All that glitters

Is Manganese.

Ever since I’ve been after oil spot glazes I’ve been seeing images of spots that are out of the ordinary. Famously, this National Treasure bowl in Osaka

which is probably not a ‘true’ oil spot in the sense that the incredibly vivid spots were somehow seeded in or over the glaze and not generated naturally from within it.

But also this National Treasure:

Which is very probably a true oil spot.

Until recently I relegated such things to the realm of National Treasures. Things that I can safely ignore in the course of my daily potting life, even though I knew about a few people in China and Japan who have such glazes and fire them in lowly electric kilns. I think Kamada Koji does, at least, but I may be wrong.

Ithaca gave me a nice ride, trying to see where oil spots would take me. Driven by a comment in Daniel de Montmollin’s book about the essential similarity between iron red and oil spot glazes, I started exploring the largely uncharted seas between them.

I was also driven to some extent by interest in the above iridescent oil spots. A few metals iridesce naturally in glazes, chief among which is the lowly iron. Iron reds often have a sheen to them that is only apparent when you hold a pot in your hand and look at it in the light. So looking at iron was a natural direction and it yielded some results that I have yet to explore fully.

The other suspect is Manganese. Unlike iron though, manganese is highly toxic. Not in a fired glaze apparently – according to some sources,, the body doesn’t process non-organic manganese compounds in large amounts. But in inhalation of dust and chiefly, of firing fumes, it can create Parkinson-like symptoms. A clear deterrent for me. I had tried adding a few percent manganese to previous rounds of oil spots, but didn’t get much except for the same kind of behaviour that you can get from cobalt (blacker, bigger spots) so I abandoned that route.

Until now. Knowing that someone closer to home is making  iridescent oil spots has awakened my interest again:

I had come across his blog before, in relation to his metallic manganese saturated glazes. So I deduced that manganese was the way to go. I started by trying out a few of his less scary manganese luster glazes (judged by the amount of time I have to spend in the presence of such highly toxic substances), both in oxidation and reduction. They were lustery as advertised. Over and under my existing oil spot glazes, of which I have quite a few, they were lustery but not spotty.

The other thing I did was take two of my existing oil spot bases and try adding in iron along one axis and manganese along the other. A tedious process known as a biaxial blend. What I got was a bit surprising. The traditional oil spot territory of 8-10% iron did not iridesce in the presence of manganese. Instead, the lower amounts of iron did, increasing with the manganese. But nothing that I would call spots.

The plot took a surprising turn, when it turned out I know someone who knows the writer of the blog above. ‘Know’ in the facebook, virtual sense. Apparently after I chatted with him about the subject he chatted with Matt about the subject. And the picture became clearer. I was apparently right to look at manganese, but not daring enough with the copious amounts that were needed.

There is another problem. Oil spots are ‘stiff’ glazes. They retain the spots because they are glazes that don’t move much, having a lot of glass formers and stabilizers relative to glass melters. Not stiff enough, you won’t get spots. Too stiff, you will get just unhealed bubbles and craters in the glaze, looking like a mini volcanic disaster. But manganese is a glass melter. Especially in such huge amounts. So my current bases, tweaked to melt at my firing range (cone 9+, or 10-, saving my kiln from weekly element changes) won’t be stiff enough to hold the spots even if there were any. Looking at Matt’s firing schedule, his firings are long and slow and hence reach cone 12 even though the final temperature is that of cone 10 only.

Where does that leave me?

With a few more biaxial blends to do, in order to figure out if I can stiffen the base just the right amount to hold spots on one hand, and not bubble on the other. And still trying to zero in on the combination of metals in the glaze – how much iron, manganese, cobalt and copper, and maybe vanadium. A multi dimensional space. As my mathematical friends know, there is no efficient way to conduct a search on such a space. So I will resort to drudgery, looking at the various planes that (don’t even) bisect the space.

Or relinquish the search. Now that I know how it’s done in principle, I am much less inclined to find those amazing glazes. My initial interest in oil spots was that they are spectacular glazes that are ‘liner glaze’ worthy – that I can handle safely, using normal safety precautions of dealing with glazes, and that will fire to a glaze that I would not hesitate to use on the food and drink bearing surfaces of my largely functional pots. But I shudder at the prospect of handling such toxic materials as manganese dioxide, vanadium pentoxide, etc, on a day to day basis in my studio, and I’m unsure about the toxicity of the final fired product.


Yes, there is more news of a less tentative kind. I have produced a number of acceptable oxidation reds, have started exploring neodymium oxide as a potential source of purples and grays, and my celadons are going strong and multiplying both in fired pots and also in kinds of glaze. I have also stumbled upon a new oil spot glaze that I like, doing a fun little experiment. Here it is:


It’s essentially a two ingredient glaze: 75 grams Alberta slip and 25 of petalite, adding just a bit (4 grams) of iron. Oh, and a few drops of darvan, which does nothing for the fired glaze but helps turn the jelly that is the alberta slip into something more liquid and usable. I also think that a bit less iron could work as well. My guess is 3%. Try it out!


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