Modification Date: 2017-07-14 12:32:47
High fluid melt glaze for reactive effects and super gloss colors
|Ferro Frit 3110||31.1||31.1%|
|Ferro Frit 3134||6.6||6.6%|
This recipe was the product of a series of tests to determine the best direction for a brilliant fluid-melt transparent base glaze for copper blues and greens. Once I selected a specific recipe (Panama Blue), I removed the colorants and made adjustments to improve its slurry properties and lower the thermal expansion to stop crazing. This type of base glaze is needed because more stable transparents lose their gloss on brown bodies and when certain colorants are added. Fluid melt base glazes also produce much more interesting visual effects. But of course, they have a down sides: they can run off the ware onto the shelf if too thick! And they have an inherently higher thermal expansion so crazing is more of an issue (but it is not impossible to solve as you will see here).
This recipe is just the base, it does not have the copper and tin to make the green color. We recommend using Copper Oxide, between 1 and 1.5% (depending on the intensity of color desired).
While the previous version, B, did not craze in my tests, its calculated thermal expansion was high enough to be a cause for concern. This adjustment lowers the expansion further while keeping the same brilliant visual appearance. Two materials have also been eliminated from the recipe (their oxides supplied by the others). The chemistry of this one has reduced high-expansion KNaO and increased low-expansion MgO. This makes it melt a little less, but visually it is the same. The higher ZnO seems to help melt the extra SiO2 I also added. As a result the calculated thermal expansion has gone from 7.7 down to 7.3.
If this crazes on your clay body then consider trying a body that has a higher percentage of silica (25% would be good). It is likely possible to adjust the recipe of this glaze to hang on to the fluidity while having a lower thermal expansion, but I have not done that because it fits on our bodies (Plainsman P300, M370, Polar Ice) as is.
I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. In the end I will pick one or two, fix the issues and provide instructions.
On dark burning medium temperature stoneware bodies, clear glazes often do not look good. These bodies contain more raw clays that contain larger particles that generate gases on decomposition during firing. These often cloud up typical clear glazes with micro bubbles, marring their appearance. There are solutions. Although more fluid-melt clear glazes have risks (e.g. running, blistering) they do clear bubbles better. If applied thinly (so they do not run during firing) they can work very well in this circumstance. Of course they do darken the body color (this body, Plainsman M390, fires red without glaze). This outside glaze is G3806C.
This is the winner of a five-way cone 6 copper blue glaze comparison that started with my dissatisfaction with Panama blue. When I compared these glazes I did not just eyeball them on a tile. I compared the melt flow, thermal expansion and slurry performance of the bases (without the copper and tin). Ball-melt tests also compared bubble and color development for very thick sections. Then I tried more copper and did more flow tests. I also did leaching tests. Where needed I adjusted recipes to increase clay content (while maintaining chemistry) so the slurries would work better. Without my account at insight-live.com to keep all of this organized it would have been so much more difficult, actually, I probably would not even have bothered with the project. The recipe is G3806C.
The first glaze is a control, a standard non-fluid clear with copper. The other three are the short-listed ones in my project to find a good copper blue recipe starting recipe and fix its problems (which they all have). The flow testers at the back and the melt-down-balls in front of them contain 1% copper carbonate. The glazed samples in the front row have 2% copper carbonate. L3806B, an improvement on the Panama Blue recipe, has the best color and the best compromise of flow and bubble clearing ability.
This is not just a typical transparent cone 6 glaze with copper added. Knowing what is different about this clear base, its trade-offs and how it was developed are important. The porcelains are Plainsman P300 and M370. The liner glossy glaze is G2926B, it has a much lower melt fluidity than the outer glaze (as a functional transparent its main job is to fit the body and be hard and durable). But in order for that outer glaze to accommodate the copper and still be super glossy it must have a much higher melt fluidity. It was tricky to develop since that fluidity comes with high sodium and lower silica, that raises the thermal expansion and moves it toward crazing. See the G3806C recipe for more info.
These two glazes are both brilliant glass-like super-transparents. But on this high-iron stoneware only one is working. Why? G3806C (on the outside of the piece on the left) melts more, it is fluid and much more runny. This melt fluidity gives it the capacity to pass the micro-bubbles generated by the body during firing. G2926B (right) works great on porcelain but it cannot clear the clouds of micro-bubbles coming out of this body. Even the glassy smooth surface has been affected. The moral: You need two base transparents in which to put your colors, opacifiers and variegators. Reactive glazes need melt fluidity to develop those interesting surfaces. But they are more tricky to use and do not fire as durable.
