Tin lined copper roasting pan
I started to roast a chicken in a tin-linned copper pan at 475 degrees before reading here that tin melts at 450. After about 15 minutes at that temperature, I turned it down to 375. There is some lemon juice in there, so it's not completely dry. But I'm worried that the tin leached into the food. Should I just throw it out and start over?
Don't toss that chicken. You can roast at an oven termperature of 475 as long as you have a good amount of food and/or liquid in the pan, although that might be pushing it. On the other hand, if you left an empty tin lined pan on a stove burner you will be sure to melt the lining. I can testify to that. And that is why you should not overheat a tin lined pan, not health concerns.
Lemon juice is irrelevant. Tin is about as harmless as iron and relatively insoluble in acids, It's used as a lining because copper is more acid soluble and somewhat toxic.