Glass Thickness

So, did an unintentional glass thickness today while varnishing.

Plate #1, 2mm thick. While I was applying the painters tape, I VERY lightly bumped the corner of a 11x14 Ambrotype on th corner of the desk. This small mistake took out a 1cm wide section about 3 cm long on the bottom corner of the plate.

Plate #2, 4-5mm thick.While taping I dropped the entire plate off the desk onto the ground at my feet. There was no damage at all!

I prefer the substance of thicker glass as well, at least 3mm, 4 mm even more so. The problem is the cost, 2mm is dirt cheap, 3mm more expensive and 4mm super expensive!

I would prefer to do all my Ambrotype work at 4mm, but I do not believe that will be possible. Unless Ii can find free sources of old glass that is not tempered.