces a smooth brilliantly glossy surface, does not cutlery mark, does not leach, accepts an opacifier well, does not blister or pinhole, does not settle like a rock in the bucket or turn to jelly.
But again, most important, is thermal expansion adjustability. Low fire glazes are simply not stuck on to the body as well as high fire glazes. If the thermal expansion of the glaze is too low it will simply flake off (especially if there is a slip between it and the body). If the expansion is too high it will craze. It is almost certain that your glaze and body just have compatible thermal expansion by accident. You must stress ware using a boiling-water-into-ice-water test (then reverse). Adjust the glaze and try it again until you get a fit (you owe this to your customers). Do you have to do this test for each clay body? Absolutely. Each temperature? Yes. Terra cotta bodies vitrify rapidly above cone 04, each temperature is a new thermal expansion animal.
What about white burning low fire bodies? These will almost definitely not fit the same clear glaze as a terra cotta. They have added talc, it is put there for the specific purpose of increasing the thermal expansion to prevent crazing. This greatly increases the chance of shivering. So, again, you must test fit and adjust the clear glaze accordingly.
Recipes that are popular do not necessarily stand up to scrutiny, especially if poorly documented. Use one that is documented, a product of much testing. And adjustable.
A well known clear for low temperature, especially terra cotta. Sources often caution against applying too thickly to avoid clouding. We found the same issue. Many use this this as an alternative to Worthington Clear. Its thermal expansion is considerably higher.
The Al2O3 is very high for a glossy glaze. High Al2O3 produces stiffer melts that do not shed bubbles as well.
The boron is very high, this is being used to crow-bar gloss out of a glaze that would not otherwise be glossy. High boron is commonly associated with clouding.
Increasing thermal expansion (to fight shivering) requires increasing the Frit 3134 at the expense of 3195. But its boron is just as high. Frit 3110 would be a better choice, a small change would be needed to increase expansion and it has low boron.
This has a high kaolin content. This could cause crawling and drying cracks. If Frit 3124 were used instead (it supplies Al2O3 and 3134 does not), less kaolin could be employed.
This recipe is a common Gerstley Borate clear base used from 04 all the way to cone 6! At higher temperatures the recipe trends toward less kaolin to more silica and a little less GB (e.g. 50:20:30).
Attractions of the recipe include its simple make-up and crystal clear fired result (low bubble population). In theory it would seem that fritted glazes should be much better at smoothing out and freezing to a crystal clear glass, but in actual practice this is not necessarily so (because common frits have a lower boron content). Potters use this glaze on terra cotta, talc and stoneware bodies that generate lots of gases of decomposition and this glaze appears to be able to pass them and heal well because of its high melt fluidity. High boron clear glazes like this are known for clouding issues (because of micro bubbles) but this has a low enough surface tension to pass them.
Problems of this recipe:
Obviously, if it melts well already at cone 04 great care is going to be needed at cone 2 to prevent it running onto shelves. Highly melt fluid glazes (that have high boron like this) are also more likely to be leachable and to crystallize on cooling (producing the boron-blue clouds).
Paramount is the dreadful problem of gelling. Even with only 1.4 specific gravity (about equal weights of dry material and water and deflocculation with Darvan) it gels badly. The high water content needed and the nature of GB means that glazed ware dries extremely slowly, the glaze shrinks and cracks during drying and crawls during firing. Gerstley Borate is very plastic, yet to make matter worse, the recipe has an additional 30% clay on top of that! This recipe would likely make a good throwing body!
If it would be possible to use another boron-sourcing material that had none of these issues, this glaze would be a winner. A frit with a very high boron content would be needed.
2931 vs 2931b
On Plainsman L215 cone 02 the original base Worthington Clear has gone on very thin on sides of mug (because of the low specific gravity necessary to prevent it from gelling it is very difficult to get it on thick enough). The fired surface is clear but not as glossy. On the rim it has bubbles. The Ulexite version (G2931B) is glossier, and went on thicker because the slurry is so much easier to use. This glaze is not recommend for L215, the latter contains talc that increases its thermal expansion, putting too much squeeze on this glaze.
