Weighing, not measuring

The discipline that makes a formula reproducible, and the one that separates a product from a recipe.

Everything by weight

Including the water, which people measure by volume out of habit.

Densities differ, a liter of one surfactant is not a kilogram, and a formula mixing volume and weight is a formula that cannot be scaled or handed to anybody.

Tare the vessel, add each ingredient by weight, record the actual figure.

Percentages of the finished product

Every ingredient as a percentage of the total, summing to 100, with water as the balance.

Water as the balance rather than a fixed number is what makes the arithmetic work when something else moves.

And the concentration that matters for compliance is the concentration in the finished product, not in the raw material. A 50 percent hydroxide solution used at 5 percent gives 2.5 percent in the bottle, which is over the child-resistant packaging threshold.

Two scales

One with the capacity for the batch, reading to a gram.

One reading to 0.01 g for preservative, dye and anything under a percent.

Trying to do both on one instrument puts a large uncertainty on the ingredient that most needs precision, and an under-dosed preservative is a failure that reaches customers.

Check both against a known weight periodically.

Recording it

Write the actual weights, not the targets, and the lot number of every raw material.

Both matter here more than in most crafts: raw materials are sold as solutions at nominal concentrations, and a lot at the other end of its tolerance moves your finished concentration.

Which is a compliance issue as well as a quality one, because the thresholds are stated in percentages of the finished product.

References