Thirukkural- On Virtue- Penance-Kural-267

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The longer the gold is fired in the furnace, the purer and brighter it gets:Even so, sufferings willingly borne, chasten the spirit.

The gold melted in the furnace loses its blemishes and shines brighter. In the same manner, men who do penance get rid of their sins and become purer in spirit and shine resplendently.

The Bible has the following parallel:

'The firing pot for silver and the furnace for gold,
but the Lord trieth the hearts'.

(Proverbs 17,3)