Each batch of cold process soap begins with twelve vegetable oils and a measure of lye precise to the gram. Six weeks later, a bar of soap is ready to use. This piece documents those six weeks.
The batch begins with the oils
A Lê Mai cold process batch uses twelve vegetable oils. Olive as the base for hardness and fine lather. Coconut for cleansing. Sweet almond for gentle conditioning. Tamanu, shea butter, jojoba, and several others. Each oil has its own chemistry.
Ratios are calculated to the gram, not estimated. Each oil family carries a saponification value (SAP value), the amount of lye needed to convert one gram of that oil into soap. A wrong number changes everything: harder, softer, more lather, or harsh on skin.
The oil blend is warmed to 38-40°C. Some oils such as shea need higher heat to melt, then come back down to 38°C before the lye is mixed in. While the oils sit warm, the lye is prepared separately.
The night of lye and saponification
NaOH dissolved in distilled water produces strong heat, up to around 80°C. It cools slowly back down to 38°C, the same temperature as the oils. This is the meeting point.
When both are at the same temperature, lye is poured slowly into the oil, stirred as it goes. Stirred to trace, the state when the mixture thickens enough to leave a trail when drizzled. Light trace for layered formulas; heavy trace for pour-once recipes.
Saponification begins. Triglycerides in the oil combine with hydroxide, forming soap and natural glycerin. The reaction needs no external heat. That is why it is called cold process.
Before pouring into the mold, additions are stirred in at heavy trace: essential oils for scent, dried botanical powders if any, exfoliants if needed. Quick, even mixing, without letting trace go too far.
Twenty-four hours in the wooden mold
The wooden mold is lined with parchment paper. Wood breathes; it holds the saponification at a steady temperature until the reaction completes. Silicon molds seal heat faster but allow no vapor out. Wood is slower and more balanced.
The mixture is poured into the mold, tapped lightly to release air bubbles. The top is gently textured with a spatula. The mold is wrapped in cloth for insulation, left overnight.
In the first hours, the reaction continues. Internal temperature rises to gel phase, around 60-70°C at the center of the mold. The soap shifts from cloudy to translucent, then sets again.
Twenty-four hours later, the soap is firm enough to unmold. A rectangular block, pale yellow to light brown depending on the formula. Cut with a wire knife into bars roughly 2.5cm thick. Each bar weighs about 110-130 grams when fresh. Soft. Still moist. Not yet usable.
Weeks one through four · cure
A freshly cut bar is too alkaline to use. The pH ranges from 9 to 10, high enough to irritate sensitive skin. Saponification is incomplete, water content is high, and the structure remains soft.
Bars are placed on wooden racks with air circulating freely between them. Room temperature, low humidity. No direct sunlight.
Each week, several things happen at once, none of them visible to the eye. Water evaporates and the bar grows lighter. Saponification completes and any remaining alkalinity neutralises. The soap molecules settle into a crystalline structure. Hardness rises. The essential oil scent stabilises and loses its initial sharpness.
By the end of the fourth week, the bar has lost 10-15% of its weight. The surface is smoother. The edges are sharper. The scent has settled. But it is not ready.
Week five · testing pH
Week five is when the ideal pH is reached. Measurement uses pH strips or a digital pen meter. Touch the surface of the bar with a wet sample, read the result.
A good cold process bar at week five reads pH 8 to 9. Above 9.5 means more cure time is needed. Below 8 is rare, found only in very high superfat formulas or specialised recipes.
A second test runs in parallel: the lather test. Rub the wet bar between both palms and observe the foam. Fine bubbles, no harshness on skin, no burning sensation. The bar is ready.
If both pH and lather pass, the bar moves into week six. If not, cure another week and test again.
Week six · the bar is ready
By week six, the bar reaches maximum hardness and stable pH. Its weight is now around 90-100 grams, 80-85% of what it weighed when cut. The structure is dense, not pliable, and the edges can chip slightly under impact.
Each bar is checked one more time. Edges. Surface. Scent. Bars that do not yet pass go back to cure. Bars that pass are wiped with dry cloth and wrapped in paper.
Six weeks have passed. Twelve oils have become soap and glycerin. The lye is fully neutralised. The scent has settled. The bar is ready to meet a user.
Six weeks is not a conventional number. It is the time the molecules need to settle. The time the water needs to leave. The time a bar of soap needs to become itself.
There is no shortcut.