Worthinton Clear at cone 01
On a terra cotta clay at this temperature was has stoneware properties. The fired surface is good.
Worthington Clear vs. Fritted Clear
Worthington (right) flows even better than the fritted glaze and does not have any more entrained bubbles even though it has an LOI of 20%. This is likely because its melting history and behavior is such that its ability handle gases of decomposition from the body and its own materials is so much better.
Entrained bubbles in Worthington Clear
This is a 16X closeup of flow test (10 gram ball melted down onto a tile) that concentrates bubbles. There are high populations of large and tiny ones. The larger ones are from the Gerstley Borate, the tiny ones from the kaolin.
GB vs Ulexite Clear glaze bubbles
These are 10 gram balls fired down onto tiles at cone 04 to compare melt fluidity and bubble populations in three clear glazes. Larger bubbles are better, they break at the glaze surface and heal. Tiny ones produce cloudiness.
Left: The original Worthington fluid melt clear glaze recipe. There are clusters of tiny bubbles and many large.
Center: A glaze of the same chemistry but sourcing its boron from Ulexite instead. Notice the lack of tiny bubbles. This fires pretty well identical to the original but has much better slurry properties.
Right: Center with with a 10% addition of lead bisilicate frit. This fires more glossy than either of the other two. Its thermal expansion is also likely lower.
Worthington Clear the next day
Even though this has a low specific gravity and is deflocculated with darvan, the next day it is still jelly. Impossible to use unless more Darvan is added, who knows where that will go!
This recipe improves the popular Worthington Gerstley-Borate-based low fire clear recipe. It targets cone 03 to work best on Zero3 stoneware and porcelain. However many low fire bodies are dramatically stronger when fired to cone 03 with this (or one of its thermal expansion variants L & H). And the vast majority of commercial glaze products will fire easily to this temperature. If you absolutely must fire lower, to cone 06 or 04, then use the G1216Q recipe instead.
Unlike Worthington something this does not become a bucket of jelly, does not crack on drying, does not go on unevenly or the wrong thickness, does not cloud up with boron blue or micro-bubbles when fired (because it has lower CaO) when used on the types of bodies discussed below. The major development work culminated in the Ulexite-fluxed G2931F. This recipe, G2931K, was formulated to have the same chemistry as F but use frits instead. It is crystal clear and super glassy.
Low fire bodies span a wide range of thermal expansions. While Worthington clear (an ancestor of this recipe) melts to a good clear, its thermal expansion was too far toward the low end of the range (it tended to shiver and fracture rather than craze). This recipe, G2931K, raises thermal expansion (by adding Na2O). The thermal expansion of Zero3 stoneware and porcelain are about the middle of the range, and they develop good glaze:glaze interfaces, so this recipe fits them well.
This glaze also works well on other low fire bodies. Low talc bodies (like Plainsman 215) also work well. But this glaze shivers on high-talc bodies and crazes on zero-talc porous ones. That is what we want. Why? As noted, the thermal expansions of low fire bodies span a wide range. And glazes are just not stuck on well to ones that have a porous and softer surface. Thus, GLAZE FIT IS VITAL AT LOW FIRE (to prevent crazing and shivering). Remember this: IT IS IMPOSSIBLE THAT ONE BASE GLAZE CAN FIT THEM ALL. Not even two can span the range! Commercial glazes only fit your clay body by accident. That is why this recipe has three variations, one of higher expansion than this one (G2931H) and one of lower expansion (G2931L). By testing glaze fit (using a thermal shock test like boiling water:ice water) and choosing the best recipe or blend of recipes, you can get the best fit. Make the effort and do that. All three fire crystal clear. And you can make make your own brushing versions (see link below). Keep in mind that adding zircon and stains changes expansion and requires re-testing and possibly changing the proportion of K, H and L to match your body. It is a hassle, but it is not rocket science.
