|Monthly Tech-Tip |
Modified: 2021-09-24 22:52:29
A super transparent low fire base clear glaze created by reverse engineering a commercial product.
|Fusion Frit F-524||850.00|
|Fusion Frit F-69||40.00|
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 its thermal expansion is too high to fit some bodies). The frit F-524 in this recipe is less common among potters but common in industry, it is high quality. We later adjusted this recipe to produce G3879C, it employs a whiter kaolin and more of the frit F-69 (to get a lower expansion to fit more bodies).
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 had it analyzed and reverse engineered it using the materials I have). I had it analyzed at a lab and then created a recipe to source the same 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. F-524 is an expensive frit, but you get what you pay for! The glaze is heads-above a number of commercial clear glazes I have tested (they are not ultra clear, they pinhole or blister or craze, they are not glossy enough, their slurries do not permit dipping, they are not adjustable). F-69 is also expensive, but it enables fitting this to any body.
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 (I glazed lots of pieces up to cone 2, it seems the higher the temperature the better this works!). 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.
I am continuing work on determining how to mix a good slurry of this recipe. It appears it may need to be gelled with VeeGum, that will also harden it and may remove the need for CMC..
This recipe also has implied adjustability (by increasing the low expansion Frit F-69 at the expense of Frit F-524).
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 insight-live.com 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 bodies are made in Montreal, Canada. The glaze was applied by dipping. The fit is still holding on both bodies after many months.
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.
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.
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.
The glaze on the left (as shown in my account at insight-live.com) 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.
Material prices are sky rocketing. Prepared glaze manufacturers have complex international supply chains. It is only a matter of time until you are affected. We are happy to sell you bottled glazes, if we can get them! But now might be the time to start learning how to weigh out the ingredients to make your own. Armed with good base glazes that fit your clay body (without crazing or shivering) you will be more resilient to supply issues. Add stains, opacifiers and variegators to the bases to make anything you want. That being said, ingredients in those recipes may become unavailable! That underscores a need to go to the next step and "understand" glaze ingredients. And even improve and adjust recipes. It is not rocket science, it is just work accompanied by organized record-keeping and good labelling.
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
Understanding your transparent glaze and learning how to adjust its melt fluidity, thermal expansion, color response, etc is a base on which to build all your other glazes.
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