How is a solid shampoo bar made, and why does it need curing?
Short answer
A solid shampoo bar is a syndet bar: dry surfactants such as SCI blended with a conditioning emulsifier and butters, melted at about 65 °C, molded, then cured for two to four weeks so the surfactants crystallize and the bar hardens.
Explanation
The book's nourishing bar is built on Sodium Cocoyl Isethionate as the primary mild surfactant at 50%, Sodium Coco-Sulfate as a co-surfactant and lather booster at 20%, BTMS-50 as the conditioning emulsifier at 15%, cetearyl alcohol for hardness at 5%, shea butter for superfatting at 5%, jojoba oil at 3%, tapioca starch as an absorbent and slip agent at 2%, fragrance at 1.5% and tetrasodium EDTA as a chelator at 0.5%.
The method is dry-mix first: combine SCI, coco-sulfate, BTMS-50, cetearyl alcohol and starch, then add the shea butter and jojoba oil and heat gently on a water bath to about 65 °C until homogeneous. Remove from heat and, at about 45 °C, stir in the EDTA, preservative and fragrance. Pour quickly into molds and tap out air bubbles.
Curing is the step most people skip. Two to four weeks in a dry, cool place lets the surfactants and butters crystallize fully. The result is a harder bar that dissolves more slowly in the shower, lasts longer and develops a better lather over time.
If a bar stays sticky or soft, the usual causes are high humidity during making or curing, too short a cure, or an imbalance between hard and soft oils. Dry the environment, extend the cure, or raise the cetearyl alcohol slightly while lowering the jojoba oil.
What to do
- Mix the dry surfactants thoroughly before any heat goes in.
- Keep the melt at about 65 °C; overheating degrades the surfactants.
- Cure for two to four weeks in a dry, cool place before judging the bar.
Source: The Big Book of Homemade Hair Care — The Nourishing Solid Shampoo Bar