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Anthracite clay


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Is it actual anthracite in the clay, or are you talking about a commercial clay body that's called 'anthracite' because it's black in color, like the one made by RPM? Any commercial body that fires black is going to contain manganese. In the US, clay bodies have to pass safety inspections, and the black clay bodies are labeled safe to use, just like the regular clay bodies. As long as you follow good hygiene practices regarding clay dust, and fire in a vented kiln, the black bodies will be safe to use.

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Anthracite is a black clay from Germany that is also available from UK suppliers. It’s used frequently by a well known potter on Instagram who is uncommonly generous with all his information, and the clay is key to many of his glaze effects.

UK suppliers seem to not post the MSDS or SDS sheets on their websites, so I can’t tell for sure if it’s  manganese based, iron based or some combination thereof. There are a a few cues I’ve seen that make me think it’s iron based, but that doesn’t mean there’s no manganese as well. There don’t appear to be any warnings against using it for food purposes, but again, that doesn’t always mean anything. If you reach out to the supplier, they should be able to provide the safety information.

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I just asked the german supplier and they said that the clay body contains iron chromate and not manganese dioxide, which makes me wonder whether or not its even worse ... sigh Is iron chromate worse than MnO2 with respect to kiln fumes?

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Ok. I’m going to paste some of my response from our DM, because this is good info for everyone to have.

I think there might be some confusion from this supplier. I don’t know that they were talking about manganese having 2 different toxicity levels. I think it’s the iron source they’re talking about.  Here’s a really important spelling distinction. Did the supplier say Iron ChromATE or Iron ChromITE?

Iron chromATE is a divalent form of chrome, which has some significant toxicity concerns via inhalation, skin contact, ingestion, all of it. It’s not as bad as some of the higher valencies are, but it’s still not good. See the movie Erin Brockovitch for references for what hexavalent chrome does. Iron chromate comes from roasting iron chromITE, which is the raw ore, and is less toxic, and is even a micronutrient. Although I’d be very cautious around any kiln fumes, especially in the bisque. Given that there are no warnings against using this clay for food use, I suspect the hazards are all involved in the production stages, especially the ones involving heat. Be sure to use good studio hygiene. 

Disclaimer: I am not a medical expert, nor should you consider me one.

Here are my references.

Iron Chromate digital fire base page

Chromium compounds toxicity from Digitalfire. Note, the toxicity levels vary significantly by valence.

Iron ChromITE MSDS via Tucker’s Pottery Supply/Laguna  Note: dust is considered a nuisance hazard. However long term exposure under normal use is bad. Use proper hygiene and PPE.

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5 hours ago, thiamant said:

I just asked the german supplier and they said that the clay body contains iron chromate and not manganese dioxide, which makes me wonder whether or not its even worse ... sigh Is iron chromate worse than MnO2 with respect to kiln fumes?

Has the clay body passed any sort of safety certification?

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3 hours ago, thiamant said:

Ok, the german supplier just confirmed that it is Chromite Fe2+Cr2O4

Is this safer to use than Manganese dioxide?

if you don't eat it, it should be fine.  Manganese dioxide is quite safe to handle, you just do not want to inhale it, or the fumes from firing it.  The same goes for iron chromite.  Don't eat it or inhale the dust or vapor.

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Is that a passive (without powered fan) setup Thiamant?

Either way, systems that pull a small stream of kiln atmosphere reduce the fumes considerably (per my limited experience), however, there are still some fumes and a great deal of heat as well coming off the kiln. If you use wax resist, that might give you an idea, for it has a strong smell when it burns off. 

I'm using a powered kiln venting system and an overhead hood fitted with a 400 cfm fan, even so, a bit of fumes do escape...

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