Crystallization (also called devritrification). You can see the tiny crystals on the surface of this copper stained cone 6 glaze (G3806C). The preferred orientation of oxides in crystalline, especially when metal oxides are present. When kilns cool quickly there is simply no time for oxides in an average glaze to organize themselves and crystals do not grow. But if the glaze has a fluid melt and it cools slowly through the temperature at which the crystals like to form, they will.
These porcelain mugs were decorated with the same underglazes (applied at leather hard), then bisque fired, dipped in clear glaze and fired to cone 6. While the G2926B clear glaze (left) is a durable and a great super glossy transparent for general use, its melt fluidity is not enough to clear the micro-bubbles generated by the underglazes. G3806C (right) has a more fluid melt and is a much better choice to transmit the underglaze colors. But I still applied G2926B on the inside of the mug on the right, it has a lower thermal expansion and is less likely to craze.
The outer green glaze on these cone 6 porcelain mugs has a high melt fluidity. The liner glaze on the lower one, G2926B, is high gloss but not highly melt fluid. Notice that it forms a fairly crisp boundary with the outer glaze at the lip of the mug. The upper liner is G3806C, a fluid melt high gloss clear. The outer and inner glazes bleed together completely forming a very fuzzy boundary.
These melt-flow and ball-melt tests compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are: G3814 - Moderate zinc, no boron G2938 - High-soda+lithia+strontium G3808 - High boron+soda (Gerstley Borate based) G3808A - 3808 chemistry sourced from frits G3813 - Boron+zinc+lithia G3806B - Soda+zinc+strontium+boron (mixed oxide effect) This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, excessive running, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually chose G3806B and further modified it to reduce the thermal expansion.
The top base glaze has just enough melt fluidity to produce a brilliant transparent (without colorant additions). However it does not have enough fluidity to pass the bubbles and heal over from the decomposition of this added copper carbonate! Why is lower glaze passing the bubbles? How can it melt better yet have 65% less boron? How can it not be crazing when the COE calculates to 7.7 (vs. 6.4)? First, it has 40% less Al2O3 and SiO2 (which normally stiffen the melt). Second, it has higher flux content that is more diversified (it adds two new ones: SrO, ZnO). That zinc is a key to why it is melting so well and why it starts melting later (enabling unimpeded gas escape until then). It also benefits from the mixed-oxide-effect, the diversity itself improves the melt. And the crazing? The ZnO obviously pushes the COE down disproportionately to its percentage.
Wrong. It is the one on the right. While the copper looks so much better in that fluid one on the left, that melt mobility comes at a cost: blisters. As a clear glaze it is no glossier than the other one, but it runs into thicker zones at the bottom and they blister. This is because the high mobility coupled with the surface tension blows bubbles as gases of decomposition travel through (in a normal cooling kiln they break low enough that mobility is insufficient to heal them). The fired glass in the one on the left is also not as hard, it will be more leachable, it will also craze more easily and be more susceptible to boron-blue devritrification. But as a green? Yes it is better.
Fired at cone 6. A melt fluidity comparison (behind) shows the G3808A clear base is much more fluid. While G2926B is a very good crystal clear transparent by itself (and with some colorants), with 2% added copper oxide it is unable to heal all the surface defects (caused by the escaping gases as the copper decomposes). The G3808A, by itself, is too fluid (to the point it will run down off the ware onto the shelf during firing). But that fluidity is needed to develop the copper blue effect (actually, this one is a little more fluid that it needs to be). Because copper blue and green glazes need fluid bases, strategies are needed to avoid them running off the ware. That normally involves thinner application, use on more horizontal surfaces or away from the lower parts of verticals.
Typical zero-boron high temperature glazes will not soften in a 1500F decal firing. But low temperature glazes will (especially those high in boron). Even middle temperature ones can soften. G3806C, for example, is reactive and fluid, it certainly will. Even G2926B, which has high Al2O3 and SiO2, has enough boron to soften and sometimes create tiny pits. In serious cases they can bubble like the mug on the right. Why? Steam. It was in use and had been absorbing water in the months since it was first glaze fired at cone 03. The one on the left was not used, but it did have some time to absorb water from the air, it is showing tiny pits in the surface. Even if moisture is not present, low fire bodies especially may still have some gases of decomposition to affect the glaze. One more thing: Fire the decals at the recommended temperature, often cone 022.
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By Tony Hansen+