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Purifying Rancid Castor Oil
To clean rancid castor oil mix 100 parts of the oil at 95° F. with a mixture of 1 part alcohol (96%) and 1 part sulphuric acid. Allow to settle for 24 hours and then carefully decant from the precipitate. Now wash with warm water, boiling for 1/2 hour; allow to settle for 24 hours in well closed vessels after which time the purified oil may be taken off.
(Taken from a book published in 1907.)