STEP 1: Get an old sponge, the type with a green scourer
STEP 2: lightly wet the sponge
STEP 3: Dip damp sponge in the cold fire ash
STEP 4: Scrub glass until you can see through it
STEP 5: Dry glass with tissue. JOB DONE!
If the glass is thick with soot you may need to apply a little elbow grease but this will still do the job.
You can buy a sponge in Mercadona, I can't help you with the elbow grease