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

Project Name

Low Fire Clear Glaze Comparison

Notes

A good base clear for low temperature ceramics is vital for success. It can become the base of all of your coloured glazes. And a kiln firing situation where you can do lots of testing is important. After all, if you can do a firing in 3 hours, it is much more practical to do testing.

At low fire, boron is the king of fluxes. And frits are the key sources (not a single commercial glaze manufacturer uses Gerstley Borate to source boron, yet many potters still do). What is the perfect combination of Ferro frits to create a super clear glossy glaze for low fire? There are an infinite number. But that number is drastically reduced when we eliminate those of too-high and too-low thermal-expansion-adjustable. And those that are not durable and resistant to leaching. And those that are not compatible with stains we need. And those that do not heal blisters and pinholes well.

Potters coming to low fire from higher temperatures expect fired strength, that is just not there at cone 06-04. But terra cotta bodies fired at cone 03-02 are dramatically better. That is why, if possible, it is better to fire at this range. Our G2931K is intended for exactly this. At lower temperatures it loses clarity and the surface is not as smooth.

Good low fire glaze can be applied in a much thinner layer than at higher temperatures and yet still look full bodied. A thinner layer will also produce a more crystal-clear product. One way to get it thinner is to bisque higher so ware is less absorbent or thin the glaze with water (to make it too watery) and then flocculate it to a slight gel (in this state it will apply in an even yet thinner layer and hang on well with no drips). If faster drying is needed or ware is very thin and easily waterlogged by the glaze, heat it to 300F before immersing it into the glaze.

But low fire can bring alot of problems. What is needed in a glaze glaze is one that does not go cloudy on firing, does not bubble over underglazes, does not crystallize or devitrify, produces a smooth brilliantly glossy surface, does not cutlery mark, does not leach, accepts an opacifier well, does not blister or pinhole, does not settle like a rock in the bucket or turn to jelly.

But again, most important, is thermal expansion adjustability. Low fire glazes are simply not stuck on to the body as well as high fire glazes. If the thermal expansion of the glaze is too low it will simply flake off (especially if there is a slip between it and the body). If the expansion is too high it will craze. It is almost certain that your glaze and body just have compatible thermal expansion by accident. You must stress ware using a boiling-water-into-ice-water test (then reverse). Adjust the glaze and try it again until you get a fit (you owe this to your customers). Do you have to do this test for each clay body? Absolutely. Each temperature? Yes. Terra cotta bodies vitrify rapidly above cone 04, each temperature is a new thermal expansion animal.

What about white burning low fire bodies? These will almost definitely not fit the same clear glaze as a terra cotta. They have added talc, it is put there for the specific purpose of increasing the thermal expansion to prevent crazing. This greatly increases the chance of shivering. So, again, you must test fit and adjust the clear glaze accordingly.

Recipes that are popular do not necessarily stand up to scrutiny, especially if poorly documented. Use one that is documented, a product of much testing. And adjustable.

May 2019: We are working on another recipe, G3879. See the link below, it could beat all of these.

URLs

L4170 Terra Cotta Casting with G3879 UltraClear glaze

Deb's Clear Cone 04-02

Total:100.00

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Notes

A well known clear for low temperature, especially terra cotta. Sources often caution against applying too thickly to avoid clouding. We found the same issue. Many use this this as an alternative to Worthington Clear. Its thermal expansion is considerably higher.

The Al2O3 is very high for a glossy glaze. High Al2O3 produces stiffer melts that do not shed bubbles as well.

The boron is very high, this is being used to crow-bar gloss out of a glaze that would not otherwise be glossy. High boron is commonly associated with clouding.

Increasing thermal expansion (to fight shivering) requires increasing the Frit 3134 at the expense of 3195. But its boron is just as high. Frit 3110 would be a better choice, a small change would be needed to increase expansion and it has low boron.

This has a high kaolin content. This could cause crawling and drying cracks. If Frit 3124 were used instead (it supplies Al2O3 and 3134 does not), less kaolin could be employed.

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Deb's Clear Cone 04-02" id="70214" key="PW2aKECy" date="2017-03-10" codenum="G2932" picturebasename=""> <recipelines> <recipeline material="Ferro Frit 3134" amount="30.000"/> <recipeline material="Ferro Frit 3195" amount="45.000"/> <recipeline material="EP Kaolin" amount="25.000"/> </recipelines> </recipe> </recipes>

Born: 2014-03-17, Modified: 2017-03-10 23:15:34

Worthington Cone 06-2 Clear

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

Total:100.00

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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

Zero3 K Low Fire Transparent Glaze

Total:95,000.00 (R)

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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

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. 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 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.

Cone 03-04 Expansion Adjustable Gloss Base

Total:100.00

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Notes

This recipe provides the greatest thermal expansion adjustability we have seen in a low fire clear glaze. It combines a middle-of-the-road thermal expansion frit with a very low and very high expansion frit. By trading 3195 for 3249 expansion can be lowered. With 3110 it can be raised. As it stands here it fits a terra cotta body we use.

Frit 3195, by itself, fires to a slightly silky surface. 3110 and 3249 are highly glossy. It does not take much of the others to move the 3195 surface toward a brilliant gloss.

Do not assume the glaze fits because a piece emerges from the kiln without crazing or shivering. Stress it (by two minute boiling-water and ice-water immersion cycles). If you find this does not fit, try a 10% adjustment. For example, if it crazes, increase the 3195 by 10% and reduce the 3110 by 10%. Do the opposite if it shivers. If you end up with pure 3195 and it still crazes, then introduce Frit 3249 at its expense.

The ball clay also imparts better working properties than the EPK and it has a lower LOI to generate less micro-bubbles in the glaze.

Pictures

Cone 01 on 3D+iron

Very nice results on L3724E red body at cone 01. Very strong.

No raw kaolin = more transparent

1916J (left) and 1916S (right). The S is much more transparent. Why? J employs raw EPK (kaolin loses 12% of its weight on firing, creating gases). S has no raw kaolin, it uses VeeGum and calcined kaolin). However it is not possible to create a good slurry with this recipe (it gels). Thus, we settled on using a clean ball clay to suspend and harden the recipe (ball clays have half the LOI or kaolins), that is recipe
1916Q.

1916J and Q fired to 1450F

These glazes are starting to melt, the surface having reached the consistency of a porcelain and have densitfied to very low porosity. Notice the iron in the ball clay really shows up at this stage (it will be less evident later).

G1916Q and J fired 1650-2000F

Ten-gram balls melted and flattened as they fired. They soften over a wide range, starting well below cone 010! At 1650F carbon material is still visible (even though the glaze has lost 2% of its weight to this point), it is likely the source of the micro-bubbles that completely opacify the matrix even at 1950F (cone 04). This is an 85% fritted glaze, yet it still has carbon; think of what a raw glaze might have! Of course, this is a thick layer, so the bubbles are expected. But they still can be an issue on a piece of ware. So to get the most transparent possible result it is wise to fire tests to find the point where the glaze starts to soften (1450F in this case), then soak the kiln just below that (on the way up) to fire away as much of the carbon as possible.

Success with cyrstal clear glaze cone 03

Uses Cone 03 soak-soak-slow cool schedule.
Left: P6282 with 3685U slip and 1916Q. Clear and very good. Glaze is thicker than the other two. Shivering on lip, the slip is not fitting the body.
Center: P6282 with G2931F Ulexite clear. Better clarity even though it is applied very thin. Shivering on lip, the slip is not fitting the body.
Right: L3724F with 3685U slip. No shivering. Very good coverage of the glaze, very clear, the best I have seen yet!