LOL! Umm, the ons I've used are like 1/2" to 3/4" thick! I've never heard of using pennies! LOL! Wait a second! Now that I think of it... I really apologize for my statement! I can't believe I thought it was copper! I believe it is bronze actually... You did'nt burn yourself did you??
Bronze! Lol, and I believe the ones I've used are 1/2". They do work pretty good to absorb the heat and keep panel distortion, but that does'nt mean you can go all out on it! Lol and after a while the bronze will get hot!