Can two polarized glass filters held at near 90° orientation serve as safe eclipse glasses? Using two polarized filters to block all (?) light is a common classroom demonstration. With the imminent north American total eclipse, the media has been pushing eclipse glasses and emphasizing the inadequacy of sunglasses to protect the eyes. This made me wonder whether perpendicular polarized filters could serve as a sort of adjustable eclipse glasses. The eclipse glasses I got seem awfully dark when looking at the midday sun.
 A: Short answer: No - don't do it. When it comes to things like this you need the real deal - there is no point risking your eyesight to see the eclipse.
Long answer: Polaroid filters only work for a specific range of wavelengths this one for example works only for 400-700nm i.e. basically the visible range. It will not protect you from e.g. the invisible uv radiation which you cannot tell how well it is being blocked. As (1) states UV flooding the eye in can cause "solar retinopathy" (aka. Photic retinopathy) which can cause blindness or dark/yellow spots.
Even if you buy polarizers specially designed for the UV range as the images on this page* show even when crossed there is a non-zero transmission of UV radiation.
References
(1) http://scienceline.ucsb.edu/getkey.php?key=3269
*About 2/3rds of the way down. I will not reproduce them here for copyright reasons.
A: As far as I know, the main problem is not the visible light, but the invisible light. So your glass pol filters should probably sufficiently block the UV (depending on their thickness and which glass was used) -- that's why people don't get sunburns if they are driving a car (not a cabrio). However, they probably do not block the IR. But both are dangerous to your eye. So don't use your self-made stuff. 
