Maybe your grandma used to use tinfoil to line her oven floor to aid cleanup. But times have changed; tinfoil is no longer made of tin. The stuff you use to shield your ham is actually aluminum foil, which has a lower heat tolerance.
A Case Study on Aluminum Foil in the Oven
Warners’ Stellian‘s expert service guy, Gene, passed on a cautionary tale after he ordered a new, $90 oven floor for a customer.
A well-meaning woman lined the bottom of her oven with aluminum foil, to catch all the food that bakes into the oven.
Instead of having to scrape it all off, she could just pull out the aluminum foil, and voila, no more mess.
Except she ended up with a bigger mess when the aluminum foil melted onto the oven. Turns out the heat tolerance between aluminum foil and tinfoil from the good old days makes a difference.
Aluminum foil-maker Reynolds warns against the practice of lining your oven.
From the Reynolds FAQ webpage:
To avoid possible heat damage to your oven, we do not recommend using aluminum foil to line the bottom of your oven. Rather, we recommend that you place a sheet of heavy duty aluminum foil on the oven rack beneath the pie or casserole you are baking. The foil should be only a few inches larger than the baking pan to allow for proper heat circulation. The foil will catch any drips before they reach the oven bottom.
There you go. Smart play on Reynolds part, right? Because you know people will end up getting rid of the sheet of foil and using a new one next time.