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Cosmetic formulas are written as percentages by weight that must total exactly 100%, with water adjusted to bring the total to 100. Phases are weighed separately: the water phase and oil phase are heated and emulsified, and heat-sensitive actives, preservatives and fragrance are added in the cool-down phase below 40 C.
Phase quantities
Phase mass = batch size x phase percentage / 100; water is adjusted so all percentages total 100
Cosmetic formulations require safety assessment and preservative efficacy testing before sale. Ingredient concentration limits are set by cosmetic regulation.
Because percentages are the formula's definition. If they do not total 100, the actual concentration of every ingredient is different from what is written, which breaks reproducibility and regulatory documentation.
Many preservatives degrade above 40-50 C, and fragrance and volatile actives evaporate. Adding them hot reduces or destroys their function.