Dipping ware in Zero3 Transparent is just so much easier than trying to paint on commercial, slow-drying clears. It produces a slurry with very nice suspension, fast drying and good application properties (if you tune it right). It naturally gels to a creamy consistency at around 1.53 specific gravity (in our circumstances). However it goes on too thick at that high a value, we have found the best all around performance at 1.43 with enough Epsom Salts to gel it back up to a creamy consistency (a few grams per gallon). Measure the SG by weighing (forget about fiddling with hydrometers). You will think it is too thin and watery, but it isn039;t. The Epsom salts will thicken it and make it thixotropic (which the 1.53 slurry was not). Use a little Darvan if you get it too thick.
As a clear overglaze to finish your decorated ware, this glaze has a number of other advantages over commercial clear glazes:
-It is much less expensive so it is practical to have a large pail of it so that pieces can be easily dip-glazed (using dipping tongs).
-It behaves like stoneware glazes, it suspends well and applies in an even layer that can dry to handling stage is seconds.
-It fires to a better crystal clear than most.
-Since the recipe is known, and contains no toxic materials, you can more confidently assure retailers that it is safe; all underglaze color decoration can be isolated from any contact with food or drink by this.
-It is compatible with most underglaze colors (including pinks).
Pay special attention to the drop-and-hold firing schedule, especially if your clay body is not fine-grained.
To mix up 5 Kg use about 4.5 kg of water to get 6.5 litres of glaze.
To mix up paint-on consistency (1 pint jar):
-113g Laguna Gum Solution (100 grams of gum solution has 6.5g powdered CMC Gum)
-Mix in 500g of powdered glaze, adding water if needed to get desired painting consistency (up to 325g total water). It is better to be a little thinner than too thick.
-To make colored versions add stains as needed (if stains cause micro-bubbling or orange peel surface include 2-3% zircopax).
Batch Ticket Notes
These notes were entered in the notes panel under "Batch Ticket Notes"
Fired cone 03. Body is Zero3 stoneware.
Surface is perfect, even where thick. Ultra clear.
Survived three boil:ice cycles and one 300F:ice cycle without crazing.
G2931F vs. G2931K on Polar Ice Low Fire
2931F was the Ulexite flused version of this recipe. The F survived three boil:ice cycles and 1 300F:ice cycle without crazing or shivering.
The K is slightly smoother, tiny dimples in the surface are fewer. It is also applied thicker.
G2931F vs G2931K fritted - terra cotta mugs cone 03
F was the Ulexite-fluxed version of this recipe.
G2931F vs G2931K flow test
These two recipes have the same chemistry, but K sources boron from frits rather than Ulexite. Notice how much less bubbles there are in the flow and how much more predictable the melting pattern is.
G2931F vs G2931K - Melted balls at cone 03
F, the Ulexite version, is obviously bubbling more, the percolation is causing the melt to spread out more on the tile. On the flow test is was less fluid.
Firing temperature is important for Zero3 glaze
This is G2931F on Plainsman Buffstone, L213, F100, L215. First column is cone 04, center is cone 03, right is cone 02. All exited the kiln without crazing except Buffstone at cone 04.
We subjected all of them to a 300F:IceWater thermal shock.
Buffstone crazed on all of them.
L215 and L212 Cone 04 crazed.
L213 was good but later the glaze was found to be under excessive compression, subject to shivering over underglazes.
At cone 02 there are some dimples and defects.
Three low fire bodies that need three clear glazes
Because of glaze fit. The left-most mug is Plainsman Buffstone, it contains no talc and fires buff colored. The centre one is L212 (about 25% talc). The right one is L213 (about 45% talc, it fires very white). 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 L215 (also Zero3 porcelain and stoneware). But it crazes on Buffstone and shivers on L213 and L212. So I adjusted it to reduce its expansion (to work on zero-talc porous bodies) and raise it (to work on high talc bodies like L213). How? By decreasing and increasing the KNaO (in relation to other fluxes). These three can be blended to fit any low fire body.
