Glass is always in motion although very slow ove time glass from the top of a window runs down to the bottom, giving a preson the idear that it is brittle but it is only thiner at the top there for it breaks easaly. tempored glass my be brittle but you don't want to try and drill tempered glass any ways. You will have to check and see if it is tempered, I know a glaser that willnot cut old glass just because it is alot harder to cut. When you cut older glass it can break easer and break lines may run on you.

Bill