That is caused by very temps when the drum bulges.

You can test it with a hydrameter.

I use it to check gasoline, alky and percentage of nitro!! Have run up to 12% in my alky injected engines, makes it cackle and stink. We had a lot of fun with the nitro and quick 8 racing.
I think alky weighes .592 or something like that and if it weighs more than either you have nitro or water mixed in.