Molten glass at red stage is generally crystal clear. you can find videos of glass blowing demenstrations on youtube and see for yourself.
On Wed, Jul 22, 2009 at 6:38 AM, Stephen A. Lawrence<sa...@pobox.com> wrote: > I ran across an explanation of a "blackbody" which I actually understood > a week or so back (totally unexpected, it was in the introductory > chapter to a QM book), and since then I've been fiddling around with > gedanken experiments involving black boxes with little holes in them and > the second law of thermodynamics. > > And it appears to me that, according to the second law of > thermodynamics, if glass is heated red-hot or orange-hot, and it's > actually seen to be glowing orange, it should also turn *opaque* to > visible light while it's at that temperature. (If its glow is weaker > than, say, steel at the same temp then it should be semitransparent > rather than totally opaque but none the less it shouldn't be > water-clear, as it is at room temperature.) > > I've seen lead-crystal (very clear) glass being worked at high > temperatures, at Corning many years ago, and as far as I can recall it > did indeed glow bright orange. > > Does anyone here happen to know if glass also turns opaque (or > semi-opaque) when it's heated to high temperature? (If it is I'll be > amused; if it's not I'll have to go figure out where my reasoning went > off the tracks.) > > I know for a fact candle flames are transparent, but I don't have the > facilities to heat a pane of glass until it produces a cheery glow while > shining a bright beam of light through it (don't even own a propane > torch at this point, and in any case hitting a windowpane with a propane > torch would probably shatter it). > >
