The bottom of a springform pan I bought recently was rusty after the first use--even though I handwashed it--and I don't want to make that mistake again. Would also appreciate brief primer (or link?) on what to look for/avoid in terms of types of metal--aluminum? nonstick? steel? In reading reviews on Amazon and elsewhere, it appears that dangerously sharp edges are common and rust is also a problem, as is uneven heating and/or burning.