I have seen a number of blogs out there explaining how to make a copper pot sit flat if for some reason it no longer does – it’s cool to see all the ideas out there. There are usually several reasons for the bubbling on the bottom, and I see it most often in the really old pieces, pieces that have been brazed with cramp seams along the bottom, and pieces that have been overheated.
Now, it’s always a bit of a tricky business to get these flat, and of course I don’t recommend doing it unless you are comfortable with trying in the first place, as well as being safe (do any of this at your own risk!). Also, some of these methods will affect the tin linings by darkening, burning, or melting the tin, so I usually only do the flattening before I do the tinning. I also don’t recommend doing this to a stainless-lined copper pot as by the time the copper has bubbled on one of those, there’s already likely internal separation and that is not reparable.
Method 1: Get a block of wood that fits the interior diameter of the pot and place it in (you can use a cloth to protect the tin from the wood). Place the pot on a flat surface. Place another block of wood vertically on top of the interior block and start to hammer on it while pressing down.
Method 2: Place the pot upside down (bottom facing up) on a fireproof surface. Start a blowtorch and heat the base. Immediately begin to hammer in a circular pattern starting in the middle and spiraling out toward the edges, then back in. Repeat the heating and hammering until the pot sits true/flat. This will discolor the copper as well as potentially ruin the interior tin, so do it only if you are OK with re-tinning or if you are OK if the tin is discolored. Remember to use all safety equipment (safety glasses, welding gloves, etc).
Method 3: Place the pot upside down (bottom facing up) on a flat, hard surface. Hammer in a circular pattern starting in the middle and spiraling out toward the edges, then back in. This is a cold hammer vs the annealing of Method 2. I do not recommend this with a piece that has any seamwork as you can split the seams if they are brittle (you won’t know this until it’s too late and will require expensive repairs).
Anyway, there’s the basics. If you want to see some videos of me heating and pulling out dents (which is basically Method 2), you can see that on the YouTube HERE.