It's probably 30 gal (we guessed 25 and undershot). It is plenty old, no idea exactly how old. It was one of my grandfather's old wine barrels. It's been empty for a while (10+ years), so we're rolling the dice here, but it's got sentimental value

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Swelled it with water and gave it a thorough rinse. Then hit it with citric/kmeta holding solution for several weeks. We pulled two barrels out of his basement and gave both the same treatment. This one smelled clean and held tight. The other one smelled pretty funky and kept pissing water, even with wax.
Dumped the solution and rinsed with plenty of hot water. Poured a couple handles of whiskey in for a few days before filling to keep it wet, then dumped that before filling.
I want to eventually turn it into a sour solera, but my buddy wanted to see what it would do with a clean beer. I guess we'll see if there are any resident bugs in there already (I'm giving it a better than 50% chance of that).