Tuesday, 8 March 2016

Oilspot tests

Oilspot originally was produced in Fujian (called Jian ware)  and the glaze was basically made from local iron-rich soil mixed with ash. Hence traditionally the glaze was viscuos, thick and not unlike the clay pots it was used on. There is no lack of recipes today; Britt's book "The Complete Guide to High-fire Glazes" contains about 10 recipes, and my old blog about twice that amount. Today, most "reconstructed" oilspot recipes contain upwards of 6 ingredients (typically potash, silica, gaolin, calcium, dolomite or talc and red iron oxide). Many, like the widely known Candace Black, Montmollin and John's Oilspot contain relatively large amounts of cobalt (2-5%) to enhance the spotting. You would not really want to use such toxic glazes for food vessels, beautiful as they may be. Manganese is another hit among oilspot potters, it almost always produces glittery, iridiscent spots. But again, it is toxic material and the fumes are very dangerous and long exposure can produce Parkinson-like symptoms - not going to happen in my kiln any time soon.

So, kind of like a dancer who limits herself to only a few moves, I have been groping around for viable, sustainable and beautiful oilspot glazes that will work in my kiln. It is a tall order.

I have a couple of recipes that always work if applied correctly on porcelain. Examples:

 I do not need a special firing schedule to get spots with these two glazes, I just need to go to 1280 degrees Celcius at 100 degrees per hour after 900 degrees. So that's handy, but I "need" more. I want to see and make all kinds of different spots - preferably on the same bowl!

A couple of potters have unknowingly been very helpful in this search. There is Australian potter Steve Harrison of http://tonightmyfingerssmellofgarlic.com .  Another is American potter Matt Fiske, who has generously shared many interesting observations on his blog https://mattfiske.wordpress.com/ . One post that intrigued me was about him sourcing rhyolite and basalt from the wilderness in Utah. Sigh! I live in an urban environment and cannot easily source materials from around me. In fact, I would not know where to look or what to even look for! My supplier did not carry any volcanic ash when I asked, so I turned to Amazon, which as you know can source just about anything... Sure enough, they had 250 mesh pumice for use in dental cabinets. I got a small stash of that, and mixed up a glaze bucket according to Matt Fiske's research - 1 part basalt to 3 parts pumice. Assumed he meant by weight rather than volume. Gave up on sieving it. Looked in my notes and saw it is my 33rd attempt at a base glaze recipe, so took a deep breath and labelled it Oil33 (A/B/C/D). By the way, this glaze smells wonderful ;-) - just like my dentist's clinic.

The glaze did not adhere well to the porcelain test tile, so I mixed in 1 part gaolin, and added 6% RIO.

Oil33 fired to 1250 degrees

I was pleased to see I got spots at both 1250 and 1282 degrees Celcius. The small test cup I glazed also fired nicely - I think the  glaze thickness accounts for the difference in appearance. The cup was glazed very thickly compared to the test tile. A few pinholes.

Next, I tested some variations, none of which I will pursue further.

OIl33 + 3% titanium

OIl33 + 1% rutile
Now, as for the use of this glaze on food vessels, I do not really have any reservations. Normally, a stabile glaze should have the right proportions of potash, calcium, silica and alumina to guarantee that will not leach, stain or get scratched in use. I shall use the little test cup for my tea and time will tell how it wears in use.

Ahem. My children tell me I am never satisfied with anything. It is just that... I think... I just need to twist it a little... some sparkle is missing... it is lacking that "wow! factor.... So, I still am not finished with this Oil33 business. If I find that "wow" ingredient, I am not sure I will be posting it here. Got to leave you guessing.

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