Here’s a Podcast I did at NCECA! Check it out if you’ve got some time.
You might notice that this one has a significant addition of Cobalt, half and half Custer (Potash) and F4 (Soda) Feldspars, calcined talc, and 2% Manganese. Typical that I changed too many things to give a really useful side-by-side comparison. But I suppose when I’m coming up with new variations, that’s always been my style.
Some observations on this one:
Cobalt goes a long way and pretty dramatically alters an oilspot. With a .25%-.5% addition you get a nice shift from brown and russet glaze matrix to a darker solid black glass. Beyond 1% you can get some really nice silvery qualities to the spots. The drawback is that the more you add, the more refractory the glaze tends to get – and the longer it takes for the glazes to heal.
I was beyond excited to work with my newest found material, a rhyolite from Topaz Mountain, in Juab Country, Utah. This time rather than choosing a handful of very large rock samples (to insure relative material consistency), I instead went to a wash and filled up a 5 gallon bucket with very fine material the size of course sand. My reasoning this time was that consistency is completely relative, and as long as I get materials from the same spots, it doesn’t matter – and I can grab material that has already been 99% processed for me. In the end I think this worked out, because I was able to run 5 gallons of sand through our ball mill with 2x 1 gal. ball mill jars in 10 batches. But I’m getting a bit ahead of myself, because I think it’s important to test fire a material before you go through the trouble of ball milling. So my new first step in dealing with materials (after identification of course) is to take a small chunk, put it in a small dish, and fire to cone 10 in reduction. Since this is my primary temperature range, that’s it, if there are chances I’ll also put similar samples into cone 6 oxidation as well as an oilspot firing schedule, which is about cone 12 oxidation. Here was the result at cone 10, in reduction:
Looks a lot like a fired chunk of granite or feldspar. Onwards with the milling!
Someone asked me about my process for ball milling, and here it is: Fill a 1 gal ball mill jar 1/3 with mixed sized media (approx 50% 1/4″ balls, 25% 1/2″ balls, 25% 1″ balls) then fill the jar with 1/2 gallon of water, then fill the the rest of the container up with material until it’s about 2/3 full.) If I had more containers I wouldn’t exceed filling the jar 1/2 way, but my circumstances are what they are, and I haven’t needed to change anything yet, such as it is.
In reduction, this rhyolite material was surprisingly similar to my ice crackle glaze. I think with very little modification (a small addition of clay, bone ash, and maybe a bit of frit) I’m nearly positive this will look and feel like a Kuan, ice crackle glaze.
Once I had all of my material milled, I let it sit overnight and then drained off the excess water, leaving me with a glaze slurry with an SPG of 1.58 (That’s 79g of material in a 50cc syringe). That’s only important if you want to know how much material you have per given volume. Since I was going to blend this with a basalt material that was also in solution, I needed this info. After taking the SPG of my basalt material, which happened to be 1.54, I did a simple line blend. On both sides are the materials by themselves, in the middle a 50/50, and on the left and right middle 25/75.
Pretty interesting results, I think. The big surprise was how sweet the 25% Basalt and 75% Rhyolite mix came out.
Finally, because I was looking for an oilspot/tenmoku type glaze with this research, I should also detail my firing schedule. Here’s my current Blaauw gas kiln firing schedule:
|0||time_temp 00:00 5|
|1||time_temp 01:30 200|
|2||time_temp 07:00 1160|
|3||time_temp 01:30 1200|
|4||time_temp 01:00 1220|
|5||time_temp 02:00 1230|
|6||time_temp 01:15 1252|
|8||time_temp 00:08 1252|
|10||time_temp 00:30 1220|
|11||time_temp 01:30 1200|
|13||time_temp 02:00 1000|
|14||time_temp 02:00 800|
|15||time_temp 02:00 700|
|16||time_temp 02:00 500|
|17||time_temp 02:00 300|
|18||time_temp 02:00 50|
|19||time_temp 04:00 50|
Blaauw kilns have the capability of firing in extremely oxidized conditions – blowing in somewhere to the tune of double the amount of air needed for complete combustion. The default, and maximum air value is 200. An neutral flame is around 100, and a smoky reduction is something like a 70.
