Pictures




A closeup of a cone 10R rutile blue (it is highlighted in the video: A Broken Glaze Meets Insight-Live and a Magic Material). Beautiful glazes like this, especially rutile blues, often have serious issues (like blistering, crazing), but they can be fixed.

An example of how a carbonate can cause blistering. Carbonates produce gases during decomposition. This glaze (G2415B) contains 10% lithium carbonate, which likely pushes the initial melting temperature down toward the most active decomposition temperatures.

These are thick pieces, they need time for heat to penetrate. Both were soaked 15 minutes at cone 6 (2195F in our test kiln). But the one on the left was control-cooled to 2095F degrees and soaked 45 more minutes. Pinholes and dimples are gone, the clay is more mature and the glaze is glossier and melted better. Why is this better than just soaking longer at cone 6? As the temperature rises the mineral particles decompose and generate gases (e.g. CO2, SO4). These need to bubble through the glaze. But on the way down this activity is ceasing. Whatever is gassing and creating the pinholes will has stopped by 2095F. Also, these are boron-fluxed glazes, they stay fluid all the way down to 1900F (so you could drop even further before soaking).

Left: 4% rutile in the Alberta Slip:frit 80:20 base. This glaze has been reliable for years. But suddenly it began firing like the center mug! Three 5 gallon buckets of glaze (of differing ages) all changed at once. We tried every combination of thickness, firing schedule, clay body, ventilation, glazing method on dozens of separate pieces with no success to get the blue back. Even mixed a new batch, still no color. Finally the 'crow bar' method worked, 0.25% added cobalt oxide (right mug). It is identical ... amazing. It is not the same mechanism to get the color and it is not exactly the same, but worked while we figured out the real issue: the firing schedule (the secret turned out to be cooling, soaking, then slow cooling to 1400F).

Left: What GA6-C Alberta Slip rutile blue used to look like. Middle: When it started firing wrong, the color was almost completely lost. Right: The rutile effect is back with a vengeance! What was the problem? We were adjusting firing schedules over time to find ways to reduce pinholing in other glazes and bodies. Our focus was slowing the final stages of firing and soaking there. In those efforts the key firing phase that creates the effect was lost: it happens on the way down from cone 6. This glaze needs a drop-and-soak firing (e.g. cooling 270F from cone 6, soaking, then 150F/hr drop to 1400F).

An extreme example of blistering in a piece fired at cone 03. The glaze is Ferro Frits 3195 and 3110 with 15% ball clay applied to a bisque piece. Is LOI the issue? No, this glaze has a low LOI. Low bisque? No, it was bisqued at cone 04. Thick glaze layer? Yes, partly. Holding the firing longer at temperature? No, I could hold this all night and the glaze would just percolate the whole time. Slow cooling? Close, but not quite. The secret I found to fix this was to apply the glaze in a thinner layer and drop-and-hold the temperature for 30 minutes at 100F below cone 03. Doing that increased the viscosity of the glaze melt to the point that it could break the blisters (held by surface tension) while still being fluid enough to smooth out the surface.

Cone 6 mugs made from Plainsman M350 (left) and M390 dark burning cone 6 bodies. The outside glaze is Alberta-Slip-based GA6-C rutile blue and the inside is GA6-A base (20% frit 3134 and 80% Alberta Slip). That inside glaze is normally glossy, but crystallizes to a stunning silky matte when fired using the schedule needed for the rutile blue (cool 100F and soak, slow cool to 1400F).

This is an admirable first effort by a budding artist. They used a built-in cone 6 program on an electronic controller equipped electric kiln. But it is over fired. How do we know that? To the right are fired test bars of this clay, they go from cone 4 (top) to cone 8 (bottom). The data sheet of this clay says do not fire over cone 6. Why? Notice the cone 7 bar has turned to a solid grey and started blistering and the cone 8 one is blistering much more. That cone 8 bar is the same color as the figurine (although the colors do not match on the photo). The solution: Put a large cone 6 in the kiln and program the schedule manually so you can compensate the top temperature with what the cone tells you.

