Zero3 Ulexite-based glaze, engobe and firing schedule

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See Also: Four Low Fire Base Clear Glazes: What is the Best Strategy?

Project Name

Zero3 Low Temperature Stoneware System


This began as a project to formulate red and white low fire stoneware bodies that could be used on each other as engobes. However, over time, it because evident that the red body far excelled in strength and the white as an engobe. So that became the focus.

The way to achieve high fired strength at low temperatures has long been known: frit additions. But frit is far more expensive than typical body materials and low-fire did not seem a logical place to make stoneware when it came so natural at cone 6 and 10. But times have changed. Now we think a lot more about energy consumption, so the prospect of firing ten or twelve cones lower is an immediate attention getter. Further, anyone who makes stoneware in electric kilns knows how hard these temperatures are on kiln elements and relays. Imagine making ware of superior strength and density only a cone or two above your usual bisque temperature! That is what we have here.

But the technical challenges are much greater to make this work. That is why we have created the Zero3 Solution. Everything you need is here. And it is open source, no secrets. The problems have been solved. Just do it according to our instructions and you will have brightly colored, high gloss, super strong and safe ware that people will often pick over even porcelain.

And there are no sacrifices in workability. The body is highly plastic so you can make ware very thin. The underglazes paint on like silk and dry enough that in less than an hour you can follow up with incising into the body. The body dries well, sets hard and can tolerate a fast bisque. The glaze glides on like a the best stoneware glaze would (on dipping).

Why was cone 03 chosen? Because 04 is too low and 02 is too high. Almost any low fire underglaze, glaze or overglaze will still work at cone 03. Also it is low enough to still develop good terra cotta red color and bubble free glazes but high enough that minimal frit is needed to densify it.

Zero3 Ulexite Transparent Glaze

For use with the Zero3 body and engobe

Code #


Materials Amt
EPK 27.000
Silica 7.000
Ulexite 29.000
Ferro Frit 3249 9.000
Wollastonite 9.000
Ferro Frit 3110 19.000


Unity Formula

CaO 0.60
MgO 0.10
K2O 0.02
Na2O 0.28
(KNaO) 0.30
B2O3 0.76
Al2O3 0.40
SiO2 2.33

Si:Al Ratio


SiB:Al Ratio







17.76 per kg


This recipe converts the popular Worthington Clear Gerstley-Borate-based recipe into something much easier to use. While Worthington melts to a good clear it has serious gelling problems (characteristic of Gerstley Borate). Normal frits cannot easily source this high level of B2O3 so I have employed Ulexite to augment (a mineral similar to GB yet gels only slightly). Another issue with Worthington is shivering and fracturing of ware when glazed only on the inside (its thermal expansion was too low). This recipe also raises thermal expansion (by adding Na2O), the result is a glaze that fits a much wider range of bodies. Worthington also has clouding issues, this yields a more crystal clear, defect free fired result.

Dipping ware in Zero3 Transparent is just so much easier than trying to paint on than commercial, slow drying clears. It produces a slurry with very nice suspension, drying and application properties. It naturally gels (at around 1.4 SG in our lab). It applies very well in an even layer to low or high porosity bisque (the gel makes it hang on without running or dripping). However the 1.4 specific gravity (which is lower than most stoneware glazes) means that it dries slower if bisque is dense.

As a clear overglaze to finish your decorated ware, this glaze has a number of 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.
-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 soak-soak-slow cool firing schedule for this glaze, especially if your clay body is not fine grained.

Note: Later we did find a way to create the chemistry of this glaze using a combination of frits (look for a link).


Terrastone, L215

The successful firing of these mugs represents two years of testing to learn how to get a crystal clear glaze and how to create slips that dry and fire bond well! The firing schedule is very important: Cone 03 soak-rise-soak-slowcool. Slips are bright colored (they contain only 10% stains).

Left: L3786E Plainsman Terrastone test mix with L3724M vitreous red slip with G2931F glaze. Wax resist exposes portions of the red slip which flashes near the glaze boundary.

Center: Terrastone with L3685U vitreous white slip inside and brushed on (and 10% stained versions of it brushed on) with G2931F glaze.

Right: L3724M vitreous red body with L3685U engobe (and stained versions of it) with G2931F glaze.

Shivers on L212

G2931F on L215 cone 04,03,02

All survived boiling water, ice water test without crazing.

L213 with G2931F glaze cone 04, 03, 02

Boiling water, ice water tested. None crazed.

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).

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.

G2931F on F100, Buffstone, L212 and L215

Out of the kiln all fit. However it crazed on buffstone after boiling water, ice water test (use 2931G instead). The others are good. L212 crazes at cone 04.

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.

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.

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.


LDW - LOI/Density/Water Content

1 21.86 11.23 48.6%

Born: 2014-03-19, Modified: 2017-06-08 10:38:17

Zero3 Vitreous Engobe

For use with the Zero3 body and glaze

Code #




Materials Amt
Nepheline Syenite 10.000 9.80%
Pioneer Kaolin 42.000 41.18%
Ferro Frit 3110 25.000 24.51%
Silica 20.000 19.61%
National Standard Bentonite 5.000 4.90%



The Zero3 Engobe finds its inspiration in the ceramic tile industry. Thousands of tons of white engobe are applied to tiles made from red or brown burning bodies, after pressing, to prepare them for glazing. This neutral surface enables the use of glazes of any color.

This engobe is not required in the Zero3 system, but like tile, it is employed for aesthetic or functional reasons. A typical use case is application to inside food surfaces of ware and covering with the clear transparent Zero3 glaze after bisque firing. Do not use Zero3 Engobe on regular non-vitreous terracotta or white earthenware, its fired shrinkage is too high.

Although it is possible to apply this at the bisque stage, we recommend doing it at stiff leather-hard. Even coverage can normally only be achieved by dipping or pouring (if you plan to paint this on we recommend using a commercial white underglaze product instead).

Since this fires white, it can be applied to outer surfaces (by dipping) as a base on which to paint colored underglazes.

To use this product it is vital that you have a good mixer. In cases where you do not need to have gallons a variable speed kitchen blender can work well. The consistency must be exactly right or it will not apply properly. The consistency can changes on storage so you may need to adjust it on each use. When properly gelled the Zero3 engobe slurry goes on to the leather hard ware evenly, hangs on very well, does not run or drip and dries without flaking off. We have found that equal parts of water and powder produce a specific gravity of about 1.4 and a little Epsom Salts will gel that so it will stop motion (and bounce back, called thixotropy) less than 2 seconds after stopping stirring. As you develop expertise in controlling the rheology of this slurry, you should be able to reduce its water content somewhat. If you over deflocculate it just thin it back out with a tiny amount of Darvan.

However, it also rewets the clay and thus sets back drying considerably.

A drop or two of Dettol might be needed to store this for long periods (to prevent bacteria or mold).

Recipe info: The 20% silica helps match body thermal expansion and glaze fit. The 25% frit makes the fired hardness more closely match the body. Nepheline Syenite is included as a filler.


Engobe applied to leather-hard mug

This was done by simply filling and rotate-pouring the engobe, then pressing the lip into it. Since the engobe gels the mug could be turned over immediately without creating any drips.

Notice that the outer surface of the handle has been waxed to slow down its drying. This is because the engobe rewets the mug body, thereby setting it behind the handle. This mug has thin wall so this this rewet adds a day to the production process. This next day the brushwork is done on the outside.

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).

Born: 2014-11-04, Modified: 2017-06-21 11:23:26

Firing Schedule Name

Cone 03 hold-rise-drop-hold

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 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.