Basically, this program fires up to cone 6 in about 9 hours, and then goes slowly up to 1252C, reduces for 8 minutes, and then goes back to oxidation, drops to 1220 over the course of 30 minutes, then drops to 1200 over the course of an hour and a half. I’m still very much tweaking this schedule, which works very well for some glazes, and not so much for others.
For a very long time now I’ve wanted to utilize some volcanic rock as glaze. In much of my research here at Utah State I’ve been looking at iridescent phenomena, both in glazes and in the natural world. It was quite fortuitous, then, when geology grad Doug Jones asked me to accompany him on an excursion just over the border into Idaho to look for Xenoliths, which at this site are very deep mantle rocks that have been blasted quickly to the surface in younger volcanic flows.
While we were poking around looking for Xenoliths, I started picking up some rather remarkable chunks of iridescent vesicular basalt. Vesicular basalt is characterized by it’s frothy, bubbly matrix… if you don’t know what I’m talking about, think red lava rock. It’s one in the same. Here’s an example:
After picking up a good pile of this stuff, we went on to find about 40 Xenoliths, as well as some other interesting stuff.
Once I got back to the studio it was time to figure out if this stuff was even viable. My standard go to for this is to break off a small chunk, put it in a dish, and fire away.
After putting theses samples in a cone 10 reduction kiln and a cone 10 oxidation kiln, it became quite evident that I had something useful.
After deciding that this was a good road to go down, the hard work of crushing and processing this stuff began. I started by breaking the boulders down into gravel sized pieces. These then went into out ball mill. I could have shaved down the time it took to mill this stuff by using intermediate crushing equipment (an impact mill, or hammer mill) but I found it easier in the long run to load up our ball mill and run for about 24 hours, sieve out the useful material, add in more course material, and repeat. By the end of 4 days I’d run all the material through and was left with tumbled lava rock:
With my material milled down fine enough to pass easily through a 100mesh sieve, I then let it sit for a few days, pouring off the water each morning, until gradually the material became thicker and started to resemble a glaze. Because it was ball milled, the particles were quite small, and suspend really well. The next step was to take this glaze material and see what happens in the kiln. I was quite pleased:
Now that I knew I was dealing with a viable glaze, I couldn’t wait to get this stuff in the kiln and firing it in a weird, experimental reduction cool cycle. Last year I discovered some really incredible surfaces by cooling a kiln in reduction, and holding at certain temperatures. In this case, the geologists have told me that basalt solidifies at about 980C, so I’ve been crash cooling the kiln to around this temperature, holding in a reduced environment, and letting the metallic compounds crystallize in reduction. My speculation is that I can somewhat re-create the conditions in which iridescent phenomena occur. Lo and Behold:
This result is remarkably similar to effects you can achieve in Raku, or Lustre firing… but it’s a different phenomena, and relies on totally different elements; namely, Iron. Whereas raku usually derives rainbow iridescence from Copper and Cobalt, and lustres from Silver, and Bismuth, these colors are coming from Iron with trace amounts (less than .5% Manganese and Titanium). It’s very interesting on the ceramics side, and the geologists are quite interested too, because the phenomena is not wholly understood. One of the perks of being a graduate student with STEM funding is that I have access to fancy analytical equipment. This analysis has allowed me to build a material profile in Insight Glaze Software.
To that end, my future plans with this research will involve more experimentation with the firing process. In fact, I’m currently working on a piece that will exhibit between 10 and 20 wall hanging tiles that all feature the exact same clay and glaze with different firing schedules. At the same time, I’ll also continue to tweak this material by adding other oxides to end up with brand new flavors of glaze.
I got a lot of responses after my last post, where I made the claim that using zinc is hard on elements. I had some really interesting discussions with both John Britt and John Tilton on this subject, and I thought I’d share some of that info with you guys. Of particular interest was a tidbit from Britt where he mentioned that firing a zinc sample in oxidation leaves it unmelted (as the melting temp is 3500) while the zinc sample in reduction almost entirely goes away. Heres a fired sample from our glaze calc class, fired in cone 10 reduction. It was a thumbnail sized lump when it went in.
