I found the melting point of tin last night.
I knowingly pushed it last night, but I'm happy to report that there is no real damage to speak of--just altered cosmetics.
I had four lamb (loin) chops to sear, and instead of using an 8" iron skillet, which would have been a more appropriate size, I used a 10.5" copper saute. I put a pat of butter and a bit of olive oil over medium high heat and waited for the first wisps of smoke to rise, which indicates to me a good searing temp that is still below the melting point of tin. I laid the four chops in the pan (they probably only occupied about 25% of the bottom of the pan) and left the flame at about 80% full bast. Normally, I'd back it down to medium to maintain the searing heat, but I wanted to see how the pan would handle it. I gave the first side about 3 minutes and the second side the same. As I lifted the chops out of the pan with tongs, I noticed the that tin was shining (like new, non-darkened tin) and smeared where my plastic tongs scraped the bottom. The end result was that I have 8 shining smears on the bottom of the pan were my tongs touched. It looks a little like a thick hand-wiped tin coat, and the rest of the pan retained its smooth tin appearance. No copper is showing through, and other than the altered cosmetics, I can't see any further issue.
So, now I know--find the smoke point of my oil/butter mix and back off. At least there was no real damage in the process. And the chops were delicious--nice seared crust on the outside and beautiful red in the middle.
Nice! Fortunately there was no tin on the lamb.
I suppose it's a bit of hyperbole to say that I hit the "melting point" of tin. As I understand it, tin softens as it approaches the melting point. I am guessing that I was well into that softening but still shy of a true melting point.
That is probably not the real melting point of tin. Tin is already a soft metal. At high temperature, it will get softer and softer. Until it hits its melting point, then it becomes liquid. You likely only have soften the tin to the point where the tin is easily move around.
Think about people who work with glass and steel. You can easily start to bend glass and steel before their respective melting points.
As for oil and butter mixture, the oil and butter smoke point is fairly low compared to many cooking oils, so this give me another reason to believe you are way lower than the true melting point of tin.
Tell me... What do you know of the pan's retinning history?
The reason I ask is that I have a saute with which I had experiences highly similar to yours--basically the tin smearing around, leaving bright steaks. Because it was doing this, it was one of the first pans I personally retinned at home. After which the issue ended abruptly and hasn't recurred.
I have a theory that a slapdash retin (or one using impure tin) is more prone to this smearing. Perhaps it has to do with insufficient stripping of the original lining (or no stripping at all), residual matter left in the pan, or some fluxing issue. There is also a remote possibility that the alloy used by an unscrupulous retinner contained enough "other metals" to change the melting point or the flux.
I recommend using the pan some more. I wager you'll find that the smearing happens in specific places and not in others.
Are you saying it was a bad idea for me to melt a couple of ounces of lead into my tin in that old Griswold skillet (which I'm now selling on ebay) before smearing my new alloy all over the bottom of my saute pan! Maybe it was the motor oil I used to keep in the cast iron skillet that caused the problem. For those interested in my auction, don't worry, I dumped the lead out and smeared on a clean coat of motor oil to make it look nice and seasoned! [All joking and sarcasm, of course. Sorry, I've read way too many motor oil and lead in cast iron comments lately. I couldn't help myself.]
This one was retinned within the year by my coast's most reputable tinner, so I doubt that is the issue. I do usually do my searing in my other saute pan, which was retinned by Peter, and I've never had this issue, but I generally try to keep about 50% of the of the pan covered by food, and I usually back the flame off a bit once the food goes in the pan. I suppose I should try searing some more in this pan with the exact same process as I do in the one Peter retinned to see if there is any difference. Given the reputation of the tinner, I doubt the content of the tin is suspect, but I suppose anyone could make a stripping or fluxing mistake. I really pushed this pan pretty hard last night though, so I am guessing I caused it.
I'll post a followup if I notice anything unusual as I continue to use the two pans.