More residue-in-carboy thread!

Acquired a couple of old carboys, which hadn't been cleaned very well by the previous user. (okay, one looked as if it hadn't been rinsed, but; for the price; I wasn't going to argue)

A few days and some elbow grease later, two of them are fine but one has what almost looks like pitting at the krausen line. I'd post a photo, but it's hard to see. They look like bubbles in the glass, but I tend to doubt it. Still, crystal clear, tiny,,, something...

but the carboy brush makes noise going across it. This stuff, whatever it is, has resisted: bleach, at 1cup/5gallons, scrubbed vigorously (several times) and left sit in a dark corner for weeks; PBW at roughly 1.5 ounces/gallon, soaked overnight and scrubbed vigorously; and Starsan at nearly 2oz/5gallon, tepid water, scrubbed vigorously a few times across several days of sitting full.

I'm willing to go on the notion that; after all that; there's probably nothing in the glass that can cause trouble, but is there anything I missed regarding cleaning agents/efforts?

 

What about going after the whatever-it-is with a scotchbrite pad? I would worry about scratching the glass but a) I have little to lose and b) I could only scrub with as much pressure as I could exert with the carboy brush. At this point, I'm rather certain that it is residue of some sort. Residue of what, I cannot say, but the carboy was only used for brewing, so it's either krausen residue or some cleaning agent.

 

If it won't come off after that much soaking, I doubt it will come off in your wort. If you're really worried I'd try a little muriatic acid. That will bring the pH way down below what any wort will be. Anything that would have dissolved in your wort will dissolve in the acid.

 

+1
It could be calcium deposits, in which case an acid should dissolve it. You could even try vinegar.

 

CLR and hot water, let it soak for a couple days. If it's a calcium deposit that'll get it off.

 

Could it be the beginning of stress fractures in the glass?