To make a long story short, the juries out on whether or not zinc directly reduces the life of kiln elements. What’s most likely is that it’s a combination of complex volatilizing compounds released during the firing coupled with high temperatures (especially if there are programmed holds). An often overlooked variable is water. It could very likely be that the extremely hard water at my current and previous studios is the culprit. If you’ve ever looked closely at old workhorse kilns, some of the deposits near the lids and spy ports are similar to calcite deposits. Its not hard to imagine Calcium and Flouride attacking the elements.
Another variable to consider is that the kilns that I’ve had access to for my entire career have been heavily used community kilns. At research universities there’s a huge range of clays and glazes that go through kilns. I’ve also heard that barium, cobalt, and copper are also hard on elements. Given all these extra variables, I suppose the only real way to quantify if and how much zinc affects elements would be to fire two brand new kilns side by side for a 100 or so firings and then compare. Let me know if you have two brand new kilns you need me to test. I’ll be happy to help you out!
On a related note, John Tilton had some great info on kiln elements. I hope he doesn’t mind me sharing the info – it seems very useful. From an email from Tilton:
The zinc thing is somewhat solved by using heavier elements. I have 11 gauge KA1s in my newest L&L, and after 104 firings they are still standing nicely. 12 gauge seems to give about 200 firings, and 13 gauge 80. Normal elements, the 16 gauge or so, give between 18 and 35 firings, not worth using. I also fire only one or two pots at a time so the total zinc load per firing is probably less than if you stacked tightly.
So it looks like 11 gauge might be the sweet spot, though they do require length to be resistant enough, and maybe that translates into coils of larger diameter.
Finally, I’ll leave you with the Material description from Digital Fire. I highly recommend consulting digital fire for material descriptions, and if you have the means, purchase access to the software. So worth it. From Digitalfire.com
Pure Source Of Zinc
Alternate Names: ZnO, Zincite
Oxide Analysis Formula ZnO 100.00% 1.000 Oxide Weight 81.40 Formula Weight 81.40 Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas).
Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.
DENS – Density (Specific Gravity) 5.6
Zinc oxide is a fluffy white to yellow white powder having a very fine physical particle size (99.9% should pass a 325 mesh screen). It is made using one of two processes that produce different densities. The French process vaporizes and oxidizes zinc metal, the American process smelts a coal/zinc sulfide mix and oxidizes the zinc fumes.
Ceramic grades are calcined, they have a larger particle size and much lower surface area (e.g. 3 square meters per gram vs. less than 1; however 99.9% still passes 325 mesh). Calcined grades are said to produce less glaze surface defect problems (although many ceramists have used the raw grades for many years without serious issues). You can calcine zinc on your own in a bisque kiln, fire it at around 815C. Calcined zinc tends to rehydrate from atmospheric water (and get lumpy in the process, calcining a mix of zinc and kaolin produces a more workable powder). Alot of zinc is used in crystalline glazes (typically 25%), because these have no clay content, they bring out the best and worst of both the calcined and raw materials. The raw zinc suspends glazes much better (the calcined settles out much more). The raw zinc takes more water, but since the glaze can thin out over time it is better to add less than needed at mixing time and mix thoroughly. The raw zinc screens better (although it can be a challenge to get either slurry through an 80 mesh screen).
Zinc oxide is soluble in strong alkalies and acids.
It can be an active flux in smaller amounts. It generally promotes crystalline effects and matteness/softness in greater amounts. If too much is used the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Other surface defects like pitting, pinholing, blistering and crawling can also occur (because its fine particle size contributes to glaze shrinkage during drying and it pulls the glaze together during fusion).
Zinc oxide is thermally stable on its own to high temperatures, however in glazes it readily dissolves and acts as a flux. Zinc oxide sublimes at 1800C but it reduces to Zn metal in reduction firing and then boils at around 900C (either causing glaze defects or volatilizing into the atmosphere; note that electric kilns with poor ventilation can have local reduction).
While it might seem that zinc would not be useful in reduction glazes, when zincless and zinc containing glazes are compared it is often clear that there is an effect (e.g. earlier melting). Thus some zinc has either remained or it has acted as a catalyst.
