Avoid putting tin foil underneath the bottom element.

This reflects heat onto the element and can cause damage.

It also throws off the temperature in the oven. The surface inside your oven is carefully designed. It must reflect and absorb heat in certain ways and at certain rates. That’s why you may notice all ovens look similar inside. Some may be more blue, grey or black, but they are always a similar shade. It is the walls which cook the food.

The elements are just there to maintain the temperature (as ballast).