The first column, G2931G, is a low expansion, non-crazing glaze for non-talc and non-vitreous bodies. The second column, Zero3 K is preferred, it is ultra-clear, for Zero3 porcelain and stoneware and talc bodies. Third is the firing schedule.
This is a low-expansion version of the Zero3 Clear glaze (code G2931K). While that glaze fits the vitreous Zero3 porcelain and stoneware and non-vitreous bodies containing talc (like Plainsman L212, L215), it crazes on others (e.g. Plainsman TerraStone, BuffStone, L210).
This version, G2931G, drops expansion drastically, from 7.4 down to 6.2 (it is a follow up to G2931B which was 6.5, this adds 0.1 ZnO at the expense of KNaO and CaO). It is thus possible adjust thermal expansion of either recipe by adding some of the other (to deal with shivering or crazing you encounter).
The soak:soak:slowdrop firing is critical for a defect free surface. This sacrifices some surface brilliance compared to G2931K. The chemistry cannot be produced using common North American frits, so Ulexite is employed (the original recipe from which the Zero3 glaze system was derived, Worthington Clear, was high in Gerstley Borate and made a jellied mess in the bucket, very difficult to use).
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.
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.
This recipe converts the popular Worthington Clear Gerstley-Borate-based low fire clear recipe into something that 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), does not craze or shiver (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 of Ulexite to source the boron. The results are excellent, actually stunning! This one also has even fewer micro-dimples in the fired surface (visible in the light on very close inspection) and is crystal clear and super glassy.
We have found this works on Plainsman low fires like L212, 215, Terrastone plus Zero3 stoneware and Zero3 porcelain. It shivers on 50:50 Talc:Ball clay bodies (like Plainsman L213).
As noted, while Worthington melts to a good clear it has serious gelling problems (characteristic of Gerstley Borate). An issue with Worthington is shivering and fracturing of ware when glazed only on the inside (its thermal expansion was too low). This recipe raises thermal expansion (by adding Na2O), the result is a glaze that fits a much wider range of bodies.
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, drying and application properties. It naturally gels at around 1.53 specific gravity (in our circumstances), but we add water to bring it down to about 1.46-1.47 and add Epsom Salts to gel it back up (google glaze thixotropy to learn more). This produces a slurry that applies very well in an even layer to low or high porosity bisque (the gel makes it hang on without running or dripping). However extra water means that it dries slower on dense bisque.
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 many.
-Since the recipe is known, and contains no toxic materials, you can more confidently assure retailers that it is safe; all underglaze color decoration is isolated from any contact with food or drink by this glaze.
-It is compatible with most underglaze colors (including pinks).
Pay special attention to the drop-and-hold firing schedule for this glaze, especially if your clay body is not fine grained.
To mix up 5 Kg:
G2931K glaze 5.0 Kg
Water: 4.5 Kg.
Mixed for approx. 1/2 hour
S.G. 100 ml. grad. cyl. 1.46
S.G. 1.0 L. grad. cyl. 1.46
S.G. Hydrometer 1.46
Viscosity: Ford Cup 10.8 seconds.
Yield: 6.5 litres.
Batch Ticket Notes
These notes were entered in the notes panel under "Batch Ticket Notes"
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.
Alternate Code Number:GS04-1
Born: 2006-03-16, Modified: 2017-06-16 14:54:57
Firing Schedule Name
Cone 03 hold-rise-drop-hold
Degrees (Fahrenheit or Celcius)
Start Time and Temperature
(no temperature specified)
The firing can start and finish within your working day. Step 2 can often be done as fast as your kiln will go. We are done cold-to-cold in three hours.
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.