Canning 101: When Black Scum Forms on the Outside of Jars

August 11, 2010(updated on October 3, 2018)

mucky jars

You slave over a batch of jam, lovingly chopping the fruit and simmering it into sweet, sticky submission. At the end of a long canning session, you leave the jars with the rings on to cool on the counter, content in the knowledge that you’ve made enough jam to last until the following summer.

However, the next morning when you go to take the rings off, you find that they’re a little tough to turn. And what’s this? Black scum has formed around the rims of your jars! Oh no! Does this mean all your hard work is lost?

Happily, the answer to that panicky question is that the jars are perfectly fine, as long as the seals are still good. That black scum you see on the outside of my jars in the picture above is a result of a bit of blueberry jam (on the left) and white peach sauce (on the right) that siphoned out the jars during processing. It was trapped by the ring and left to sit for a day (sometimes it takes me a day or two to get around to removing the rings and labeling). During that time, that bit of jam and sauce reacted with the metal of the ring and formed that scum.

The way you handle a situation like this is simple. You just clean it off. Typically a simple wipe down with a sponge or damp cloth will remove any traces of it. However, if it’s resistant to removal, fill your sink with tepid water and let the jars soak for a few minutes. It should rub right off after that. Once the jars are clean, carefully dry them completely and gently, using the edge of the towel to wipe off that tiny bit of lid overhang.

Finally, when the jars are clean and dry, double check the seals to ensure that they’re still good. Make a thorough visual inspection to ensure you got all of it. Remember also to be careful when cleaning full, sealed jars as too much jostling and rough treatment can potentially break those carefully constructed seals. Oh, and while you’re cleaning your jars, do make sure to also clean the rings. They’ll have some of that residue on them and cleaning will extend their lives.

(I promised a few of you a Canning 101 post on pickling salt and how to substitute other types of salt in canning. I couldn’t quite get that post together tonight, but look for it next week.)

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18 thoughts on "Canning 101: When Black Scum Forms on the Outside of Jars"

  • Thank you for setting my mind at ease. I had this happen but didn’t realize it until much later. And it was only on one jar. But I just about threw it out for fear the seal had been compromised. Everything indicated the seal was good other than the jam that was on the outer rim. My husband convinced me it was fine by telling me that if I wouldn’t eat it, I needed to stop canning all together because I’m always afraid of botulism. I decided to eat it and quit worrying because I don’t want to quit canning. And I lived to tell this short story.

  • Marisa, why would I want to take the rings off the next morning?

    If the seal is good when the jars are righted, wouldn’t it just be time to label and store?

  • Thank you for this. I get a bit nervous if the jars don’t look “just so,” so this post was a relief. I know that the “thwack” is the key, but… well, it’s good to have reassurance.

  • I have also learned to wash the rings after canning. I never thought of siphoning in the past and it is not alway apparent that it occurred. I noticed my rings were sticky and put two and two together.

    Natasha: if you have a can go bad and looses the seal it is more obvious when stored with the rings off.

  • Thanks for this black-scum post, I just had issues with it the other day! (And thanks for the timely FB response on it, too!)
    Natasha, another reason to remove the rings before storage is that if something has got on the rims and is corroding it, removing them right away and washing them can stop the corrosion. And, if you’re short on rings, you can use them for canning over and over and you won’t need to store them all!

  • Thank you for the reminder – I managed to avoid spillage on honey-peach jam, but the canned peaches that I made late last night were a big sticky mess today. They cleaned up easily with a damp sponge. Can I put in a request for a white peach sauce recipe – that sounds delicious!

  • Peach sauce? Is that like applesauce, just made with peaches? Do you sweeten/flavor it at all? It sounds delicious!!

  • I am glad to run into this post. I was trying to google it, but my terms never turned this up. I had some siphoning from a blackberry jam and from some tomatoes, and I was a bit concerned. I never remembered my folks’ stuff doing that.

    I have another question. What should I do when a jar turns over in the canner? It has only happened with jam and peach butter. I lowered the rack handles the other day and prevented it, but did some half pints of peach butter this evening and it happened again. I use my jar lifter to turn them back upright if I see it right when it happens, but this time I came up to turn the canner off and found one on its side. I turned it upright for the five minutes of resting time, and set it in a spot I would remember, just to check it more carefully tomorrow. It was actually the first to seal.

  • Dani, I’m planning on devoting a whole post to why you should store your jars without the rings, but the short answer is that it is best to leave them off. You know sooner if something has gone wrong with your jars.

    Vanessa, as long as the jar seals, it doesn’t matter if it turned over in the canner.

  • I’ve just found similar looking black stuff on a few jars of applesauce I canned last fall. However I just noticed it so it’s been sitting there for about 10 months, I didn’t realize I was supposed to remove the bands and wipe the rims after processing (I used water bath not pressure), is it still safe to eat?

    1. As long as it’s only on the outside of the jar (really, only on the outside) it is safe to eat. But if it’s not, you should toss it.

  • Hello & TY! I didn’t make jam though. This was the only post I could find addressing the black residue on my jars. I’m making lacto-fermented krauts. They’ve been fine for a week, but today there was very black residue leaking down the jar. It didn’t seem like mold though and seemed to just be on the seal lid and outer threads of the jar. It cleaned off easily and I put new lids on. But now I feel better about it not being mold. They have 11 more weeks to go! But in case someone wasn’t making jam, I thought I’d comment.

  • I have a ceramic jar (purchased used) and I am trying to make sauerkraut.
    Ten days have gone by and I am noticing a black substance seeping out of the sides of jar. It looks like tobacco juice.
    It is slightly sticky but doesn’t seem to have any odor.
    Is this a common problem? Has this happened to anybody here?

    1. Sauerkraut shouldn’t be releasing black liquid. It sounds like it is spoiling rather than fermenting.

  • I just canned my first and second batches of chicken, when I took the jars out of the canner they had a blackish film on them in spots, particularly on the lids. Should I be concerned? Everything else looks fine and all of them sealed but one.
    Thank you😊