Castile is an old name. A bar of soap made only from olive oil, lye, and water. No other oils. This piece records the classic Castile formula that Lê Mai still follows.
Castile · a name over five hundred years old
The name comes from the Castile region of Spain in the twelfth century. The soap was made for the royal court, using local olive oil. By the sixteenth century, the formula had spread across Europe and become the standard for premium soap.
Over the next five centuries, hundreds of soap formulas were developed: coconut added for lather, butters for conditioning, fragrances for scent. Castile kept its single oil.
Today, Castile is still the benchmark for sensitive-skin soap. The simplest. The most durable.
Why 100% olive
Oleic acid makes up 70 to 75 percent of olive oil. It is a monounsaturated fatty acid. The oleic molecule is long, giving the saponified bar its hardness and producing a fine lather in water.
Compared to coconut, Castile does not dry the skin. Compared to sweet almond, it is not greasy. Compared to palm, it does not harden too quickly. One oil holds the balance between hardness, conditioning, and gentleness.
Infant skin, sensitive skin, and reactive skin tolerate Castile better than multi-oil formulas. The reason is fewer variables. The fewer fatty acid types in a formula, the lower the sensitisation risk.
Lye ratio and saponification
For one thousand grams of extra virgin olive oil, 127 to 130 grams of NaOH are needed. The saponification value of olive oil is 0.134. That is, one gram of olive needs 0.134 grams of NaOH to fully saponify.
Distilled water for the lye solution is 280 to 300 grams. The water-to-lye ratio is around 2.3 to 1. This ratio is enough for the lye to fully dissolve without being so dilute as to slow the reaction.
Superfat is set at 5 to 8 percent. That is, oil is deliberately used in excess so it does not fully saponify, leaving free oleic acid in the bar. This free oleic is the conditioning portion that remains after the soap is rinsed.
Cold mixing at 32 to 35°C
Olive at room temperature is already liquid, no melting needed. This is different from formulas with coconut or palm, which need heating to melt.
Lye after mixing with water produces heat up to 80°C. It cools naturally to 32 to 35°C before being added to the oil. Olive sits at this temperature too. Lye is poured slowly into the oil, stirred evenly with a stick blender.
Castile reaches trace more slowly than other formulas. It takes five to eight minutes of blending to hit light trace. Wait a few more minutes to reach medium trace. No need to go thicker.
Cutting after 48 hours
Pour into a wooden mold lined with parchment paper. Wrap lightly for insulation, not too warm. Castile saponifies more slowly than multi-oil formulas; gel phase is gentle and prolonged.
Leave the mold 48 hours instead of 24 as with standard recipes. After 48 hours, the bar is firm enough to unmold and cut with a wire knife. The bar is still moist but holds its shape.
Each bar is 2.5 to 3 centimetres thick, weighing 110 to 130 grams. The surface is pale yellow, not the chalky white of coconut-heavy bars.
Cure four to six months
Castile cures the longest of any cold process formula. In the first week, pH hovers around 10 to 10.5. Too alkaline to use.
By the second month, pH drops to 9. Hardness rises. The original olive scent softens into a subtle base.
By months three and four, pH stabilises at 8.5 to 9. The bar is firm enough but not yet at its peak. Usable, but not at full quality.
By months five and six, pH reads 8 to 8.5. Hardness is at its maximum. The lather is finest. The bar is ready for sensitive skin and for infants.
Castile is an exercise in patience. One oil. One bar. Four to six months.
There is no shortcut. Each month the oleic molecules settle a little denser, a little gentler.