Some tricks which may help avoid cracking your glass whilst trying to remove it from the brittle(r) old gasket and hard cement is:
After extracting any chrome strips (and A LITTLE BIT of silicone lubricant may help with that in the channels, as well, but try to keep it off your glass)
a. Use a sharp boxcutter (the disposable plastic retractable types with the snap-off blades work fine - nice and long and flexible) to run ALONG the glass on the outside, between the glass and gasket, cutting through to the slot/channel which held the chrome strips. [If you want, use a little bit of dishwashing detergent soapy solution to add lubrication and avoid any scratches - but keep arteries clear of and out of line of the blade if it slips...
] Also, do not use silicone as a lubricant, here -
b. Remove the cut-off outer strip so that you can see the old cement/sealant and clean that down until you can see the glass's outer edges - [soapy solution and a single-edge razorblade scraper may work best.]
c. GENTLY run the tip of the cutting blade approx 1/4" deep around the edges, perpendicular to the glass surface, to cut cement along that edge, as well, but try to not catch/chip the glass edge, which would make a "fault" for cracks to start...
d. Take the boxcutter, with blade extended a ways out (to take advantage of the flex), and run it along the entire INNER glass/gasket boundary to loosen that sealant line, as well.
The glass should have nearly fallen out, by now...
e. Use the scraper to clean all the old cement/sealant off the glass - but again, be careful not to scratch/chip edges - work blades "off" the glass, not "into" it...
f. Thoroughly wash off any silicone residue, as well as watery soap solution.
You want to keep it dry for safer handling until you're ready to apply soap solution, goop, or whatever lube you select for installation.
g. Finally, once the glass is out and the gasket remnants pulled out of the shell, use the old inner portion of the gasket to protect the edges of the glass until it's ready to reinstall (strips of masking tape will hold it in place nicely)
The string method still works fine.
Happy replacement, without any nasty (cracking) noises....
Stan