The use of zinc in standard glazes is limited by its price, its hostility to the development of certain colors and its tendency to make glazes more leachable in acids (although zinc itself is not considered a hazardous substance).
Zinc oxide is used in glass, frits, enamels and ferrites. Zinc oxide is also used in large quantities in the rubber and paint industries; in insulated wire, lubricants, and advanced ceramics. Credit: Tony Hansen
Thanks to Tony, and John Tilton. Also a special thanks to John Britt for the feedback on my last post; the clarification on Feldspar (I was mistaking K200 for Minspar 200 – ignorant to the fact that K200 has been out of production for 20 years!) I’ve since edited my post on Cone 6 with this info.
I get asked a lot about this recipe, and for good reason. It’s pretty indistinguishable from the best cone 10 recipes out there. For those purists out there, I’m referring to Pinnell Clear, Deller Chun, Cushing’s LungChun, and any number of Robert Tichane’s recipes from his book Celadon Blues. In any event, I often point people to an older post, but in the years since I mistakenly transcribed a recipe wrong and happened on the winning formula, I’ve learned quite a bit working with this glaze; Things like it working reasonably well in soda and atmospheric kilns, looking very nice from a range of cone 5 to cone 12, and readily taking most mason stains.
Fiske 6/10 Clear Base:
F4 (Or MinSpar) Feldspar 34.9
Zinc Oxide 11
OM4 Ball Clay 13.8
(Pictured: Add 1.75% Robin’s Egg Blue Mason Stain)
Notes on materials, mixing, and application:
Feldspar:Since F4 is no longer widely available, Minspar 200 will work. Custer works as well, but the bubble matrix that really gives this glaze it’s character is different with custer, g200 (now g200hp), or nepheline syenite. Experiment first, because milage may vary. Of the ingredients, this is probably the 3rd most important.
Fluxes: Whiting and Zinc. This glaze is not kind to kiln elements. (See my post on zinc for clarification!) It’s my opinion that the relatively high % of zinc is caustic to electric kiln elements. If you must, ventilate the kiln, but expect a short life on the elements. Sometime in the near future I’ll be eliminating zinc and trying to use a frit to solve this, but until then I can only recommend firing in a gas kiln. It’s the cost of firing.
Clay: Probably the most important element of this recipe. When I was testing for cone 6 glazes, I made a mistake transcribing to a batch recipe. The result was that I had doubled the clay. After the firing I went back over the notes and realized why the glaze looked the way it did. One of the side effects of the higher clay content is that application is sometimes difficult. The higher % of clay makes thick applications crawl. To get around this I calcine 10 of the 15%.
Thus, my recipe looks like this:
Fiske 6/10 Clear Base: Minspar200 Feldspar 38, Whiting 14, Zinc Oxide 12, Calcined OM4 Ball Clay 10, OM4 Ball Clay 5, Silica 30. [H20 60%]
Silica: I use 200mesh sil-co-sil. I’ve tried 325 mesh, but it didn’t look right.
Colors/Mason Stains: I use Robin’s Egg Blue, Bermuda Green, and Canary yellow. Most colors I’ve tested, and usually 1.5-3% is pretty nice, but some take as much as 5-10%. I haven’t had much luck with purples, Pinks, and oranges, (they don’t play nice with the zinc) but honestly I’m largely done tweaking this one and haven’t tried in earnest to figure out those other colors. Metallic oxides will also work, cobalt at like .3% for a not overpowered blue color.
Application: Can’t stress this enough. It’s gotta be thick. I tell people to glaze “Thicker than you think thick is, and then just a little thicker.” I’ve taken to adding just a touch of deflocculent to the glaze batch so that it needs less water to become liquid. I then add a bit of Epsom salt to thicken the batch up. Again, this is to taste. Dipping is absolutely the way to go with this one, but I’ve gotten accustomed to spraying it. Usually takes about 15 minutes to spray glaze something appropriately.
