These words come from forging metal into shape. Iron is heated until it is red hot, then placed on an anvil. The forge has only a short period of time to hammer it into shape. Once the iron cools, it is no longer malleable.
The Forge
Francisco de Goya [Public domain],
via Wikimedia Commons
More English Proverbs
More American Proverbs
16, POTD, Proverb of the Day,