G2931B cone 03 clear glaze for Plainsman Buffstone and low-talc bodies

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Worthington Clear with Ulexite:Wolly

Code #


Materials Amt
EPK 25.100 0.06%
Silica 10.600 0.02%
Ulexite 26.500 0.06%
Ferro Frit 3249 16.900 0.04%
Wollastonite 15.500 0.03%
Nepheline Syenite 5.200 0.01%


Unity Formula

CaO 0.69
MgO 0.17
K2O 0.01
Na2O 0.13
(KNaO) 0.15
B2O3 0.76
Al2O3 0.41
SiO2 2.19

Si:Al Ratio


SiB:Al Ratio







26.80 per kg


This is a lower expansion version of G2931K, an ultra-clear for porcelain and talc bodies at low fire. This sacrifices some surface brilliance for the low thermal expansion. The chemistry of this recipe cannot be produced using common frits, the Ulexite is essential (the boron high).

The original recipe from which this was derived, Worthington Clear, was high in Gerstley Borate and made a jellied mess in the bucket, almost impossible to use. This has the same chemistry but a very different recipe having much better slurry properties.

Do not try to use this with too little water. Mix it to 1.4 specific gravity and add vinegar or Epsom salts to gel it so that it stops moving (after vigorous stirring) in less than 2 seconds. It naturally gels so little vinegar should be needed. It applies very well to low or high porosity bisque (in a very thin layer if needed), it does not drip or move but does take a minute to dry.

The soak:soak:slowdrop firing schedule is critical to a defect-free surface.

Although this recipe has more ingredients than Worthington clear, the range of properties and chemistry they supply presents many more adjustment possibilities.

Links with Schedules

To: Cone 03 soak-soak-slow cool


Ulexite Worthington Clear to cone 02

No pinholes, thin application. Some bubbles. This was fired to temperature and soak 10 minutes (and cool of 100 degrees and soak there would have yielded even better results). Cone 03 and 02 do produce a more brilliant result than cone 04.

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.

Ulexite Worthington clear cone 02

This is a terra cotta stoneware body, the body contains enough frit to produce 1% porosity at cone 02. Although it appears to work well, it is under too much compression because its thermal expansion is too low. We noted a tendency to shiver on the rims of mugs, especially with the white engobe present.

Worthington Original vs. Ulexite varitation flow test

Gas generation during melt is obviously more active in the Gerstley Borate version (left) than in the Ulexite version (right). This is fired to cone 03.

Entrained bubbles in Ulexite Worthington

16x photo of a 10 gram glaze ball melted down onto a tile, it was fired to cone 03. This has both the large and small bubbles populations.

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.

G2931B and F fit different bodies

The fritted vitreous body on the left shivers using G2931B. The same body (having the same white slip (G3685U) does not shiver the G2931F version of the glaze (it has more KNaO).

Born: 2014-03-19, Modified: 2017-01-20 15:11:03

Ulexite Worthington Clear - Low Expansion

Code #


Materials Amt
EPK 27.296 0.06%
Silica 13.265 0.03%
Ulexite 26.085 0.06%
Ferro Frit 3249 18.622 0.04%
Wollastonite 13.180 0.03%
Zinc Oxide 2.551 0.01%


Unity Formula

CaO 0.61
MgO 0.18
Na2O 0.10
(KNaO) 0.10
ZnO 0.10
B2O3 0.76
Al2O3 0.40
SiO2 2.15

Si:Al Ratio


SiB:Al Ratio







33.10 per kg


This is an even lower expansion version of G2931B (see its notes for more information, especially about mixing with water and specific gravity). This is only needed if 2931B crazes.

The soak:soak:slowdrop firing is critical to a defect free surface.


Terra cotta and thermal shock

This terra cotta cup is glazed with G2931G clear glaze and fired at cone 03. It survives 25 seconds under direct flame against the sidewall before a crack occurs. Typical porcelains and stonewares would survive 5-10 seconds!

This is a key advantage of earthenware. Sudden changes in temperature cause localized thermal expansion, this produces tension that easily cracks most ceramics. But the porous nature of earthenware absorbs it much better.


P6318 Terrastone with G2931G Clear Glaze cone 03

G2931G clear shivering on 3724N1 (fritted terra cotta) at cone 03

G2931F (left), G2931G (right) on buffstone

Cone 03. Did boiling water, ice water test on both. F crazes (notice the piece is waterlogged). G does not. Buffstone has a high porosity at this temperature so glazes must fit well.

P580 Pioneer cone 10R bisqued with 2931F and G glazes

F crazes. G does not. Both need to be applied thicker.

P580 with cone 03 clear vs cone 10 clear

G2931G clear on cone 10R bisqued piece (left).
G1947U cone 10R glaze (right).
The G need a tiny amount of blue stain.

Born: 2014-03-19, Modified: 2017-01-20 15:13:14

Firing Schedule Name

Cone 03 soak-soak-slow cool

Degrees (Fahrenheit or Celcius)


Schedule Type


Start Time and Temperature

8:01 am (no temperature specified)


Step Degrees/Hr Temperature
Time Note
1 400 240 60 1:36 9:37 am
2 800 1850 30 4:06 12:07 pm
3 108 1950 10 5:11 1:12 pm
4 500 1850 30 5:53 1:54 pm


The firing can start and finish within your working day. Step 2 can often be done as fast as you kiln will go.

The soak at 240F does not fracture ware even though it is above the boiling point of water. We find this is needed to be sure ware is sufficiently dry to withstand the rapid ascent to 1850. If your ware is thicker a slow ascent may be needed.

The 1850 soak on the way up clears the clouds of microbubbles. The 1850 soak on the way down heals the defects (blisters, pinholes) because the increasing viscosity is enough to overcome the surface tension holding bubbles from breaking).

It may be necessary to alter the last step if any imperfections are present. Try dropping to 1800 or 1750 and holding there. An additional step could be added to cool at 100F/hr down to 1500.