Possible Clues:

• It is not commonly something that causes pollution
• Sully, tarnish
• "'Tis" opposite
• 'Tis opposite
• Opposite of ''purify''
• For the illiterate, it's not a flaw
• Opposite of purify
• Corruption? They say not
• Seems a little tea is enough to spoil this in bad taste
• It isn't commonly a corrupt condition