it would be much easier to use a chemical to rid the glass of existing silicone rather than using a razorblade by hand.
go to an auto parts store and look for a paint stripper in an aerosol can.
specifically, one i have experience with is called aircraft coating remover.
trust me, it works. it eats through anything that is not metal (or glass.)
this stuff will turn epoxy resin into jello. it will liquify silicone.
spray it on your silicone and stick the tank somewhere where it can (fall apart) safely. like outside on the grass on top of plastic trash bags and newspaper. unless you want to kill your grass.
just spray it on, wait 10 minutes, the silicone will start bubbling up and start coming off. it will loosen up the whole thing and you will be able to pull it apart and wipe off any silicone left with paper towels.