How to fix Worthington Clear

Share from (Lab Documentation and Calculation System) by Digitalfire

This is a popular glaze but flawed in several ways. You can do better using these alternatives. Frits are needed and more expensive up front, but you'll save money later.

Worthington Cone 06-2 Clear

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

Code #


Materials Amt
Gerstley Borate 55.000
EPK 30.000
Silica 15.000


Auto Unity Formula

CaO 0.69
MgO 0.17
K2O 0.01
Na2O 0.13
(KNaO) 0.14
B2O3 0.76
Al2O3 0.41
SiO2 2.20
Fe2O3 0.01


Si:Al: 5.3:1
SiB:Al: 7.2:1
R2O:RO: 0.1:0.9






37.79 per kg


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!

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

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!

G2922G, G2931 flow tests


SHAB - Shrinkage/Absorption

1 92.24 7.8%
2 92.11 7.9%

LDW - LOI/Density/Water Content

1 13.52 10.26 24.1%

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Worthington Cone 06-2 Clear" keywords="Gelling, High LOI, Gerstley Borate difficult to sub, High Boron" id="56711" key="qBgRC24J" date="2017-09-05" codenum="G2931" picturebasename=""> <recipelines> <recipeline material="Gerstley Borate" amount="55.000"/> <recipeline material="EPK" amount="30.000"/> <recipeline material="Silica" amount="15.000"/> </recipelines> </recipe> </recipes>

Born: 2014-03-17, Modified: 2017-09-05 13:01:38

Zero3 K Low Fire Transparent Glaze

Code #


Materials Amt Units
Ferro Frit 3195 25.000 KG 26.32%
Ferro Frit 3134 33.000 KG 34.74%
EPK 20.000 KG 21.05%
Ferro Frit 3249 10.000 KG 10.53%
Ferro Frit 3110 7.000 KG 7.37%

Total:95,000.00 (R)

Auto Unity Formula

CaO 0.59
MgO 0.10
Na2O 0.31
(KNaO) 0.31
B2O3 0.76
Al2O3 0.41
SiO2 2.45


Si:Al: 6.1:1
SiB:Al: 7.9:1
R2O:RO: 0.3:0.7






0.20 per kg


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)
-200g water
-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"


Glaze fit test

How to tune the thixotropy of a glaze

How to convert a d..o a brushing glaze


Zero3 body with G2931F Zero3 glaze

The F version of the glaze employs Ulexite to source the boron (instead of frits). These Zero3 stoneware mugs were fired to cone 03 with underglazes. The right mug has the Zero3 engobe inside (under the glaze).

G2931K Fritted version of G2931F - Cup

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 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.

G2931K glaze precipitates things on storage

G2931K On L212 after a year

Some crazing starting.

K on L215 at cone 04

The thicker version is clouding. The thinner one has micro-pinholes. It need a higher temperature.

G2931K o L215 - Cone 03, thick application

G2931K on L215 - Cone 03 fired in 30 minutes

Very transparent. No crazingafter many months.

G2931K on L215 - Cone 06

Milky but only a little crazing after a year.


ST-Untitled typecode

Alternate Code Number:GS04-1

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Zero3 K Low Fire Transparent Glaze" id="95671" key="7MgzDWoi" date="2018-12-06" typecodes="ST" codenum="G2931K" picturebasename=""> <recipelines> <recipeline material="Ferro Frit 3195" amount="25.000" unitabbr="KG"/> <recipeline material="Ferro Frit 3134" amount="33.000" unitabbr="KG"/> <recipeline material="EPK" amount="20.000" unitabbr="KG"/> <recipeline material="Ferro Frit 3249" amount="10.000" unitabbr="KG"/> <recipeline material="Ferro Frit 3110" amount="7.000" unitabbr="KG"/> </recipelines> </recipe> </recipes>

Born: 2006-03-16, Modified: 2018-12-06 09:14:37

524 UltraClear - Cone 04 to 1

Code #


Materials Amt
Fusion Frit F-524 850.000 82.93%
Fusion Frit F-69 40.000 3.90%
EPK 90.000 8.78%
Silica 45.000 4.39%


Auto Unity Formula

BaO 0.04
CaO 0.44
MgO 0.05
K2O 0.10
Na2O 0.12
(KNaO) 0.21
SrO 0.25
B2O3 0.66
Al2O3 0.47
SiO2 3.91


Si:Al: 8.4:1
SiB:Al: 9.8:1
R2O:RO: 0.2:0.8






11.06 per kg


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.

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 not 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.


B - Tin White

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="524 UltraClear - Cone 04 to 1" id="154451" key="KXpA9u4n" date="2019-05-16" codenum="G3879"> <recipelines> <recipeline material="Fusion Frit F-524" amount="850.000"/> <recipeline material="Fusion Frit F-69" amount="40.000"/> <recipeline material="EPK" amount="90.000"/> <recipeline material="Silica" amount="45.000"/> </recipelines> </recipe> </recipes>

Born: 2019-04-15, Modified: 2019-05-16 22:25:03