Strange discoloration discovered after storing antique African pot

I am confused. This pot is decades old and looked fine when I purchased it. It might have had a small dark spot on one side... don't recall exactly. I stored it in my attic which while sometimes hot is pretty dry. Recently I took a look at the thing and the side facing the wall has turned mostly black! I am at a loss. My cat was up there and at first I thought maybe it was cat urine that he sprayed on it but that hypothesis seems unlikely as the distribution of the discoloration suggests otherwise and there is no odor. I tried vigorous rubbing with mild soap and a rag and it is impossible to get off. My friend suggests "oxidation" but how did that occur so rapidly? This color was not on this pot to anywhere near this extent a few months ago.

 

@2manybooks
@Taupou

maybe a mold that's spreading..?

 

I see the red pigment is what's turning black.

Red Oxide can be made to turn black, as in Bluing. I don't know if conditions of attic storage would do this with pottery.

Just thinking aloud.

 

Considering it seems to only be effecting the red pigmented areas, and on a side facing presumably an but outside wall I agree it was temp/humdity/environment change effecting the pigment itself.

Always a good reminder to never store objects you care about in spaces that aren't climate controlled.

 

If I had not been told the story about the attic, I just would have assumed it was a "fire cloud" - not unusual in low fired earthenware like this. It looks like a type of pottery made in Mali, which is coated with a slip containing red iron oxide before firing. When there is enough oxygen in the firing, the slip stays red. If there is insufficient oxygen in some areas, the slip will be "reduced" and turn gray/black.

I can only speculate that the attic became hot enough, and the air flow was restricted enough around that side of the pot ("smothered"), that the iron oxide changed color. But I admit this seems unlikely.

What does the inside of the pot look like? Another possibility is that if the pot was actually used, there may be residues left inside. Under uneven conditions, these may have migrated to the surface, causing a stain.

There is also the possibility that in the decades since you bought it, you might have forgotten what it actually looked like.

Here is a similar example in the collection of the Art Institute of Chicago, with a brief explanation of how such pottery is made and by whom -


https://www.artic.edu/artworks/185654/water-container

 

But mold should rub off at least a bit I imagine?

 

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That is interesting... Attic can get hot but I was not clear that I made the purchase only a year ago. The guy who sold it to me had in a storage facility but I examined it and it DID NOT have this stain. Somehow this happened in the year it was up in my attic. What would trigger Red Oxide to turn black and can it be reversed? Or at least stopped?

 

Yes, but I have literally dozens of other antique or even ancient pots up there and not a one ever had any problems until this one. There must have been something in the clay.. but then as it is an obviously old pot why would that occur in the year I had it. And only on the side facing the wall. A mystery.

 

This was not on the pot when I purchased it a year ago. So it cannot be from the firing.. although the example you posted of the same style from Mali has some extended black areas. The attic does get very hot on occasion but I have dozens of other pots up there in my collection - African, Peruvian, Native American and none ever had a problem. Inside looks fine. it is a thick clay so if anything was on the interior I have a hard time imagining it leeched to the exterior. But maybe? So weird...

 

Sorry, I misunderstood. I took your first post to mean that it had been in your attic for decades.

Substances can, indeed, migrate through porous pottery. It is one of the features that made such jars good for water storage. As small amounts of water moved to the surface and evaporated it helped keep the water in the jar cool, similar to a modern evaporative "swamp cooler".

Mold is not fluffy in all stages of growth, and can leave stubborn stains. I believe it would only grow if there was some type of organic residue present, as well as a localized source of moisture.

It is an interesting phenomenon. Explaining it might require a careful examination and analysis of the surface, as well as an assessment of the conditions in the specific place it was stored. Is there a university or museum near you that has a conservation department?

 

I understand it's frustrating. I live in a high humidity environment and it's a constant struggle even with climate control.

The particular environment of the wall of your attic just wasn't like how this piece was kept prior to your purchase. I've been in the ancient pot storage of one of our local museums. There's a warehouse full of other items, but the pots got a specially made climate controlled storage all for themselves.

I think if the item is moved to inside your home, the blackening shouldn't continue, but if it's a change in the pigment it might not be reversible. It would probably require analysis of what it's made of to know more.