Four Cone 6 (1200C) Casting/Throwing Porcelain Recipes (Grolleg and #6 Tile)

Project Name

Dialing the desired plasticity in four cone 6 porcelains

Project Codenumber

UnAssigned

Notes

White porcelains are made from kaolin, feldspar and silica. Super-white porcelains employ expensive, low titanium, low plasticity kaolins (e.g. Grolleg, New Zealand) and thus require the help of white-burning bentonites like Veegum (3-5% in throwing bodies, ~1% in casting bodies). Ivory porcelains employ cheaper, but more plastic kaolins and can tolerate a less expensive bentonite. White stonewares employ a mix of kaolin and ball clay (to mix with the feldspar and silica).

Modern potters want high plasticity, few would be willing to tolerate the flabby porcelains used in the past. Since plasticity determines plastic strength, it is possible to "dial-up" just the right amount to enable anything from throwing a giant vase to casting a super-thin-walled piece (capable of pulling itself from the mold, as it shrinks, without splitting). The bentonite addition for this whole range of working properties is from 5% down to 0.5%.

Grolleg kaolin does not fire quite as white as New Zealand kaolin but requires less feldspar to produce a vitreous body. And it has better drying strength. By employing VeeGum as the plasticizer, the feldspar can be dropped even further.

Assuming that 20-25% silica is needed for glaze fit, we did a series of tests (L3778) to discover what blend of feldspar and kaolin (in the remaining 75% of the recipe) is needed to achieve a degree of vitrification where zero porosity is reached a little before cone 6. Fired shrinkage and porosity data is shown for each.

If you are adding stains note that maturity and firing shrinkage will be affected (depending on the stain).

URLs

Every potter needs a plaster table

Convert a Grolleg throwing body to a casting body

What is deflocculation

Mixing the Casting Slip (section on Polar Ice Page)

Cone 6 Translucent Grolleg Casting

Total:100.00

Notes

Because this employs Grolleg kaolin, it fires much whiter than bodies made using North American kaolins. And it will be more vitreous.

Substituting New Zealand kaolin for Grolleg Kaolin will whiten the body, but if you use it be ready to adjust the percentage of Nepheline to control changes in degree of vitrification. Plainsman Polar is made this way. It has 4% Veegum and 25% silica, so deriving the nepheline/NZK ratio is just a matter of testing to determine the best compromise between highly vitrified and resistance to warping in the kiln (nepheline with be between 20 and 30%, the nepheline/kaolin combination should total 74).

The 1% VeeGum is needed for casting, without it the clay shrinks too little and lacks the leather hard strength to pull itself away from the mold.

You can convert this to a plastic throwing version by simply adding more VeeGum. It has unparalleled plasticizing power, fires completely white (but its addition can drastically reduce the amount of feldspar/nepheline needed, Veegum is a superflux). For plastic bodies 3% VeeGum is enough to get good plasticity, 4% gives super plasticity.

For slurry-mixing instructions use the ones found on the Polar Ice data sheet at plainsmanclays.com.

Pictures

L3778G cone 6 Grolleg casting body vs. others

Top is this body.
Next one is a whiteware casting body made using Pioneer Kaolin and KT1-4 ball clay (M370 casting)
The bottom one is a buff stoneware (M340 casting)

L3778G Grolleg cone 6 casting

The glaze is GA6-B. The body fires extremely white and dense, producing stunning pieces. It is porous at bisque state and it is easy to get glazes on too thick.

L3778G with GA6-B glaze fired at cone 6

Fired strength is very good (under the hammer). Rather than shatter, it simply broke cleanly into to pieces.

L3778G Cone 6 Translucent Grolleg Casting fired bars

L3778G vs Polar Ice - Fired bars

P300 Cone 6 Casting Body

Total:603,277.60 (R)

Notes

P300 is a cone 6 plastic kaolin-only porcelain made by Plainsman Clays. This is a casting version of it. This recipe employs Pioneer Kaolin instead of #6 Tile, this only needs 1% bentonite (instead of 5% for the plastic version) and the Nepheline here has been adjusted to fine tune the maturity.

