Five Low Fire Base Clear Glazes: What is the Best Strategy?

Project Name

Low Fire Clear Glaze Comparison

Project Codenumber

UnAssigned

Notes

Don't look for a good transparent recipe for low temperature, look for a good "starting recipe". Expect to adjust it, at a minimum, for thermal expansion. This is because poor or marginal fit are not tolerated like they are with stoneware (crazing or shivering happen much easier).

At low fire, boron is the king of fluxes (melters). And frits are the key source (I mention Ferro frits here, but your supplier will have equialents). A transparent can be as simple as 75:25 frit:kaolin for cone 04 or 85:15 for cone 06 (using a balanced one like Frit 3195 or Frit 3124).

Most important is thermal expansion adjustability. Low fire glazes are simply not stuck onto the body as well as high fire glazes, there is a narrower range of divergence from ideal that can be tolerated without crazing or shivering. A glaze fits your clay body? Does it really? Stress testing ware using a boiling-water-into-ice-water test (e.g. the ICWT test) may show otherwise. Then adjust the expansion up or down by substituting some of the main frit for high expansion Frit 3110 or low expansion Frit 3249 (or available equivalents).

Low fire glaze can potentially be applied in a much thinner layer than at higher temperatures (the thinner, the more crystal-clear). One way to get it thinner is to bisque higher so the ware is less absorbent, or make it thixotropic (in this gelled state it will apply evenly and hang on with no drips).

As noted, a target is a glaze that fits, fires transparent and produces a smooth glossy surface. Icing on the cake would be resistance to cutlery marking and leaching. And a body that does not generate gases that produce blisters, pinholes or clouding (at times, firing as low as possible (e.g. cone 06-04) keeps gases to a minimum).

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 (or similar material like dolomite, calcium carbonate), these are employed to increase the thermal expansion to prevent crazing. This greatly increases the chance of shivering. So, again, glaze-fit must be rechecked.

What about adjusting melting temperature? Not needed. You always want the best melt possible. If it melts well at cone 06 then it will be fine at 02 (but the reverse is not the case).

Popular recipes do not necessarily stand up to scrutiny, especially if poorly documented. Ones that employ Gerstley Borate to source the B2O3 just are not as good as the fitted versions. And, they are not adjustable. And Gillespie Borate has its own issues.

URLs

L4170 Terra Cotta Casting with G3879 UltraClear glaze

Deb's Clear #1 Cone 04-02

Total:100.00

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Notes

A well-known clear for low temperature, especially terra cotta. Sources often caution against applying too thickly to avoid clouding.

Many use this as an alternative to Worthington Clear. Its thermal expansion is considerably higher (because of the high 3134), so crazing is more likely.

The Al2O3 is very high for a glossy glaze. High Al2O3 (from high kaolin) produces stiffer melts that do not shed bubbles as well. And matte surfaces. However the very high boron crow-bars the gloss. 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.

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 would be needed.

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Deb&amp;#039;s Clear #1 Cone 04-02" id="70214" key="icwaeCm5" date="2025-04-06" codenum="G2932" picturebasename=""> <recipelines> <recipeline material="Ferro Frit 3134" amount="30.000" tolerance=""/> <recipeline material="Ferro Frit 3195" amount="45.000" tolerance=""/> <recipeline material="EP Kaolin" amount="25.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2014-03-17, Modified: 2025-04-06 20:10:52

Deb's Clear #2

Total:119.13

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Notes

This adds 20% silica to the original Debs clear (likely done to reduce thermal expansion, but not a good way to do it.
-First, melting would not be as good (although it would be better with 325 mesh silica).
-Second, it would be much better to trade off frits for each other to tune thermal expansion.

This recipe has plenty of kaolin, the Veegum is detrimental, it will slow down drying, increase water demand and increase drying shrinkage and cracking.

The CMC gum is not enough to make this a brushing glaze. Why is it here?

