Jonathan has hit the nail on the head.
Most glass, unless it has been designed to screen out the wavelengths that
interest us, will adequately pass near UV and light suitable for our
purposes. Once one accepts that, freedom from imperfections and flatness
are the.important qualities.
The need for freedom from imperfection is obvious and this can be ensured
by buying one's own glass from the glass merchant.and inspecting it first,
( isn't the man who fits it the glazier?). I am told that one
manufacturer, who included 'optical' glass at an enhanced price in a
specification, just made sure that there were no bubbles in the glass.
As to contact and vacuum frames, one's prints are limited in size by the
size of the frame. As float glass is a very flat commodity, two pieces of
5 mm float glass, with the print and the negative as the contents of the
sandwich, will give even contact . One is then only limited by the size of
the glass.
As glass becomes brittle with aging, it will crack when put under stress
after a couple of years. Sometimes it will crack when not in use.
Terry King