Connecting to Collections Care

Hi Nikki, 

Thanks for reaching out to the C2C Care Community! What an interesting question, I'm not sure what the black material is, so I'm going to reach out to our experts who hopefully have a better idea. I would think it is some kind of reaction between the sugar, corrosion of the metal cap, and the acidity of the soda, but I'm really not sure. 

I'll update with any follow-up from the experts, but other members should feel free to chime in!

All the best,

Liz

Hi again Nikki,

I've heard back from some of our experts and they agree with Toms's explanation for what's going on. Thanks, Tom, for sharing!

Best,

Liz

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Elizabeth Peirce
C2C Care Monitor 7/8-7/21
Conservator (Objects)
Library of Congress
Washington DC
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I presented a paper on a similar topic at the 2019 AIC Meeting in Uncasville (it appears the Postprints are still in progress).  What probably happened was that someone incorrectly assumed this was like a bottle of wine with a wooden cork, and leaning it on its side would help keep the cork wet and sealed, rather than drying out.  Instead however, the acid and the water in the soda caused the rubber or cork gasket in the cap to leak, which caused the metal cap to corrode, which eventually weakened the cap such that it allowed the CO2 to push out the contents, probably relatively slowly (thankfully not explosively!).  The brown color is probably a combination of iron corrosion (rust) and the deterioration/drying of a sugar solution, which often causes it to blacken.  Tom Braun

Thank you Thomas! That is very interesting but makes a lot of sense. 

Hi Nikki!

Totally agree with Tom's assessment. I did bring this incident forward during the AIHA's Museum and Cultural Heritage Working Group for their thoughts and they said that the phosphoric acid in the soda came into contact with the metal cap (likely the way Tom described tbh) which caused a chemical reaction. While that doesn't explain the "tar", the chemical reaction combined with the sugar in the soda is the most likely suspect. 

Wear gloves, but you should be safe to clean off the display and the bottle with conservator approved cleaning products. You'll probably want to find a replacement cap too.

Thank you for asking around Jessica. I appreciate the help!