Modified: 2020-07-24 10:55:55
An expansion-adjustable cone 04-02 transparent glaze made using three common Ferro frits (low and high expansion) and a suspension strategy that produces an easy-to-use slurry.
|Ferro Frit 3195||65.00|
|Ferro Frit 3110||20.00|
|Ferro Frit 3249||0.00|
|No. 5 Ball Clay||15.00|
This recipe contains a very high percentage of frit and thus has the potential to produce a super-transparent surface of high quality. It also has good application properties (if mixed properly, see below) and melts to a clear at cone 04-02. The high frit content also means it will fire to an equally good surface at all three cones. Although the glaze will melt also at cone 06 its bond with the body is poor at that low a temperature, it is much better to fire higher (cone 04 or 03). Additionally, this recipe is thermal-expansion-adjustable (using the method described below).
Frit 3195 is actually a complete glaze on its own (although it fires slightly silky rather than completely glossy). But it is middle-of-the-road for thermal expansion (an 85:15 frit:clay mix will shiver on some bodies and craze on others). We initially experienced shivering on our native clay talc bodies so incorporated another frit to make the recipe up-or-down expansion-adjustable. The advantage of Frit 3110 is that is also shines up the surface, producing a super-gloss.
To raise expansion (and fix shivering): Super-glossy Ferro Frit 3110. This recipe has Frit 3110:3195 ratio of 20:65 (while G1916R is 40:65). Of course, using too much Frit 3110 will induce crazing.
To reduce expansion (and fix crazing): Glossy Ferro Frit 3249. Although the amount is zero in this recipe, it is here as the expansion-down-adjustable option (use this instead of Frit 3110). For example, a 65:20 mix of 3195:3249 (G1916T) works on Plainsman Buffstone. Using too much will induce shivering.
This frit-juggling strategy affords a wide range of adjustment for tuning the fit to a body. Stress-test the fit by subjecting a piece of thin-walled ware to boiling-water-into-ice-water (and vice versa) immersion. This will reveal misfit that will happen with time (and if further adjustment is needed).
We have had some issues with clouding when it is applied thicker. Of course, the goal with a transparent glaze is to produce a crystal clear (without micro-bubble clouds). Low fire glazes are generally applied thinner than with stoneware, especially on white burning clays. To achieve this, try bisquing your ware higher (to get a less absorbent surface). Or try tuning the glaze viscosity and specific gravity to be able to apply it thinner and evenly. A very good way to do this is to gel the slurry a little (by flocculating it; see the thixotropy link below for more info).
If you do not mix this with the amount of water needed to make a creamy slurry it will apply too thick, too thin or unevenly.
This recipe employs ball clay, feel free to substitute a kaolin (we have used EPK). The 15% clay is plenty to suspend the slurry (added bentonite is not needed). The best slurries for pottery are gelled slightly so they hang on in a even layer without dripping. Achieving this is a matter of specific gravity and the addition of a small amount of vinegar or epsom salts (see the thixotropy link below).
G1916J (85% Ferro Frit 3195, 15% EPK) fired to cone 04 on Plainsman Buffstone and then 300F-Ice-water shocked (IWCT test). It tests better than 1916Q (which has some Frit 3110) but not as good at 1916T (which contains Frit 3249).
I have switched to a Redart based red burning low fire stoneware. You can see all my test data on an insight-live share here: http://goo.gl/DOd23J What are these? The white body matures to less than 2% porosity at cone 03. The red: 4%. The white can also be used as a slip on the red and the red as a slip on the white. The glaze (it is column three on the above share) works on both. They are both really plastic. This is really cool, I cannot wait to get down to the lab tomorrow to trim and slip these.
Two clear glazes fired in the same slow-cool kiln on the same body with the same thickness. Why is one suffering boron blue (1916Q) and the other is not? Chemistry and material sourcing. Boron blue crystals will grow when there is plenty of boron (and other power fluxes), alumina is low, adequate silica is available and cooling is slow enough to give them time to grow. In the glaze on the left B2O3 is higher, crystal-fighting Al2O3 and MgO levels are alot lower, KNaO fluxing is alot higher, it has more SiO2 and the cooling is slow. In addition, it is sourcing B2O3 from a frit making the boron even more available for crystal formation (the glaze on the right is G2931F, it sources its boron from Ulexite).
