The Snow Queen/Headscratchers

How are you supposed to make windows and glasses out of a mirror?

 * They're pieces of a Magic Mirror.
 * Perhaps a tiny shard rarely gets into furnace (directly, deliberately to recycle it or simply with sand), but when it does, the whole batch of glass ends up contaminated. Given that the whole problem is that this enchantment clings to all little pieces and retains full power, it looks like something that could be replicated in right conditions rather than fizzle out through dilution or simple heating.