Firing: As I mentioned earlier, it’s got a pretty wide range. It will be fully melted, albeit slightly pin holed at cone 5. Ideally, I like to go to a perfect 6, but taking it to 7 or programming a hold in the schedule makes for some nice movement that suits carving and texture very well. Most of my work is completely smooth, so I prefer it to stay thick and not run down too much. It takes some getting used to, but when you do, it behaves very predictably. It can also go into reduction, but the colors change quite a bit. Less change with Bermuda Green, but quite a bit with the Robin Egg Blue. Its been fired every which way, and needs to be tested before full comittment.
Hello Again! It’s been quite some time since my last post. Gotta thank those of you who have contacted me with interest and suggestions! With so many summer projects and school stuff, it’s been very difficult to put my full efforts into any one thing… but life is what happens while you’re making plans. Anyways, enough with the excuses.
Over the summer I had the time and energy to figure out an acceptable firing schedule in our new Blaauw kilns. For as much as I love their sleek and sexy design, computer controllers, and top of the line hardware… you can’t look in the damn things while they’re firing. This poses several challenges for control freak oil spotters. Usually, the idea is to firein complete and total oxidation, going slowly through cone 7,8,and 9 to allow thermally reducing iron to bubble up through the glaze and cause the surface to crater or foam. By carefully monitoring the situation inside the kiln, and by pulling out glazed pull rings, the firer can increase the temperature slowly and fire until the glazes have significantly ‘healed over’. This isn’t really an option, so as a result a much more empirical approach was needed to find a good fit.
After 5 firings, I settled on a more or less acceptable firing schedule (the way this programming works is that the kiln starts at 0, take 1:30 to get to 200C, then 2:30 to get to 700C, etc). In Celcius;
time_temp 00:00 5
time_temp 01:30 200
time_temp 02:30 700
time_temp 03:00 1115
time_temp 02:00 1190
time_temp 02:30 1230
time_temp 02:30 1253
time_temp 02:00 1000
time_temp 02:00 500
time_temp 02:00 300
time_temp 02:00 50
time_temp 04:00 50
Once that was established, I began with some of my favorite tiles from my initial 2 rounds of oilspot base glaze recipes. My favorites:
NoCo OS: (NC)
K200 Feldspar 57.3
200m Silica 24.2
Spanish FeOx 10
Candace Black: (CB)
K200 Feldspar 60
200m Silica 20
Spanish FeOx 8
Cobalt Carb 5
Local Black Dolomite 10
Red Iron 8
Fake Mashiko: (FM)
Calcined Redart 35
Bone Ash .5
Red Iron 4
With these base glazes I began mixing, blending, and layering, and combining glazes with dipped, poured, and sprayed application. On a whim I decided to experiment with some of my manganese saturate glazes, and that’s when things started to get really interesting. There is admittedly one glaze in particular that I’m not sharing, but with a little diligence and some wet blending, a seriously motivated glaze experimenter can discover this glaze by looking at my old posting on my OSII series. Blend them all in 50/50 proportions and you’ll get the elusive but beautiful GF glaze. Hell, it might even be on my blog somewhere. That’s all I’m saying for now – I’d hate to rob anyone of the learning experience… Hah! =)
Recently I was contacted by the British potter Allen Richards who has done some pretty extensive research into lustrous gold glazes. He suggested that I try small additions of Vanadium Pentoxide. These glazes feature 2 amended manganese saturate glazes in combinations with the usual oilspot suspects.
Here are some videos of some of my latest results. None of these particular tiles have Vanadium pentoxide. As time goes by I’ll try to annotate the combinations MS corresponds to Manganese Saturate.
Back in the studio after a great trip over to Telluride, CO via Moab, UT. You better believe I scooped up some of this clay, ball-milled it, and made a slip. I also packed the jeep with a load of rocks and shit. More on that as the story develops!
