Peter Parry wrote in message
Only with care. You must add caustic soda to water (not water to
caustic soda) because when it dissolves it generates considerable
heat. If you simply pour the crystals in the pan the heat generated
locally around the pile of crystals can crack the ceramic pan. You
need to mix it a separate vessel or take great care to stir as you
are adding the crystals.
I think that's a bit alarmist. Caustic heats cold water to boiling for a few
seconds, but hardly a threat to a ceramic pan.
The main problem with pouring it straight into water is that the crystals
form a solid mass which then takes a long time to dissolve.
In any case it's highly unlikely to unblock a WC, which IME usually has
something solid stuck in the soil pipe. Last one I did was a combination of
a large cooking apple, a pair of St.Michael underpants, and a piano castor.
The mind boggles.