Making Cold-Process Soap from Lye and Oils — Saponification Chemistry

Every oil has a specific saponification value (SAP value) — the amount of lye needed to completely convert that oil to soap. Using a lye calculator, determine the exact amount of sodium hydroxide needed for your specific oil blend. For this recipe (350g olive oil, 150g coconut oil, 100g shea butter), the calculated NaOH is approximately 83g with a 5% superfat (meaning 5% of the oil remains unreacted, making the soap milder). Weigh all ingredients precisely on a digital scale. Even small errors in lye quantity produce soap that is either lye-heavy (caustic and unsafe) or excessively oily (soft and does not clean). Use distilled water only — minerals in tap water can cause DOS (dreaded orange spots) during curing.

Wearing safety goggles and chemical-resistant gloves, slowly add the weighed sodium hydroxide to the measured distilled water in a heat-resistant container (HDPE plastic or stainless steel — never aluminium, which reacts with lye). Always add lye to water, never water to lye, as the reverse can cause a violent exothermic boil-over. Stir gently until all crystals dissolve. The solution will heat to approximately 80-90 degrees C and release caustic fumes — work in a well-ventilated area or outdoors. Set aside to cool to approximately 38-45 degrees C. The lye solution is extremely corrosive at this stage and will cause chemical burns on skin contact.

Melt the solid oils (coconut oil and shea butter) gently and combine with the liquid olive oil in a large mixing container. Both the oil blend and lye solution should be at approximately 38-45 degrees C (within 5 degrees of each other). Pour the lye solution into the oils in a thin stream while blending with an immersion blender. Alternate between blending (5-10 seconds) and stirring (5-10 seconds) to avoid overheating the motor. The mixture will gradually thicken and turn opaque as saponification begins. Continue until you reach light trace — the consistency of thin custard where a drizzle from the blender leaves a faint line on the surface before sinking back. This indicates sufficient emulsification and the beginning of saponification.

At light trace, add any desired essential oils (approximately 2-3% of total oil weight — roughly 12-18g for this recipe) and stir thoroughly. Common soap-safe essential oils include lavender, tea tree, peppermint, and cedarwood. Pour the traced soap batter into a prepared mould (silicone loaf mould or a wooden box lined with parchment paper). Tap the mould firmly on the counter several times to release trapped air bubbles. If desired, swirl a contrasting colour through the top using a skewer. Cover the mould with cardboard and insulate with a towel to retain heat — this allows saponification to proceed uniformly through the entire batch (a process called gel phase).

After 24-48 hours, the soap is solid enough to unmould. If using a loaf mould, turn it out and cut into bars using a sharp knife or soap cutter. Each bar should be approximately 2.5-3cm thick. Place the cut bars on a rack in a well-ventilated area, spaced so air circulates around all sides. Cure for a minimum of 4-6 weeks. During curing, excess water evaporates (making the bars harder and longer-lasting) and saponification completes fully. Fresh soap still contains unreacted lye and is harsh — cured soap is mild and gentle. Test the pH of the finished soap: it should read 8-10 on a pH strip (mildly alkaline, which is normal and safe for skin). A pH above 11 indicates excess lye and the batch is unsafe for use.