Modification Date: 2017-08-03 14:00:51
|Ferro Frit 3195||25.0||26.3%|
|Ferro Frit 3134||33.0||34.7%|
|Ferro Frit 3249||10.0||10.5%|
|Ferro Frit 3110||7.0||7.4%|
This recipe converts the popular Worthington Clear Gerstley-Borate-based low fire clear recipe into something that 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), does not craze or shiver (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 of Ulexite to source the boron. The results are excellent, actually stunning! This one also has even fewer micro-dimples in the fired surface (visible in the light on very close inspection) and is crystal clear and super glassy.
We have found this works on Plainsman low fires like L212, 215, Terrastone plus Zero3 stoneware and Zero3 porcelain. It shivers on 50:50 Talc:Ball clay bodies (like Plainsman L213).
As noted, while Worthington melts to a good clear it has serious gelling problems (characteristic of Gerstley Borate). An issue with Worthington is shivering and fracturing of ware when glazed only on the inside (its thermal expansion was too low). This recipe raises thermal expansion (by adding Na2O), the result is a glaze that fits a much wider range of bodies.
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, drying and application properties. But, when dipping, there is a danger of getting it on too thick (meaning it could run downward during firing). It naturally gels at around 1.53 specific gravity (in our circumstances) and can be thinned with water down to 1.51-1.52 and appears like it would work well. But if the porcelain is bisque fired to 1600F, the absorbency will make this glaze go on too thick, even after a quick dip. There are two solutions: Bisque higher to reduce absorbency (however this approach will increase drying time). Or, add water to bring it down to about 1.46-1.47 specific gravity and add Epsom Salts to gel it back up (google "glaze thixotropy" to learn more). This produces a slurry that applies very well in an even layer to low or high porosity bisque (the gel makes it hang on without running or dripping).
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 many.
-Since the recipe is known, and contains no toxic materials, you can more confidently assure retailers that it is safe; all underglaze color decoration is isolated from any contact with food or drink by this glaze.
-It is compatible with most underglaze colors (including pinks).
Pay special attention to the drop-and-hold firing schedule for this glaze, especially if your clay body is not fine grained.
To mix up 5 Kg for glazing bisque ware:
G2931K glaze 5.0 Kg
Water: 4.5 Kg.
Mixed for approx. 1/2 hour
S.G. 100 ml. grad. cyl. 1.46
S.G. 1.0 L. grad. cyl. 1.46
S.G. Hydrometer 1.46
Viscosity: Ford Cup 10.8 seconds.
Yield: 6.5 litres.
To mix up paint-on consistency (1 pint jar):
-113g Laguna Gum Solution (100 grams of gum solution has 6.5g powdered CMC Gum)
-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 (include 2-3% zircopax so stains to not micro-bubble).
This is an all-fritted version of G2931F Zero3 transparent glaze. I formulated this glaze by calculating what mix of frits must be employed to supply the same chemistry of the G2931F recipe. The mug is made from the Zero3 porcelain body (fired at cone 03) with this glaze. This glaze fits both the porcelain and the Zero3 terra cotta stoneware. The clarity, gloss, fit and durability of this glaze are outstanding.
I melted these two 9 gram balls on tiles to compare their melting (the chemistry of these is identical, the recipes are different). The Ulexite in the G2931F (left) drives the LOI to more than 14%. That means the while the ulexite is decomposing during melting it is creating gases that are creating bubbles in the glass. Notice the size of the F is greater (because it is full of bubbles). While this seems like a serious problem, in practice the F fires crystal-clear on most ware.
These two glazes have the same chemistry but different recipes. The F gets its boron from Ulexite, and Ulexite has a high LOI (it generates gases during firing, notice that these gases have affected the downward flow during melting). The frit-based version on the right flows cleanly and contains almost no bubbles. At high and medium temperatures potters seldom have bubble issues with glazes. This is not because they do not occur, it is because the appearance of typical glaze types are not affected by bubbles (and infact are often enhanced by them). But at low temperatures potters usually want to achieve good clarity in transparents and brilliance in a colors, so they find themselves in the same territory as the ceramic industry. An important way to do this is by using more frits (and the right firing schedules).
This is Zero3 porcelain made using Dragonite Halloysite (instead of New Zealand Halloysite). It is the L2934C recipe. It was fired to cone 03 and glazed with G2931K clear glaze (which has fired crystal clear and flawless). I fired at 1200F/hr to 1950F, held it for 15 minutes, cooled at 999F/hr to 1850F and held it for 30 minutes, then dropped as fast as the kiln would do. It has some translucency and fires with a purplish hue (the NZ burns blue-white and is more translucent).
Want to make this incredible porcelain and glaze yourself? Read on. The mug on the left is a cone 10R (2350F/1290C) porcelain (#6 Tile kaolin and Nepheline Syenite) with G1947U clear glaze. The other is a fritted cone 03 (1950F or 1065C) porcelain (NZ Kaolin, Ferro Frit 3110) with G2931K clear glaze. We call the body/glaze/firing system "Zero3" (google it or use the links here). The Zero3 porcelain is blue-white instead of grey, the glaze is crystal clear, underglaze colors are so much more vibrant. The Zero3 mug was fired in 3 hours (cold-to-cold). It also withstands thermal shock better, it is as strong or stronger and much more translucent. How is this possible? The magic of the frit, it melts so much better than nepheline. The recipes and method are linked here. It is the most expensive body you will ever make. But from it you will create the highest quality ware you have ever made using the most plastic body you have ever thrown! Follow the instructions carefully.
On the left is G2931J, a zinc alkali fluxed and high Si:Al ratio glaze. Those look like micro-bubbles but they are much more likely to be micro-crystals. High-zinc and high-silica is the mechanism for crystalline glazes, so it appears that is what they are. G2931K on the right has much more boron, double the Al2O3, less SiO2 and is magnesia-alkali instead of zinc-alkali. It is the product of dozens of tests to find an ultra-clear having a glassy smooth surface. This particular chemistry, although having only a 6:1 SiO2:Al2O3 ratio is ultra-gloss. In addition is has low expansion, will fast fire and the boron is not high enough to compromise the hardness.
The mug was decorated with underglazes at the leather hard stage, then bisque fired to 1650F, then dip-glazed and fired using a cone 03 drop-and-hold firing schedule. This glaze is exceptionally brilliant and clear, it transmits the color of the underglazes better than any other glaze at any other temperature I have seen. This glaze was later upgraded to one having the same chemistry but a higher frit content and firing even better, G2931K.
It runs downward during firing and the thicker sections have micro-bubbles that clouds the otherwise crystal-clear glass.
On Zero3 porcelain. Between 10 and 15% stain with 2-3% zircon added (to prevent bubbling). The firing schedule approached cone 03 (1950F) at 100F/hr for the final 100 degrees, held for 5 minutes, dropped to 1800 and held for 30 minutes.
Out Bound Links
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside g...
A cone 04-02 clear glaze developed from Worthington Clear. The latter was based on Gerstley Borate, this one employs Ulexite as the melter an is easier to use.- 2015-10-19
In Bound Links
A cone 04-02 clear glaze developed from Worthington Clear. This employs Ulexite instead of Gerstley Borate and has a lower thermal expansion.- 2016-10-11
A base glaze is one having no opacifiers, variegat...
A fully transparent glaze is simply one that does ...
By Tony Hansen+