Why are telescope mirrors made of glass? Optical telescope mirrors are typically made from endlessly polished glass surfaces that have been aluminised to provide a very smooth surface of reflective aluminium.

But why the need for glass at all? Why can't mirrors be made of pure metal, or of metal deposited onto some other surface?

Is there something special about the glass grinding process that means no other surface can be made as smooth and still have aluminium deposited on the surface?

Or is it to do with the thermal expansive properties of the glass used?
you are partially correct. Glass is used for several reasons. It is a very stable material and will hold its shape well for thousands of years. Glass can also be polished to a high degree of accuracy without having defects.  Another major reason is the expansion properties are very favorable as well. Glass doesn't corrode and is easier to mold into shape than metal mirrors. Glass is relatively cheap and readily available.

Some useful references are found here ,here and here.