I've been using mirror tiles since I started. They are 3mm thick and I've never broken one. I've never been completely clear about the "borosilicate" glass and why it is any better than what I use. Heat-stable? I don't think that even matters at 100C. Maybe at 1000C. What is the difference in expansion in a 3mm mirror vs. a 3mm borosilicate? Even though borosilicate has like 1/3 the expansion coefficient, what are we talking about in real numbers? Will the differences between the two make any difference at all if you set your bed height after everything is heated up? The math is a bit beyond me now but somebody please give it a go and let's talk about real numbers. Borosilicate glass is expensive....maybe that's why vendors want us to buy it? I think it is all gimmick.