To make olive oil soap (traditional Castile soap), you must combine olive oil, caustic soda (sodium hydroxide), and water in a precise ratio to initiate saponification. A standard recipe for 500 ml of olive oil requires 64 grams of caustic soda and 150 ml of distilled water.
The process involves the following critical steps:
Safety First: Always wear gloves and safety glasses, and work in a well-ventilated area, as caustic soda is highly corrosive.
Mixing Lye: Slowly add the caustic soda to the water (never water to soda) in a heat-resistant container; the mixture will become extremely hot.
Temperature Control: Allow the lye solution to cool to approximately 40–45°C, then heat the olive oil to the same temperature to ensure a proper chemical reaction.
Saponification: Slowly pour the lye mixture into the oil while stirring until the mixture reaches a thick „trace” consistency (similar to pudding).
Curing: Pour the mixture into molds and let it rest for 24–48 hours, then cure the solid bars in a cool, dry place for 4 to 6 weeks to ensure the pH stabilizes and the soap is safe to use.
While some sources suggest a 1:1 volume ratio (3 liters water, 3 liters oil, 500g soda), the most precise method uses a lye calculator to determine exact weights based on the oil’s specific saponification value, often targeting 0% superfat for a hard bar.
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