This recipe provides the greatest thermal expansion adjustability we have seen in a low fire clear glaze. It combines a middle-of-the-road thermal expansion frit with a very low and very high expansion frit. By trading 3195 for 3249 expansion can be lowered. With 3110 it can be raised. As it stands here it fits a terra cotta body we use.
Frit 3195, by itself, fires to a slightly silky surface. 3110 and 3249 are highly glossy. It does not take much of the others to move the 3195 surface toward a brilliant gloss.
Do not assume the glaze fits because a piece emerges from the kiln without crazing or shivering. Stress it (by two minute boiling-water and ice-water immersion cycles). If you find this does not fit, try a 10% adjustment. For example, if it crazes, increase the 3195 by 10% and reduce the 3110 by 10%. Do the opposite if it shivers. If you end up with pure 3195 and it still crazes, then introduce Frit 3249 at its expense.
The ball clay also imparts better working properties than the EPK and it has a lower LOI to generate less micro-bubbles in the glaze.
Cone 01 on 3D+iron
Very nice results on L3724E red body at cone 01. Very strong.
No raw kaolin = more transparent
1916J (left) and 1916S (right). The S is much more transparent. Why? J employs raw EPK (kaolin loses 12% of its weight on firing, creating gases). S has no raw kaolin, it uses VeeGum and calcined kaolin). However it is not possible to create a good slurry with this recipe (it gels). Thus, we settled on using a clean ball clay to suspend and harden the recipe (ball clays have half the LOI or kaolins), that is recipe
1916J and Q fired to 1450F
These glazes are starting to melt, the surface having reached the consistency of a porcelain and have densitfied to very low porosity. Notice the iron in the ball clay really shows up at this stage (it will be less evident later).
G1916Q and J fired 1650-2000F
Ten-gram balls melted and flattened as they fired. They soften over a wide range, starting well below cone 010! At 1650F 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, this is a thick layer, so the bubbles are expected. But they still can be an issue 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 (1450F in this case), then soak the kiln just below that (on the way up) to fire away as much of the carbon as possible.
Success with cyrstal clear glaze cone 03
Uses Cone 03 soak-soak-slow cool schedule.
Left: P6282 with 3685U slip and 1916Q. Clear and very good. Glaze is thicker than the other two. Shivering on lip, the slip is not fitting the body.
Center: P6282 with G2931F Ulexite clear. Better clarity even though it is applied very thin. Shivering on lip, the slip is not fitting the body.
Right: L3724F with 3685U slip. No shivering. Very good coverage of the glaze, very clear, the best I have seen yet!
I developed this for cone 04 to fit as many clay bodies (without crazing) as possible (my Zero3 clear works well on specific bodies but does not melt enough at cone 04 and it's thermal expansion is too high to fit some bodies). I am developing a recipe for a terracotta casting body at the same time as this and I am working to make it compatible also.
I found the inspiration for this on testing many commercial clears and finding one that stood above the others regarding fit and clarity at 04. I have always been under the impression that low fire bodies have a wide enough range of thermal expansions that one glaze cannot be expected to fit them all. And that if a glaze melts well at cone 04 it will have too much melt fluidity past cone 02. But this glaze has made me question both, I am shocked at how it is possible that it can fit so many bodies and work across such a wide temperature range. In fact, I have not found a body that it does not fit! So I had it analyzed at a lab and then created a recipe to source its chemistry. A stroke-of-luck was that a frit we already use to produce another glaze, Fusion Frit F-524, is close to the complete chemistry needed.
I have been surprised by other aspects of this recipe. It is crystal-clear on any body at any thickness (at cone 04 and above)! Additions of tin and zircon produce a white that melts just as well as the clear. It is amazing how the mobile the melt is, how it runs right off my fluidity checker! Yet it is not significent more mobile at cone 1 than cone 04. There is some kind of magic with this chemistry that I am anxious to learn more about.
While it hardens to a powdery surface, it is amazing how little gum solution is required to make it dry hard and hang on to the bisque when other layers are added on top.
