How to fix Worthington Clear

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Worthington Cone 06-2 Clear

Gelling, High LOI, Gerstley Borate difficult to sub, High Boron

Code #

G2931

Materials Amt
Gerstley Borate 55.000
EPK 30.000
Silica 15.000

Total:100.00

Auto Unity Formula

CaO 0.69
MgO 0.17
K2O 0.01
Na2O 0.13
(KNaO) 0.14
B2O3 0.76
Al2O3 0.41
SiO2 2.20
Fe2O3 0.01

Ratios

Si:Al: 5.3:1
SiB:Al: 7.2:1
R2O:RO: 0.1:0.9

Expansion

6.5

LOI

20.7

Cost

37.79 per kg

Notes

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).

Attractions of the recipe include its simple make-up and crystal clear fired result (low bubble population). In theory it would seem that fritted glazes should be much better at smoothing out and freezing to a crystal clear glass, but in actual practice this is not necessarily so (because common frits have a lower boron content). Potters use this glaze on terra cotta, talc and stoneware bodies that generate lots of gases of decomposition and this glaze appears to be able to pass them and heal well because of its high melt fluidity. High boron clear glazes like this are known for clouding issues (because of micro bubbles) but this has a low enough surface tension to pass them.

Problems of this recipe:

Obviously, if it melts well already at cone 04 great care is going to be needed at cone 2 to prevent it running onto shelves. Highly melt fluid glazes (that have high boron like this) are also more likely to be leachable and to crystallize on cooling (producing the boron-blue clouds).

Paramount is the dreadful problem of gelling. Even with only 1.4 specific gravity (about equal weights of dry material and water and deflocculation with Darvan) it gels badly. The high water content needed and the nature of GB means that glazed ware dries extremely slowly, the glaze shrinks and cracks during drying and crawls during firing. Gerstley Borate is very plastic, yet to make matter worse, the recipe has an additional 30% clay on top of that! This recipe would likely make a good throwing body!

The thermal expansion is too low for talc bodies, it will shiver on them.

If it would be possible to use another boron-sourcing material that had none of these issues, this glaze would be a winner. A frit with a very high boron content would be needed.

Pictures

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.

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.

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

Testdata

SHAB - Shrinkage/Absorption

DLEN DSHR FSHR ABS
1 92.24 7.8%
2 92.11 7.9%

LDW - LOI/Density/Water Content

WWGT DWGT H2O LOI DENS
1 13.52 10.26 24.1%

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="a4sdzsBb" date="2017-09-05" codenum="G2931" picturebasename=""> <recipelines> <recipeline material="Gerstley Borate" amount="55.000"/> <recipeline material="EPK" amount="30.000"/> <recipeline material="Silica" amount="15.000"/> </recipelines> </recipe> </recipes>

Born: 2014-03-17, Modified: 2017-09-05 13:01:38

Low Expansion Zero3 Clear

Code #

G2931B

Materials Amt
EPK 25.100 25.15%
Silica 10.600 10.62%
Ulexite 26.500 26.55%
Ferro Frit 3249 16.900 16.93%
Wollastonite 15.500 15.53%
Nepheline Syenite 5.200 5.21%

Total:99.80

Auto Unity Formula

CaO 0.69
MgO 0.17
K2O 0.01
Na2O 0.13
(KNaO) 0.15
B2O3 0.76
Al2O3 0.41
SiO2 2.19

Ratios

Si:Al: 5.3:1
SiB:Al: 7.2:1
R2O:RO: 0.1:0.9

Expansion

6.5

LOI

13.2

Cost

26.80 per kg

Notes

This is a lower expansion version of G2931K, an ultra-clear for porcelain and talc bodies at low fire. This sacrifices some surface brilliance for the low thermal expansion. The chemistry of this recipe cannot be produced using common frits, the Ulexite is essential (to source the high boron).

The original recipe from which this was derived, Worthington Clear, was high in Gerstley Borate and made a jellied mess in the bucket, very difficult to use. This has the same chemistry but a very different recipe having much better slurry properties.

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.

Although this recipe has more ingredients than Worthington clear, the range of properties and chemistry they supply presents many more adjustment possibilities.

Links with Schedules

To: Cone 03 hold-rise-drop-hold

Pictures

Ulexite Worthington Clear to cone 02

No pinholes, thin application. Some bubbles. This was fired to temperature and soak 10 minutes (and cool of 100 degrees and soak there would have yielded even better results). Cone 03 and 02 do produce a more brilliant result than cone 04.

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.

Ulexite Worthington clear cone 02

This is a terra cotta stoneware body, the body contains enough frit to produce 1% porosity at cone 02. Although it appears to work well, it is under too much compression because its thermal expansion is too low. We noted a tendency to shiver on the rims of mugs, especially with the white engobe present.

Worthington Original vs. Ulexite varitation flow test

Gas generation during melt is obviously more active in the Gerstley Borate version (left) than in the Ulexite version (right). This is fired to cone 03.

Entrained bubbles in Ulexite Worthington

16x photo of a 10 gram glaze ball melted down onto a tile, it was fired to cone 03. This has both the large and small bubbles populations.

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.

G2931B and F fit different bodies

The fritted vitreous body on the left shivers using G2931B. The same body (having the same white slip (G3685U) does not shiver the G2931F version of the glaze (it has more KNaO).

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Low Expansion Zero3 Clear" id="56872" key="6z9K8G88" date="2017-06-08" codenum="G2931B" picturebasename=""> <recipelines> <recipeline material="EPK" amount="25.100"/> <recipeline material="Silica" amount="10.600"/> <recipeline material="Ulexite" amount="26.500"/> <recipeline material="Ferro Frit 3249" amount="16.900"/> <recipeline material="Wollastonite" amount="15.500"/> <recipeline material="Nepheline Syenite" amount="5.200"/> </recipelines> </recipe> </recipes>

Born: 2014-03-19, Modified: 2017-06-08 21:20:13

Zero3 K Low Fire Transparent Glaze

Code #

G2931K

Materials Amt Units
Ferro Frit 3195 25.000 KG 26.32%
Ferro Frit 3134 33.000 KG 34.74%
EPK 20.000 KG 21.05%
Ferro Frit 3249 10.000 KG 10.53%
Ferro Frit 3110 7.000 KG 7.37%

Total:95,000.00 (R)

Auto Unity Formula

CaO 0.59
MgO 0.10
Na2O 0.31
(KNaO) 0.31
B2O3 0.76
Al2O3 0.41
SiO2 2.45

Ratios

Si:Al: 6.1:1
SiB:Al: 7.9:1
R2O:RO: 0.3:0.7

Expansion

7.4

LOI

3.1

Cost

0.20 per kg

Notes

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.

Unlike Worthington something this 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) 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. It is 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). 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 crystal clear. And you can make make your own brushing versions (see link below). Keep in mind that adding zircon and stains changes expansion and requires re-testing and possibly changing the proportion of K, H and L to match your body. It is a hassle, but it is not rocket science.

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

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 need 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 but only a little crazing after a year.

Typecodes

ST-Untitled typecode

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 Low Fire Transparent Glaze" id="95671" key="ezdybPiq" date="2018-12-06" typecodes="ST" codenum="G2931K" picturebasename=""> <recipelines> <recipeline material="Ferro Frit 3195" amount="25.000" unitabbr="KG"/> <recipeline material="Ferro Frit 3134" amount="33.000" unitabbr="KG"/> <recipeline material="EPK" amount="20.000" 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>

Born: 2006-03-16, Modified: 2018-12-06 09:14:37