Steps
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Image titled Make Lye Soap from Biodiesel By products Step 1
Place the glycerin in a stainless steel pot and heat it to 150 °F (66 °C) to remove any traces of methanol. Heat it to 175 °F (79 °C) if you made your biodiesel from ethanol.
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Strain the glycerin to remove any impurities and then return it to the pot.
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Measure 1 qt. of water for every gallon of glycerin and heat the water to 100 °F (38 °C). This works out to a 1-to-4 ratio of water to glycerin.
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Stir approximately 5.5 oz. of lye (38.5 grams per liter) into the heated water until it is completely dissolved.
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Add the water and lye mixture to the glycerin.
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Continue to heat for another 10 minutes, stirring occasionally.
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Remove the mixture from the heat and stir for another 10 minutes. You may see it start to foam.
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Pour the mixture into a shallow container. A plastic storage bin with a lid works well. The size and number of containers needed depends on the amount of glycerin you are using and the desired thickness of the soap. For 1 ½ gallons of glycerin, use a 28-qt. container to make 1 ½-inch-thick soap.
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Cover the liquid with a piece of plywood or cardboard to help hold the heat in so the soap cures properly. Let it cool and solidify for 24 hours.
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Run a knife around the edge of the soap to loosen it and then flip the container over to remove the soap.
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Cut the soap into 45 bars that measure 3-by-2 inches (5.1 cm).
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Wait at least 4 more days before using the soap. During this additional drying time, the soap will turn from a dark brown to a lighter tan color.
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Store the soap in zipper plastic bags with wax paper between the bars so they don’t stick together.
