Melt and pour
No lye to handle: the soap is already made. What is left is arithmetic that goes wrong in small ways, so this works out how much base to melt for the mould you have, with the fragrance counted as a share of the finished soap, the way IFRA counts it.
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The formula, and where it comes from
finished = mould volume × base density (or bars × grams per bar) fragrance = finished × fragrance % # % of the finished soap, as IFRA counts it base = finished − fragrance − additions melt = base × (1 + extra %) rebatch: liquid = grated soap × liquid % added superfat = stirred-in oil ÷ (soap × oil share) × 100
- IFRA Standards express every limit as a percentage of the finished product; a supplier Certificate of Conformity states the maximum for category 9 (rinse-off soap) the same way.
- Glycerin melt-and-pour bases weigh a little more than water, about 1.03–1.10 g/mL depending on the base. The default of 1.06 g/mL is the middle of that. — Weigh a filled cavity of your own mould once and use that: it replaces this and the cavity volume in one go.
- Working convention for melt-and-pour: fragrance around 1–3 % of the soap, added oils and butters no more than about 1 tablespoon per pound (≈ 3 %), and dry additions such as clay or exfoliants a few percent. Past these a base sweats, softens, loses lather or does not set. — A habit of the craft rather than a measured threshold; the base maker’s own data sheet comes first.
- Working convention for rebatching: liquid of about 10–25 % of the grated soap’s weight, the less the fresher the soap. Too little and it scorches and will not come together; too much and it stays soft for weeks. — A habit of the craft rather than a measured threshold.
- Oil stirred into finished soap is not saponified: there is no free lye left to react with it, so every gram adds to the bar’s superfat, as a percentage of the oils in the original recipe.
Melt and pour
Melt the base gently, in short bursts in the microwave or in a double boiler, and keep it under about 70 °C: hotter and it scorches, turns yellow and loses the water that keeps it clear. Stir the fragrance and additions in as it cools, spritz the surface with alcohol to break the bubbles, and pour. Wrap the bars once they are hard: the glycerin in the base draws water from the air, and that is the sweat on an unwrapped bar.
Rebatch
Grate the soap, add the liquid, and heat it gently with a lid on until it turns to a thick, translucent mash. Take it off the heat, stir in the oil and the fragrance, and press it into the mould. It will not pour smooth like fresh soap. Give it a few days to firm up before you unmould it and a couple of weeks before you use it.
Common questions
Why is the fragrance a percentage of the finished soap?
Why do my melt-and-pour bars sweat?
Does rebatching change the superfat?
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