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Sprinkle the powder liberally and don’t worry about using too much. Both baking soda or corn starch are absorbents that will “soak up” moisture, especially oil. They will not stain or damage carpeting.  One advantage of baking soda or corn starch is that they are very inexpensive. Another advantage is that they are non-toxic and made from organic materials. Neither baking soda nor corn starch will have a negative effect on the environment or on your body. Avoid rubbing too softly or too harshly. Use just enough force for the baking soda or corn starch to penetrate into the carpet’s fibers. Use a utility brush for larger oil stains, and an old toothbrush for small stains. This means leaving the powder alone for at least 15 minutes. Now that the baking soda or corn starch has absorbed the oil, use a vacuum cleaner to suck it out of your carpeting. Vacuum very thoroughly to remove every last bit of powder. Rub the detergent into the carpeting with a utility brush or an old toothbrush. Pour a small amount of warm water over the area and immediately blot up the detergent with a clean cloth or sponge.  Don’t be alarmed if the process creates soap suds, just keep blotting until all of the detergent is removed and the carpet is relatively dry. The more detergent and water you use, the longer the process will take. Inspect the fibres carefully for traces of oil. Repeat the process if the stain is still visible.
Cover the stained area with baking soda or corn starch. Rub the baking soda or corn starch into the carpeting. Allow the baking soda or corn starch to set, and then vacuum it up. Squirt a few drops of liquid dishwashing detergent onto the stained area. Allow the carpeting to dry completely.