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Make sure the fire is completely out before taking from it. Check the ashes and pick out any bits of plastic or garbage that might have fallen into the fire. The water should come halfway up the height of the bottle so that it boils quickly. You might need a larger pot if the bottle is tall or oddly shaped. Do this gradually. Leave the bottle in the water to simmer for 30 minutes. Once cooled, wash out the ashes under cold running water. Rinse it out two or three times to make sure there are no wet ashes left in the bottle. Leave it to air dry after rinsing out the soapy water.

Summary:
Fill the oily bottle with fine ashes from a spent fire. Place the bottle into a pan of cold water. Heat the water until it boils. Turn off the heat and leave the bottle to cool. Wash the de-oiled bottle in hot soapy water.