#4 Recipe & Schedule
Fisker Bronze Custer Feldspar............. 57.000 Alberta Slip................ 7.000 Sil-co-sil.................. 2.500 F-4 Feldspar................ 1.500 Calcium Carbonate........... 0.500 Dolomite.................... 0.500 OM #4 Ball Clay............. 2.000 MnO......................... 23.000 Copper Carbonate............ 5.500 Iron Oxide Red.............. 0.500 ========= 100.000 Oxide Formula Analysis Molar% CaO 0.043* 1.129%w 1.376%m MgO 0.019* 0.348%w 0.590%m K2O 0.143* 6.237%w 4.528%m Na2O 0.071* 2.052%w 2.264%m P2O5 0.000* 0.007%w 0.003%m TiO2 0.001 0.055%w 0.047%m Al2O3 0.252 11.952%w 8.013%m SiO2 1.783 49.756%w 56.619%m CuO 0.099 3.670%w 3.156%m Fe2O3 0.013 0.955%w 0.408%m MnO 0.724* 23.839%w 22.995%m Cost: 0.273 Calculated LOI: 3.521 Imposed LOI: Si:Al: 7.066 SiB:Al: 7.066 Thermal Expansion: 6.848 Fired in Blaauw Reduction Schedule (in Celcius): time_temp 00:00 5 time_temp 00:54 140 time_temp 01:12 260 time_temp 01:10 550 time_temp 00:30 600 time_temp 01:12 900 oxidation 83 time_temp 00:45 900 oxidation 93 time_temp 03:06 1210 oxidation 98 time_temp 01:24 1270 cooling time_temp 02:15 1000 time_temp 01:00 900 time_temp 02:00 500 time_temp 01:00 300 time_temp 02:30 50 time_temp 04:00 50
#5 Recipe & Schedule
Nepheline Syenite........... 65.500 MnO......................... 22.000 Silica...................... 12.500 ========= 100.000 Oxide Formula Analysis Molar% CaO 0.018* 0.465%w 0.559%m MgO 0.004* 0.067%w 0.112%m K2O 0.071* 3.036%w 2.175%m Na2O 0.228* 6.461%w 7.033%m Al2O3 0.330 15.361%w 10.163%m SiO2 1.913 52.523%w 58.975%m Fe2O3 0.001 0.072%w 0.030%m MnO 0.680* 22.014%w 20.953%m Cost: 0.312 Calculated LOI: 0.065 Imposed LOI: Si:Al: 5.803 SiB:Al: 5.803 Thermal Expansion: 7.492 Formula Weight: 218.874 Strike Reduction Hold Firing Schedule in Small Test Gas Kiln in F 3:30 -> 1500F (^012) Body Reduction 1:00 -> 1700F (^04) Adjust to Moderate reduction, fast climb 3:30 -> 2300F (^9 flat, ^10 down) Crash Cool 0:15 -> 1840F Cut secondary air, minimize primary air, damp in, gas low to strong reduction and stalled holding temp 3:00 -> 1840F (Hold) Off, Natural Cool 6:00 -> 300F
#6 Recipe & Firing Schedule
Custer Feldspar............. 69.000 OM #4 Ball Clay............. 1.500 MnO......................... 27.500 Granular Manganese.......... 2.000 ========= 100.000 Oxide Formula Analysis Molar% CaO 0.007* 0.210%w 0.256%m MgO 0.000* 0.006%w 0.010%m K2O 0.140* 6.961%w 5.057%m Na2O 0.064* 2.091%w 2.308%m TiO2 0.000 0.018%w 0.016%m Al2O3 0.226 12.223%w 8.201%m SiO2 1.533 48.751%w 55.512%m Fe2O3 0.001 0.123%w 0.053%m MnO 0.789* 29.617%w 28.588%m Cost: 0.297 Calculated LOI: Imposed LOI: Si:Al: 6.769 SiB:Al: 6.769 Thermal Expansion: 7.120
Fired in Blaauw Reduction Schedule (in Celcius): time_temp 00:00 5 time_temp 00:54 140 time_temp 01:12 260 time_temp 01:10 550 time_temp 00:30 600 time_temp 01:12 900 oxidation 83 time_temp 00:45 900 oxidation 93 time_temp 03:06 1210 oxidation 98 time_temp 01:24 1270 cooling time_temp 02:15 1000 time_temp 01:00 900 time_temp 02:00 500 time_temp 01:00 300 time_temp 02:30 50 time_temp 04:00 50