"Chris Bacon" wrote in message

Get some brick cleaner from Jewsons, which is dilute
hydrochloric acid (check the label). Bale out the
pan, top up with the cleaner, leave for a few hours,
flush repeatedly, repeat if you need to. Don't try &
chisel the scale off, or you may damage the glaze.

No don't !!!!! Strong Hydrochloric acid can damage the glaze! I know, I've
done it!

Look in Yellow Pages under Janitorial Supplies to find your nearest
commercial cleaning supplies place, Call in & buy a good toilet descaler.
Hydrochloric is good, but in a toilet cleaner it will be the right strength
and it will be buffered to make it less aggressive.

Might take repeated applications but it will work without damage.

Dave