Von Gruff, once you try a glass platen you won't go back. I used hardened steel for a while then purchased a glass platen - no comparison.
What are the benefits of a glass platen?
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first they are quite a bit lower friction than a steel platen so they generate less heat and waste less torque for your motor.
Mostly they wear slower than steel and they are modestly easier to replace. If you are doing flat grinds you want a flat platen. All platens wear and get small grooves. Trying to get a nice even grind line on a platen with wear grooves is exceptionally frustrating. Run your fingers across your platen, if you can feel slight indentations, shallow spots or grooves, time to replace.
If you can't seem to get a nice even line on your flat grind, it's highly likely you have a worn platen. Make it a point to know where the low spots are on your platen and use other parts of it until it needs to be replaced.
If you try and do a flat grind with a platen that is worn trying to get an even grind line you will almost consider giving it up. I change mine the minute I notice ripples. I do expect a ripple where I profile a blade using the tool rest. I make it a point to never flat grind over that part of the platen.
Ceramic glass, fireplace glass, Pyroceram(tm) is basically the same thing. It isn't "regualr" window glass. It handles expansion way better than say typical glass or laminated safety glass. I've tried both. Those nearly always crack from localized heat when grinding.
The down side to glass is you can't slam your tool rest into it when setting up and it will generate static electricity when humidity is low. The solution to knock down the static, spray your belt with
Static Guard laundry anti-stat spray, grinder and a little on you (cause it smells better than you) and the annoying little sparks will go away. Some guys will attach an extra ground lead to their grinder. Some guys will run a ground (computer safety static guard wrist strap) to them selves which I have tried and hated.