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Zero3 Clear Glazes With Firing Schedule
The first column is Zero3 L, the low expansion version. The second is the regular Zero3 K recipe and the third column is the original Worthington Clear from which they were derived.
Zero3 G Low Expansion Low Fire Clear
Auto Unity Formula
*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).
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.
XML (to paste into Insight)
<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Zero3 G Low Expansion Low Fire Clear" id="76114" key="cXAknG1B" date="2017-06-16" codenum="G2931G" picturebasename=""> <recipelines> <recipeline material="EPK" amount="27.296" tolerance=""/> <recipeline material="Silica" amount="13.265" tolerance=""/> <recipeline material="Ulexite" amount="26.085" tolerance=""/> <recipeline material="Ferro Frit 3249" amount="18.622" tolerance=""/> <recipeline material="Wollastonite" amount="13.180" tolerance=""/> <recipeline material="Zinc Oxide" amount="2.551" tolerance=""/> </recipelines> </recipe> </recipes>
Zero3 K Cone 03 Transparent Glaze
Auto Unity Formula
*This recipe improves the popular Worthington Gerstley-Borate-based low fire clear recipe. It targets cone 03 to work best on Zero3 stoneware and porcelain. However many low fire bodies are dramatically stronger when fired to cone 03 with this (or one of its thermal expansion variants L & H). And the vast majority of commercial glaze products will fire easily to this temperature. If you absolutely must fire lower, to cone 06 or 04, then use the G1216Q recipe instead.
Batch Ticket Notes
These notes were entered in the notes panel under "Batch Ticket Notes"
G2931K on Zero3 Stoneware
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, the Ulexite version, 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. Left: Plainsman Buffstone, contains no talc, fires buff. Center: L212 (about 25% talc). Right: L213 (about 45% talc, fires whiter). Talc raises thermal expansion. The centre glaze is G2931K (Insight-live reports COE 7.4), it fits L215 (also Zero3 porcelain and stoneware). It crazes on Buffstone and shivers on L213 and L212. G2931L has lower expansion (to work on zero-talc porous bodies). G2931H is higher (for talc bodies like L213).
G2931K glaze precipitates things on storage
G2931K On L212 after a year
Some crazing starting.
K on L215 at cone 04
The thicker version is clouding. The thinner one has micro-pinholes. It needs a higher temperature.
G2931K o L215 - Cone 03, thick application
G2931K on L215 - Cone 03 fired in 30 minutes
Very transparent. No crazingafter many months.
G2931K on L215 - Cone 06
Milky (because cone 06 is underfired for this glaze). But only a little crazing after a year.
G2931K running on Zero3 Casting
This happened at cone 03 and 04. The G3879 (left) did not run. For some reason G2931K easy applies to thickly on this body.
This 1000 gram batch of glaze powde
This 1000 gram batch of glaze powder screened 80 mesh immediately after making, and allowed to sit for 3 months prior to usage. The material in photo was screened out once again after the 3 month time lapse, (80 mesh once again). The largest balls are approx. 2.5 mm. in diameter, and when these particles were screened out, and still perhaps slightly wet?, they were quite easy to break in half with your fingernail.
Alternate Code Number:GS04-1
XML (to paste into Insight)
<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Zero3 K Cone 03 Transparent Glaze" id="95671" key="rN2FuY4y" date="2020-12-02" typecodes="ST" codenum="G2931K" picturebasename=""> <recipelines> <recipeline material="Ferro Frit 3195" amount="25.000" tolerance="" unitabbr="KG"/> <recipeline material="Ferro Frit 3134" amount="33.000" tolerance="" unitabbr="KG"/> <recipeline material="EPK" amount="20.000" tolerance="" unitabbr="KG"/> <recipeline material="Ferro Frit 3249" amount="10.000" unitabbr="KG"/> <recipeline material="Ferro Frit 3110" amount="7.000" unitabbr="KG"/> </recipelines> </recipe> </recipes>
Worthington Cone 06-2 Clear
Gelling, High LOI, Gerstley Borate difficult to sub, High Boron
Auto Unity Formula
*This recipe is a common Gerstley Borate clear base used from 04 all the way to cone 6! At higher temperatures the recipe trends toward less kaolin to more silica and a little less GB (e.g. 50:20:30).
Worthinton Clear at cone 01
On a terra cotta clay at this temperature was has stoneware properties. The fired surface is good.
Worthington Clear vs. Fritted Clear
Worthington (right) flows even better than the fritted glaze and does not have any more entrained bubbles even though it has an LOI of 20%. This is likely because its melting history and behavior is such that its ability handle gases of decomposition from the body and its own materials is so much better.
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.
Entrained bubbles in Worthington Clear
This is a 16X closeup of flow test (10 gram ball melted down onto a tile) that concentrates bubbles. There are high populations of large and tiny ones. The larger ones are from the Gerstley Borate, the tiny ones from the kaolin.
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.
Worthington Clear the next day
Even though this has a low specific gravity and is deflocculated with darvan, the next day it is still jelly. Impossible to use unless more Darvan is added, who knows where that will go!
G2922G, G2931 flow tests
SHAB - Shrinkage/Absorption
LDW - LOI/Density/Water Content
XML (to paste into Insight)
<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Worthington Cone 06-2 Clear" keywords="Gelling, High LOI, Gerstley Borate difficult to sub, High Boron" id="56711" key="stRyT8gm" date="2017-09-05" codenum="G2931" picturebasename=""> <recipelines> <recipeline material="Gerstley Borate" amount="55.000" tolerance=""/> <recipeline material="EPK" amount="30.000" tolerance=""/> <recipeline material="Silica" amount="15.000" tolerance=""/> </recipelines> </recipe> </recipes>
Firing Schedule Name
Cone 03 hold-rise-drop-hold
Degrees (Fahrenheit or Celcius)
Start Time and Temperature
8:01 am at 80F
Used to glaze fire Zero3 porcelain and stoneware. However, there is a caution about bisquing these frit-containing bodies: They will vitrify in a typical firing, you must reduce the temperature to around 1500F (if ware is not absorbent enough fire lower). Note: this schedule is for the glaze firing, the previous two sentences simply referenced what is needed for the bisque.