Zero3 Low-fire Ultra-Clear Glaze Recipe

Zero3 Ulexite Transparent Glaze

For use with the Zero3 body and engobe

Total:100.00

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Notes

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).

Pictures

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

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.

G2931F vs 2931K flow test

G2931F on Zero3 Porcelain

The mug was decorated with underglazes at the leather hard stage, then bisque fired to 1650F, then dip-glazed and fired using a cone 03 drop-and-hold firing schedule. This glaze is exceptionally brilliant and clear, it transmits the color of the underglazes better than any other glaze at any other temperature I have seen. This glaze was later upgraded to one having the same chemistry but a higher frit content and firing even better, G2931K.

Zero3 K Low Fire Transparent Glaze

Total:95,000.00 (R)

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Notes

This recipe converts the popular Worthington 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. The results are excellent, actually stunning! 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). Links to them can be found below. 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 perfectly crystal clear, no commercial glaze can match your own fitted one. And you can make make your own brushing versions (see link below).

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"

URLs

Glaze fit test

How to tune the thixotropy of a glaze

How to convert a d..o a brushing glaze

Pictures

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

Firing Schedule Name

Cone 03 hold-rise-drop-hold

Degrees (Fahrenheit or Celcius)

Unspecified

Schedule Type

Rate

Start Time and Temperature

8:01 am (no temperature specified)

Program

Notes

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.