You say heavier. Is that because you think weight will add more pressure? The pressure comes from the springs (usually three) on the back of a contact frame or the suction of a vacuum frame.
Originally Posted by revaz_kv
If you contact print often or all the time there is wear and tear on the glass. Once or twice a year I replace the window grade glass in my contact print frame. Scratches, pin holes and other such damage are just a part of the work for me. It happens. These of course show on the print. I keep two spare glass pieces wrapped in paper in case I need a replacement suddenly, such as breaking it or scratching it. If glass is so easily damaged that is another good reason not to use plastic.