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G3879 - Cone 04 UltraClear Glossy Base

Modified: 2020-07-01 13:32:23

A super transparent low fire base clear glaze created by reverse engineering a commercial product.

Material Amount Percent
Fusion Frit F-524850.00
Fusion Frit F-6940.00


I developed this for cone 04 to fit as many clay bodies (without crazing) as possible (my Zero3 clear works well on specific bodies but does not melt enough at cone 04 and it039;s thermal expansion is too high to fit some bodies). I am developing a recipe for a terracotta casting body at the same time as this and I am working to make it compatible also.

I found the inspiration for this on testing many commercial clears and finding one that stood above the others regarding fit and clarity at 04. I have always been under the impression that low fire bodies have a wide enough range of thermal expansions that one glaze cannot be expected to fit them all. And that if a glaze melts well at cone 04 it will have too much melt fluidity past cone 02. But this glaze has made me question both, I am shocked at how it is possible that it can fit so many bodies and work across such a wide temperature range. In fact, I have not found a body that it does not fit! So I had it analyzed at a lab and then created a recipe to source its chemistry. A stroke-of-luck was that a frit we already use to produce another glaze, Fusion Frit F-524, is close to the complete chemistry needed.

I have been surprised by other aspects of this recipe. It is crystal-clear on any body at any thickness (at cone 04 and above)! Additions of tin and zircon produce a white that melts just as well as the clear. It is amazing how the mobile the melt is, how it runs right off my fluidity checker! Yet it is not significantly more mobile at cone 1 than cone 04. And it has the amount of SiO2 and Al2O3 that a cone 6 glaze has! There is some kind of magic with this chemistry that I am anxious to learn more about.

While it hardens to a powdery surface, it is amazing how little gum solution is required to make it dry hard and hang on to the bisque when other layers are added on top.

If you want to make bright colored glazes this should be an excellent base (remember, this is the clear glaze recipe currently used by at least one commercial supplier of bright coloured glazes). Encapsulated stains are fantastic at low fire and they can be mixed to fine tune to color. If you are targeting a specific or exotic color then the ability to achieve it will be determined by your ability to source an exotic stain!

For the first mix I added 3000g of powder to 2400g of water to produce a specific gravity of 1.48SG. This produced a watery slurry. I added epsom salts to the point where further additions did not thicken the slurry (up to a total of 7g). This improved it considerably but it was still a little thin (although it covered and applied like a typical dipping glaze, drying in seconds on bisque ware). However the surface was too powdery so I removed 400g of water and replaced it with 400g gum solution. This slowed dip time to about a minute (waiting for the dripping to stop) but it now tolerates thick overlaying of Majolica colors (without them pulling it away from the bisque). For a single-coat dipping (where no overglaze work will be done) I would use about half the amount of gum solution.

Joe: Made some this glaze up in March of 2020 and checked June 2020 ( 3 months later), and glaze has hard panned quite badly, but was able to remix with a little effort. The next day it was easier to remix. The Specific Gravity was adjusted to 1.50 and a moisture content was taken at this point in time and found to be 54.25% glaze powder and 45.75% water content. This glaze was used on the low temperature sample boards for the L215 "bottom" side samples, as well as the L210 "top" side samples. The samples were bisqued in the cone 06-05 range and glaze fired at the same temperature more or less.
If you want to make this into a air brushing glaze, which actually applies better to the sample board side discs, the glaze must be screened 200 mesh and then specific gravity adjusted to 1.40 and at this S.G. the powder to water ratio would be approx. 54% glaze powder and 46% water content.

Related Information

Reverse-engineer a commercial transparent glaze to get the recipe

Side-by-side melt flows of a commercial glaze and my duplicate

The commercial cone 04 clear brushing glaze on the left works really well on our bodies so I sent it away to be analyzed (about $130). From that information and using my account at I was able to create a recipe, having the same chemistry, employing two Fusion frits (which amazingly supplied all of the fluxing oxides). In this cone 04 melt fluidity comparison they are almost identical (mine, G3879, has a little more surface tension). The Al2O3 and SiO2 levels would make, even a cone 6 glaze, jealous! So it should be very durable. And it has low thermal expansion (no crazing). With the bucket of dipping-slurry I made I can glaze a piece perfectly evenly in seconds rather than the normal 10 minutes of fiddling with a brush and three coats! I have used it on dozens of pieces, it's amazing. One caution: It is possible to duplicate a transparent glaze like this but not a coloured one (a lab could not analyze the complexities of the color, stain colors are about more than chemistry (firing method, particle physics). For colored glaze you have to do trial-and-error testing with stain additions to this base.

SIAL 10F and 25F with G3879 clear glaze at cone 04

SIAL bodies are made in Montreal, Canada. The glaze was applied by dipping. The fit is still holding on both bodies after many months.

This mug has waterlogged because it is not vitrified

Notice the water has wicked up to about 1 cm from the rim (the piece sat in water overnight). The glaze fits so there are no cracks for the water to seep through. However, being fired at cone 04, the body is quite porous. The piece has a unglazed base. Notice the water even travelled up the handle. Less exposed bare clay on the base would improve the situation somewhat, however it would be much better to choose a body that vitrifies sufficiently dense so that it does not absorb water (or fire to a higher temperature). There is a not-so-obvious issue here also: Although this piece did not explode in the microwave, it got incredibly hot. Amazingly, through all of this, the glaze has not crazed. It is G3879.

Casting Zero3 Porcelain

Look at how translucent this is! I can fire one of these in three hours, cold-to-cold. I am casting them with molds made using the 3D-printing process. Anyone could do this. Incredibly exciting. And with the G3879 glaze it looks awesome, just like bone china. Notwithstanding this, the Zero3 recipe has to be altered for casting. Initially I have reduced the VeeGum to 1% but it is still casting too slow. And it is not shrinking enough to pull away from the mold well. I am considering strategies on how to increase drying shrinkage and am going to add more frit to take it down to cone 04.

Low fire glaze with the Al2O3 and SiO2 of a cone 6 glaze

Look at how fluid G3879 is at cone 06 even though it has the Al2O3 and SiO2 of a cone 6 (or even cone 10 glaze)! It have found that glazes with lots of boron can tolerate amazingly high levels of Al2O3 and SiO2 and still melt very well. And they create many options to lower thermal expansion that would not otherwise be available. The G3806N recipe has the amazing ability to tolerate large additions of kaolin. Each addition sacrifices some melt fluidity but the glaze stays glossy and gets more durable (because of the increased Al2O3 and SiO2). And the thermal expansion drops even more. A highly melt fluid, super gloss with low thermal expansion is super difficult at cone 6, but here it is. The secret is high boron. From frits.

Why do these cone 04 and 6 clear glazes have so similar a chemistry?

The glaze on the left (as shown in my account at is a crystal clear at cone 04. The high frit content minimizes micro-bubbles. The high B2O3 melts it very well (this has 0.66 B2O3, that is three times as high as a typical cone 6 glaze). The recipe on the right is the product of a project to develop a low-thermal-expansion fluid-melt transparent for cone 6 (with added colorants fluid melts produce brilliant and even metallic results and they variegate well). While the balance of fluxes (the red numbers in the formula) is pretty different, look how similar the B2O3, Al2O3 and SiO2 levels are (yellow, red and blue backgrounded numbers in the formula), these mainly determine the melting range. That means that a fluid-melt cone 6 glaze is actually just a low temperature glaze being overfired to cone 6.


Glossary Zero3
The designation for a group recipes for body, glaze and engobe (by Tony Hansen), that potters can use to make low fire stoneware and porcelain

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