White Film on the bottom of carboy

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Senior Member
Feb 21, 2009
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I found two 5gal. carboys. One basement and one in the garage (that one was in a box). I washed them both out with PBW and rinsed them well with hot water. The one that was used for bottled water (coolers) I saw had a white film on the bottom of the bottle. Does anyone know what that might be?

I'm hesitant to use it for anything now except maybe as tank to hold sanitizer.

Do you have hard water in the environment.
It might be precipitated chalk. Like in the tea kettle.

Fill the carboy with a layer of water and add a few teaspoons citric, tartaric or malic acid and look if the chalk will dissolve.

I'm with Luc, i bet its just some mineral deposits.
I was gonna suggest white vinegar (another acid) to see if that removes it.