I enter (and tune) programs manually and document them in my account at insight-live.com. This controller can hold six, it calls them Users. Whatever program I last entered or edited is the one that runs when I press "Start". When I press the "Enter Program" button it asks which User: I key in "2" (my cone 6 test bar firing program). Then it asks how many segments: I press Enter to accept the 3 (I am editing the program). After that it asks questions about each step (rows 2, 3, 4): the Ramp "rA" (degrees F/hr), the Temperature to go to (°F) to and the Hold time in minutes (HLdx). In this program I am heating at 300F/hr to 240F and holding 60 minutes, then 400/hr to 2095 and holding zero minutes, then at 108/hr to 2195 and holding 10 minutes. The last step is to set a temperature where an alarm should start sounding (I set 9999 so it will never sound). When complete it reads "Idle". Then I press the "Start" button to begin. If I want to change it I press the "Stop" button. Those ten other buttons? Don't use them, automatic firing is not accurate. One more thing: If it is not responding to "Enter Program" press the Stop button first.

This is also a common problem at low fire on earthenware clay (but can also appear on a buff stonewares). Those white spots you see on the beetle also cover the entire glaze surface (although not visible). They are sites of gas escaping (from particles decomposing in the body). The spots likely percolate during soaking at top temperate. Some of them, notably on the almost vertical inner walls of this bowl, having not smoothed over during cool down. What can you do? Use the highest possible bisque temperature, even cone 02 (make the glaze thixotropic so it will hang on to the denser body, see the link below about this). Adjust the glaze chemistry to melt later after gassing has finished (more zinc, less boron). Apply a thinner glaze layer (more thixotropy and lower specific gravity will enable a more even coverage with less thickness). Instead of soaking at temperature, drop 100 degrees and soak there instead (gassing is much less and the increasing viscosity of the melt overcomes the surface tension). Use a body not having any large particles that decompose (and gas) on firing. Use cones to verify the temperature your electronic controller reports.

Medium temperature transparents do not shed micro bubbles as well, clouds of these can dull the underlying colors. Cone 6 transparents must be applied thicker. The stains used to make the underglazes may be incompatible with the chemistry of the clear glaze (less likely at low fire, reactions are less active and firings are much faster so there is less time for hostile chemistry to affect the color). However underglazes can be made to work well at higher temperatures with more fluid melt transparents and soak-and-rise or drop-and-soak firing schedules.

This glaze creates the opaque-with-clear effect shown (at cone 7R) because it has a highly fluid melt that thins it on contours. It is over fired. On purpose. That comes with consequences. Look at the recipe, it has no clay at all! Clay supplies Al2O3 to glaze melts, it stabilizes it against running off the ware (this glaze is sourcing some Al2O3 from the feldspar, but not enough). That is why 99% of studio glazes contain clay (both to suspend the slurry and stabilize the melt). Clay could likely be added to this to increase the Al2O3 enough so the blisters would be less likely (it would be at the cost of some aesthetics, but likely a compromise is possible). There is another solution: A drop-and-soak firing. See the link below to learn more. One more observation: Look how high the LOI is. Couple that with the high boron, which melts it early, and you have a fluid glaze melt resembling an Aero chocolate bar!

The boron and zinc fluxes make the melt of this glaze highly fluid at cone 7R. That comes with consequences. Notice the Al2O3 and SiO2 in the calculated chemistry. They are at cone 04 levels. The significant ZnO increases surface tension of the melt, this helps bubbles form at the surface (like soap in water). Al2O3 and SiO2 could be added (via more clay), this would stiffen the melt so the large bubbles would be less likely to form (this glaze melts so well that it could accept significantly more clay without loss in gloss). A drop-and-soak firing is another option, in this case a drop of more than 100C might be needed (see the link below to learn more).



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