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A swatch of paint made from bituminous coal.

Maybe it isn’t sandstone… Bituminous Coal?

I’m often surprised at the colors I end up with when making paints. Sometimes the rock powder looks a lot different than the rock itself led me to think it would. But today I found a new puzzle. I usually use sandstones to get pigments, and this one black sandstone has always been a bit of a mystery. The first time I saw it, the thought that it isn’t sandstone did run through my mind. Other possibilities include bituminous coal.

I'm not sure what kind of rock this is, but maybe it's bitumen, a sort of poor grade coal.

At first glance it does look like sandstone. But get it cracked open and look with a magnifying glass … and there’s shiny black flakes and bits inside. Once I grind it up, it feels gritty enough like a sandstone, though. For lack of any other rock-type description that matched it, I decided to just call it ‘sandstone’ until I learned differently. Perhaps that day is today.

I used to use it to make a nice earthy brown color. But that was before I’d tried washing it. I had never washed and separated it before, just crushed and mulled. It is one of the hardest rocks I’ve ever crushed. However, after mulling it for a little while, the grittiness seemed to magically disappear, as though it all dissolved suddenly in the binder. It has been one of the greatest rock source mysteries I’ve found in my paint-making adventure.

Recently I crushed a good sized chunk of this rock with the glossy bits and decided to try washing the pigment to see if it changes the color. Well, it did. Not only that, the lites aren’t behaving the way sandstone lites do. Lites are the part of the rock powder that stays suspended for a while when I wash it. When they settle out, I pour off the water and use them to make a nice, smooth paint. These lites are tarry and doesn’t appear to be drying out even in the dehydrator.

So maybe this isn’t sandstone, after all. Maybe those glossy bits are bitumen – bituminous coal.

I didn’t get a brown color from the heavies, I got black. Which is nice, because sometimes the shale isn’t black enough, and bone is not always available. But what about the brown? I actually needed that color, and all of the other stone sources have a reddish tint to them that I don’t always want.

The tarry stuff might be useless… I’m not sure. (Update: the tarry stuff eventually dried and produced the nicest black paint I’ve made to date! Better than the bone, with a slight gloss. Bone black is very matte.)

A swatch of paint from washed pigment from what is most likely bituminous coal.
A swatch of paint from washed pigment from what is most likely bituminous coal.

2 thoughts on “Maybe it isn’t sandstone… Bituminous Coal?”

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  1. I am frequently amazed at what lovely colors you get from exploring rocks and other organic forms. Although I am not a painter, your site is a welcome change from many more common discussions.

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