Talc is employed in low fire bodies to raise their thermal expansion (to put the squeeze on glazes to prevent crazing). These dilatometer curves make it very clear just how effective that strategy is! The talc body was fired at cone 04, the stoneware at cone 6. The former is porous and completely non-vitreous, the latter is semi vitreous. This demonstrates something else interesting: The impracticality of calculating the thermal expansion of clay bodies based on their oxide chemistry. Talc sources MgO and low fire bodies containing it would calculate to a low thermal expansion. But the opposite happens. Why? Because these bodies are composed of mineral particles loosely sintered together. A few melt somewhat, some change their mineral form, most remain unchanged. The body's COE is the additive sum of the proportionate populations of all the particles. Good luck calculating that!
I used a binder to form 10 gram GBMF test balls and fired them at cone 08 (1700F). Frits melt really well, they do not gas and they have chemistries we cannot get from raw materials (similar ones to these are sold by other manufacturers). These contain boron (B2O3), it is magic, a low expansion super-melter. Frit 3124 (glossy) and 3195 (silky matte) are balanced-chemistry bases (just add 10-15% kaolin for a cone 04 glaze, or more silica+kaolin to go higher). Consider Frit 3110 a man-made low-Al2O3 super feldspar. Its high-sodium makes it high thermal expansion. It works in bodies and is great to incorporate into glazes that shiver. The high-MgO Frit 3249 (for the abrasives industry) has a very-low expansion, it is great for fixing crazing glazes. Frit 3134 is similar to 3124 but without Al2O3. Use it where the glaze does not need more Al2O3 (e.g. it already has enough clay). It is no accident that these are used by potters in North America, they complement each other well. The Gerstley Borate is a natural source of boron (with issues frits do not have).
This cone 04 mug has survived the thermal shock of a 300F to Ice-water IWCT test, one which a similar recipe failed badly. This is G1916T glaze on Plainsman Buffstone fired to cone 04 (the failing recipe was G1916Q). The difference? This one switches the Frit 3110 for Frit 3249.
This is a cone 04 clay (Plainsman Buffstone) with a transparent glaze (G1916Q which is 65% Frit 3195, 20% Frit 3110, 15% EPK). On coming out of the kiln, the glaze looked fine, crystal clear, no crazing. However when heated to 300F and then immersed into ice water this happens (IWCT test)! At lower temperatures, where bodies are porous, water immediately penetrates the cracks and begins to waterlog the body below. Fixing the problem was easy: Substitute the low expansion Frit 3249 for the Frit 3110.
This is L3724E terra cotta stoneware. The inside slip is L3685S, a frit-fluxed engobe that is hard like the body and attaches well to it (engobes are often insufficiently fluxed). The glaze (G1916Q) is Frit 3195, Frit 3110 and 15% ball clay. The body has about 3% porosity, enough to make very strong pots. However that porosity is still enough to absorb water (and coffee). Although not too visible here, the pinhole in the inner surface has enabled absorption and there is a quarter-sized area of discoloration below the glaze. The piece could possibly be fired a cone higher, but testing would be required to see if the slip is still firing-shrinkage and thermal-expansion compatible with the body and that the body would not be over-fired. A better solution is adjust the firing curve to heal the glaze better. High temperature stoneware can easily have a 3% porosity also, so this is not just a low fire issue.
Left: G1916Q transparent fired at cone 03 over a black engobe (L3685T plus stain) and a kaolin-based low fire stoneware (L3685T). The micro-bubbles are proliferating when the glaze is too thick. Right: A commercial low fire transparent (two coats lower and 3 coats upper). A crystal clear glaze result is needed and it appears that the body is generating gases that cause this problem. Likely the kaolin is the guilty material, the recipe contains almost 50%. Kaolin has a 12% LOI. To cut this LOI it will be necessary to replace some or all of the kaolin with a low carbon ball clay. This will mean a loss in whiteness. Another solution would be diluting the kaolin with feldspar and adding more bentonite to make up for lost plasticity.
