Weighing, and why percentages

A cosmetic formula is a percentage composition, and every problem of scale, substitution and repeatability is easier if it is written that way from the first batch.

Percentages, not quantities

Write every component as a percentage by weight of the total, with the batch weight as a separate line. Actual quantities are then derived rather than remembered.

This survives a change in batch size, a change in packaging and a change in the person making it. A formula in grams is a formula that will be scaled wrongly by somebody eventually, usually by getting five components right and one wrong.

The water phase is conventionally the balance, adjusted so the total reaches 100.

Everything by weight

Including the water, and including things it feels natural to measure by volume. Densities differ enough that a milliliter of oil and a milliliter of water are not comparable quantities, and a formula that mixes units will not reproduce.

The resolution you need

The constraint is set by your smallest component. A preservative at half a percent in a one-kilogram batch is five grams, which any scale handles. The same preservative in a hundred-gram development batch is half a gram, and weighing that on a scale reading to a gram is guesswork on a safety-critical ingredient in the formula.

A scale reading to 0.01 g for small components, and a larger one for the water phase. Two scales, because one instrument rarely covers both ranges well.

Recording as you go

Write the actual weighed amount next to the target, at the time, not afterwards. The difference between the two is the only evidence you will have when a batch behaves unexpectedly.

And record the lot number of every raw material as you weigh it. That is the link that turns a customer's batch code into an answer about which supplier lot was involved, and it takes seconds at the bench and is impossible to reconstruct later.

References