This is a kaolin-only (no ball clay), it has the whiteness that comes with using only kaolin (but only as white as North American kaolins can achieve). Understanding this recipe will enable you to adjust it for your needs and incorporate your name-brand materials. Our bentonite is from bentonite.com. Pioneer Kaolin is moderately plastic, be careful about substituting ones that claim to be whiter but are not plastic enough. Nepheline is A270. Use a soda feldspar if you do not have Nepheline (better quality feldspars will produce a whiter fired color). Talc is Silverline but yours should work also.

Fired maturity: In our circumstances this recipe has about 0.5% porosity at cone 6 (measured using the SHAB test), not a true porcelain but getting close. The talc, a super-flux in this setting, makes this body possible, without it the Nepheline would need to be 35%. Try increasing the talc if you need it to be more vitreous, but beyond 2% it will cease having an effect.

Plasticity: The 1% bentonite is needed here to give casts enough green strength to release and pull away from the mold without splitting. If your shapes cast easily, you may be able to use less bentonite (meaning pieces will cast faster).

Glaze fit: The 25% silica is essential. If you reduce it for high kaolin, glazes may craze.

Deflocculation: For 1000 powder use 394 water, 3.6 Darvan to produce about 1.8SG. Start with 3.0 Darvan and add more if needed to get a slurry that gels slightly on standing for a few minutes.

Pictures

G2926B after 300F:Icewater test 300:370

On P300 test mix (made with Pioneer kaolin, left), 370 next recipes. Neither crazed on the sidewalls. But P300 crazed on the bottom (as shown).
Slight crazing on L3905 M370 but this did not get worse after a year.

P300 using Pioneer kaolin - mug bottom

P6346 vs. L3906 Lab test with PIoneer P300s

P6346 P300 vs L3906 P300 test bars

G2926b clear not crazed after two weeks

L3906 P300 Using Pioneer Kaolin fired bars

M370-like Cone 6 Faster Casting

Total:453,592.00 (R)

Notes

M370 is a plastic throwing kaolin/ball clay porcelain made by Plainsman Clays. This is a casting version of it (this has no bentonite whereas the plastic version has about 4%). This recipe employs Pioneer Kaolin and KT#1-4 ball clay for faster casting than M370C (which uses more plastic fine particles ball clay and kaolin).

This is not quite as vitreous as M370C, consider adding 1% talc to reduce the porosity to 1% at cone 6.

For 2000 powder used 790 water, 6.6 Darvan. For us this produces 1.8 specific gravity (higher than practical with M370C).

Pictures

L4217G, L3798G2, L3778G fired bars

Fired from cone 8 to 4 (top to bottom).
G2 is a remake of the L3798G mix (to double check accuracy).

M370-like Super-Fast Casting Porcelain

Total:100.01

Notes

This body is a cone 6 variation on the standard 25-Porcelain recipe, using the more powerful Nepheline Syenite as the flux instead of feldspar. While 30% is normally needed, this adds talc, a super-flux in small percentages (we adjust its amount to target 1% fired porosity). This also has 5% more kaolin and 5% less ball clay than our typical plastic body for use on the potters wheel (that also has 3-5% bentonite).

For use in casting this recipe trades the plastic ball clay for KT#1-4 ball clay, a larger particle material recommended for slip casting. It also switches the hyper-plastic and fine particled #6Tile kaolin for Optikast, a large particle kaoli intended for slip casting. The result is dramatic.

-Deflocculation is easier.
-Casting time is drastically reduced (less slip time in the mold, fast removal after pour out).
-Faster drying of ware and less risk of cracking and warping.

However fast casting comes at a cost: Fettling the extracted ware is more difficult if it is allowed to dewater too much. A sharp knife and extra care are needed. Drying and dried ware also has less strength.

To enable fine-tuning the trade-off between high production and less breakage edge crumbling we add a small amount of bentonite (starting with 0.5%), this slows down the casting rate and increases workability and dry strength.

To make 4000 powder: 1500 water, 20 Darvan. Adjust for your water and deflocculant.

Pictures

L4807 Casting slip - A mini beer bottle