These three cups are glazed with G1916S at cone 03. The glaze is the most crystal clear achieved so far because it contains almost no gas producing materials (not even raw kaolin). It contains Ferro frits 3195 and 3110 plus 11 calcined kaolin and 3 VeeGum. Left is a low fire stoneware (L3685T), center is Plainsman L212 and right a vitreous terra cotta (L3724F). It is almost crystal clear, it has few bubbles compared to the kaolin-suspended version. These all survived a 300F/icewater IWCT test without crazing!
Two transparent glazes applied thickly and fired to cone 03 on a terra cotta body. Right: A commercial bottled clear, I had to paint it on in layers. Left: G1916S almost-zero-raw-clay glaze, a mix of Ferro frit 3195, 3110, calcined kaolin and a small amount of VeeGum T. The bubbles you see on the left are from the gas generated by the body. The ones on the right are from body and glaze. How can so many more bubbles be generated within a glaze? Raw kaolin. Kaolin loses 12% of its weight on firing, that turns to gas. Low temperature glazes melt early, while gassing may still be happening. So to get a crystal clear the raw clay content has to be as low as possible. Obviously, a white burning body made from refined materials would be even better. A good compromise: A red slip (or engobe) over a white burning body, it would generate far less gases because of being much thinner and still exhibit the nice red color.
These are two 10-gram balls (formed by dewatering the glaze on plaster) of low temperature glazes (G1916J, G1916Q) containing only frit and kaolin fired to 1250F. The carbon is part of the LOI of the kaolin (that hardens and suspends the glaze). Yet these glazes have much lower carbon content than ones made from raw materials.
G1916Q and J low fire ultra-clear glazes (contain Ferro Frit 3195, 3110 and EPK) fired across the range of 1650 to 2000F (these were 10 gram GBMF test balls that melted and flattened as they fired). Notice how they soften over a wide range, starting below cone 010 (1700F)! At the early stages 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, these specimens test a very thick layer, so the bubbles are expected. But they still can be an issue, even in a thin glaze layer 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 (in this case 1450F), then soak the kiln just below that (on the way up) to fire away as much of the carbon as possible. Of course, the glaze must have a low enough surface tension to release the bubbles, that is a separate issue.
The back flat side of balls of 1916J and Q low fire glaze that melted into a dome shape after being fired to 1550F. They have been turned over to see the back side (the front side is still stained by volatilizing carbon). Clearly they have reached zero porosity and are beginning to melt, yet they have not adhered to the vitreous porcelain tile! This demonstrates the degree to which an engobe must melt to secure itself well to the underlying body.
Hard to believe, but this carbon is on ten-gram balls of low fire glazes having 85% frit. Yes, this is an extreme test because glazes are applied in thin layers, but glazes sit atop bodies much higher in carbon bearing materials. And the carbon is sticking around at temperatures much higher than it is supposed to (not yet burned away at 1500F)! The lower row is G1916J, the upper is G1916Q. These balls were fired to determine the point at which the glazes densify enough that they will not pass gases being burned from the body below (around 1450F). Our firings of these glazes now soak at 1400F (on the way up). Not surpisingly, industrial manufacturers seek low carbon content materials.
L3685U - Cone 03 White Stoneware/Engobe
A white burning body with enough added frit to produce a cone 03 stoneware or white slip for use on a matching red stoneware.
L3724F - Cone 03 Terra Cotta Stoneware
An experimental Zero3 using Plainsman 3D clay
Thixotropy is a property of ceramic slurries. Thixotropic suspensions flow when you want them to and then gel after sitting for a few moments. This phenomenon is helpful in getting even, drip free coverage.
Shivering is a ceramic glaze defect that results in tiny flakes of glaze peeling off edges of ceramic ware. It happens because the thermal expansion of the body is too much higher than the glaze.
In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
The flocculation process enables technicians in ceramics to create an engobe or glaze slurry that gels and goes on to the ware in a thick yet even layer that does not drip.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
G1916M Cone 06-04 Base Glaze
This is a frit based boron base glaze that is easily adjustable in thermal expansion, a good base for color and a starting point to go on to more specialized glazes.
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