For the first mix I added 3000g of powder to 2400g of water to produce a specific gravity of 1.48SG. This produced a watery slurry. I added epsom salts to the point where further additions did not thicken the slurry (up to a total of 7g). This improved it considerably but it was still a little thin (although it covered and applied like a typical dipping glaze, drying in seconds on bisque ware). However the surface was too powdery so I removed 400g of water and replaced it with 400g gum solution. This slowed dip time to about a minute (waiting for the dripping to stop) but it now tolerates thick overlaying of Majolica colors (without them pulling it away from the bisque). For a single-coat dipping (where no overglaze work will be done) I would use about half the amount of gum solution.
Mike ODonnell and Fusion Frits says many customers use F280 and F38. He suggested F5 might be most similar to this. But I found that F524 was by far the closest.
G3879 on Plainsman L210, L215 at cone 04
These are 42 mesh low fire bodies. They normally have issues with pinholing but using this glaze the results are stunning. The L210 contains no talc, the L215 has 10%, yet this glaze does not craze on either one (over time it shivers on the L215).
Melt fluidity comparison with #1 commercial clear
We tested half-a-dozen commercial clears and found G3859 to be the best all-around one. This one has a very similar melt fluidity.
G3879 Clear glaze on Plainsman L211 - Cone 04
Glossy, crystal clear, no crazing! And this is a 42 mesh body containing zero talc.
G3878 has a high surface tension
As can be seen in the way it has melted here.
G3879 on SIAL 25F, Plainsman J2, L4170 TerraCotta
These are very different bodies. The leftmost contains talc to raise the thermal expanison to help prevent crazing with commercial glazes. The center one contains nepheline syenite (for the same purpose). The terra cotta on the right is just Redart and ball clay. This glaze fits are all three!
Sial 10F, 25F with G3879 clear glaze at cone 03
The glaze is applied double-thickness on the top half. Yet there are no more bubbles or crazing. Perfect fit.
G3879 Clear on L4170 TerraCotta Casting
The clear glaze is G3879. The white on the outside of the one on the left has 10% added zircopax. The overglaze colors are Spectrum Majolica colors.
G3879 with 5% Tin Oxide on SIAL 10F
When mixing Tin (as an opacifer), it is very important to mix it well. The one on the left was mixed poorly (at high speed with my propeller mixer but not for long enough). The one on the right was mixed much better and so produces better opacity. Tin is expensive so this is important.
G3879 on Plainsman Buffstone - cone 04
Buffstone is an entry-level low-price body not intended to fit commercial glazes. Yet the glaze fits! And without any surface defects.
G3879 Zircon White on SIAL 25F, 10F - cone 03
10% zircopax has been added. It is melting well so the percentage could be increased for great opacity on red burning bodies.
G3879 at cone 1 on SIAL 10F, 25F
Crystal clear, no running. Perfect!
GBMF test on G3879 at cone 1
It is not running and flowing nearly as much as expected. The melt surface tension holds it in place, so it should be able to fire to cone 2 and beyond.
G3879 on terra cotta at cone 04, 02, 1
This is on the L4170 body, it is a lighter firing product, 25F, from SIAL.
G1916M, G3879, G2931K on L215 - Thick
Thickly applied encourage poor fit to show up.
Clearly, as shivering and cracking demonstrate, G3879 is under excessive compressive on L215.
The other two are not showing any issues (other than heavy bubbling because of the thickness).
Survived 325F:IceWater test with almost no crazing. However there was a little shivering on the rim after a month. Another mug had no crazing on the inside after several months.
G3879 on L4115J2 buff body at cone 04
Glaze is ultra clear.
It was refired at cone 022 to apply a decal.
It was 325F to icewater tested without crazing.
It was waterlogged (the bottom is bare clay) and then put in a microwave for two minutes. Despite getting incredibly hot it did not fracture or craze!
G3879 on SIAL 10F at cone 02
No crazing after several months. Flawless service.