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Deb&amp;#039;s Clear #2" id="263105" key="7KQmdejS" date="2025-04-06" codenum="G2932A"> <recipelines> <recipeline material="Ferro Frit 3134" amount="29.000" tolerance=""/> <recipeline material="Ferro Frit 3195" amount="44.000" tolerance=""/> <recipeline material="EPK" amount="24.000" tolerance=""/> <recipeline material="Silica" amount="20.000" tolerance=""/> <recipeline material="Veegum T" amount="1.550" added="true"/> <recipeline material="CMC Gum" amount="0.580" added="true"/> </recipelines> </recipe> </recipes>

Born: 2024-10-15, Modified: 2025-04-06 20:19:18

Worthington Cone 06-2 Clear

Gelling, High LOI, Gerstley Borate difficult to sub, High Boron

Total:100.00

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Notes

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 its simple make-up and crystal clear fired result (low bubble population). Although it can also fire with clouding, variegation and boron blue depending on thickness, the clay body and firing.

The high boron content of Gerstley Borate, enabling high melt fluidity, make this glaze possible. Potters use it on terra cotta, talc and stoneware bodies that generate lots of gases of decomposition and this can often pass and heal bubbles well. 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 with 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 matters worse, the recipe has an additional 30% clay on top of that! This recipe can actually be thrown on a potters wheel!

The thermal expansion is too low for talc bodies, it will shiver on them.

Pictures

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.

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.

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!

G2922G, G2931 flow tests

Cone 05+ Expansion Adjustable Gloss Base

Total:100.00

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Notes

Although similar to the previous G1916M, this recipe provides greater thermal expansion adjustability. It combines a middle-of-the-road thermal expansion frit with a very low and very high expansion frit (they cancel each other out but increase gloss of the otherwise silky Frit 3195). The frits in this recipe (or similar ones) are common. We adjusted this in a later version, G1916QL1, it employs a kaolin for better clarity and more frit 3249 for glaze fit on more bodies.

If you can't get Frit 3249: Try 3195 by itself. If it shivers, add frit 3134 until it passes stress testing. If it crazes, use Frit 3124 as the base and add a little talc.

As is, this melts well from cone 05 and is stable to cone 2 or higher.

Do not assume the glaze fits because a piece emerges from the kiln without crazing or shivering. Thermally stress it (e.g. ICWT test).
If this crazes, try 65:20:15 (3195:3249:EPK).
If it shivers, use 65:20:15 (3195:3110:EPK).
If the problem persists reduce the 3195 and increase the other (e.g. 55:30:15).

The ball clay also imparts good working properties and it has a lower LOI than EPK (to generate less micro-bubbles in the glaze).

Add 2% iron oxide to this when using on terra cotta bodies to enhance their red, the iron particles also act as a fining agent to remove the microbubbles they produce.

Because this employs ball clay, it the glass will have a slightly amber color (from the Fe2O3 in the clay). If that is an issue, try using a kaolin instead (we recommend New Zealand or Grolleg, the G1916Q2 and G1916Q3 variations do this).

Although drop-and-soak firing schedule helps clarity of transparent glazes at higher temperatures this glaze can produce crystal clear results with fast firing.

We find that 3800 water for 4000 powder produces ~1.44SG. The slurry gels more than with EPK, Grolleg or NZK.

Pictures

G1916Q at Cone 01 on 3D+iron

Very nice results on L3724E red body at cone 01. Piece is very strong.

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 Cone 04 using 04DSSC schedule

Counterclockwise: L212, Raku, Buffstone, L213, L210, L215
Crazing out-of-the-kiln on Raku and buffstone.
Bisque 04-03.

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!

G1916Q on L215, L212, L210, L213, Buffstone at cone 03

All exited from the klin without crazing. The L215, L213, L210 and L212 samples subsequently survived a 300F/Icewater test without crazing, but the Buffstone did not. The L213 would not likely survive a cold-to-hot test without shivering.

1916Q cone 04, 03

Both were slow cooled. While the cone 04 version is glassy and ultra-gloss, it has significant clouding of micro-bubbles. The cone 03 version, right, is completely transparent.

