Not exactly Oil Spots in the truest sense, these two variations were concocted and tested along with the rest of my oilspots glazes. Both of these recipes utilize some local dolomitic limestone I found and incorporated into the recipes. If you want to re-create these, using a mix of 1/2 and 1/2 Dolomite and Limestone will get you very close.
Here’s my Lava Oil spot firing schedule, which always looked best fired in a neutral/oxidation atmosphere in a gas kiln.
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.
Just spent a bit of time inputting this old recipe into Glazy.org. Pretty Sweet!
For those of you who are really interested in Oil Spots, there’s an article from 2014 that I think is worth a long look. This particular article was what got me interested in SEM microscopy when I was in Grad School:
Ancient Jian wares are famous for their lustrous black glaze that exhibits unique colored patterns. Some striking examples include the brownish colored “Hare’s Fur” (HF) strips and the silvery “Oil Spot” (OS) patterns. Herein, we investigated the glaze surface of HF and OS samples using a variety of characterization methods. Contrary to the commonly accepted theory, we identified the presence of ε-Fe2O3, a rare metastable polymorph of Fe2O3 with unique magnetic properties, in both HF and OS samples. We found that surface crystals of OS samples are up to several micrometers in size and exclusively made of ε-Fe2O3. Interestingly, these ε-Fe2O3 crystals on the OS sample surface are organized in a periodic two dimensional fashion. These results shed new lights on the actual mechanisms and kinetics of polymorphous transitions of Fe2O3. Deciphering technologies behind the fabrication of ancient Jian wares can thus potentially help researchers improve the ε-Fe2O3 synthesis.
Friend and fellow Oilspot fanatic Leonard Smith has put together some great videos on Chinese glazes. I’d highly recommend checking out his Youtube Channel and taking a look. This one shows a Chinese potter reduction cooling for iridescent oilspots!
My favorite way to systematically tweak a ceramic “Base Glaze”. John does a great job explaining it!
He can be followed on YouTube @ https://www.youtube.com/user/johnbrittpottery
And you can find more on his website @ https://johnbrittpottery.com/
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.