G1916Q on L210 fired at cone 04

Has survived for a year without crazing.

G1916Q+2%Iron on L212 talc body fired at cone 05

This will likely shiver over time. But the speckle that happens on white bodies is clearly visible.

G1916Q applied thickly on L215 fired cone 04

No crazing after 2 months.

G1916Q on L4170 cone 04

Thin application is clearly the way to get the best transparent. The glaze eventually craze where application is thicker.

Cone 04+ UltraClear Glossy Base

Total:1,025.00

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Notes

We developed this for cone 06-04 to fit many clay bodies (without crazing), melt well at cone 05 and produce a more transparent glaze (rather than the amber of G2931K and G1916Q). The frit F-524 in this recipe is uncommon among potters but common in industry, it is high quality. We later adjusted this recipe to produce variations (e.g. with whiter kaolin, more of the frit F-69 to get a lower thermal expansion).

We found the inspiration for this on testing many commercial clears and finding one that stood above the others regarding fit and clarity at 04 (we had it analyzed and reverse-engineered it using the materials we have). Frit F-524 is expensive, but this glaze is heads-above a number of commercial clear glazes we have tested (they are not ultra-clear, they pinhole or blister or craze, they are not glossy enough, they do not have dipping versions, they are not adjustable). The F-69 is also expensive, but it enables fitting this to any body.

Additions of tin and zircon produce a white that melts just as well as the clear. To make bright-colored glazes this should be an excellent base (remember, this is the clear glaze recipe currently used by at least one commercial supplier of bright-colored glazes). Encapsulated stains are fantastic at low fire and they can be mixed to fine-tune the color. If you are targeting a specific or exotic color then the ability to achieve it will be determined by your ability to source an exotic stain!

The melt is very mobile at 04 yet not significantly more mobile at cone 1 (even up to cone 2). And it has the amount of SiO2 and Al2O3 that a cone 6 glaze has yet melts well at 06! There is some kind of magic with this chemistry!

It hardens to a powdery surface, some gum solution is required to make it dry hard for dipping (for each 100g of powder mix start with 73g water and 7g Laguna gum solution). To make a brushing glaze: For each 100g of powder mix start with 125 water, 1.5 CMC gum and 1.5 Veegum and blender mix (add water if it gels too much). We find the about 330g of powder produces a 500ml (about 1 pint) jar.

This recipe also has implied adjustability (by increasing the low expansion Frit F-69 at the expense of Frit F-524).

Pictures

G3879 Clear glaze on Plainsman L211 - Cone 04

Glossy, crystal clear, no crazing! And this is a 42 mesh body containing zero talc.

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 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).

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!
After a year, both the 25F and J2 were difficult to break, very strong!

Sial 10F, 25F with G3879 clear glaze at cone 03

Tile like these were done on a variety of bodies and fired at different temperatures. After a year:

Sial 10F
Cone 02: No crazing
Cone 04: Moderate crazing over time
Cone 06: Severe crazing
Cone 03: No crazing
Sial 25F
Cone 1: No crazing
Cone 03: No crazing
Cone 04: Crazing badly
L215
Cone 04: No crazing
L210
Cone 04: No crazing
L212
Cone 04: Crazing
Cone 1: No crazing
L213
Cone 04: No crazing

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.
This was not crazed after a year (cone 03).

G3879 on Plainsman Buffstone - cone 03

Buffstone is an entry-level low-price body not intended to fit commercial glazes. Yet this glaze fits at cone 03 (still fitting after a year)! And without any surface defects.

At cone 04 it does craze over time.

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. After use on various bodies, it was clear that fit at cone 03 is better than at 04 and much better than 06.

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).

G1216M is a blend of 3124/3124 with kaolin. G2931K is the Zero3 clear.

L213 with G3879 glaze at cone 04

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! Was still fitting after a year+.

G3879 on SIAL 10F at cone 02

No crazing after several months. Flawless service.