When the addition is too small to weigh
Every formula has a smallest line, and that line decides whether the batch can be made accurately at the size written. If the addition is under about 0.50 g, a 0.01 g balance is working at the edge of what it can defend: a target of 0.05 g is five digits of display and one or two digits of real confidence. There are only two honest answers. Make a larger batch, so the same percentage becomes a weighable mass, or make a premix, so a larger mass of a diluted stock delivers the small amount of active. Both answers are recorded; neither is a workaround hidden in the notebook.
A larger batch is the simpler answer when the product will be used anyway. At 0.10%, a 50 g batch asks for 0.05 g, which is too small, while a 500 g batch asks for 0.50 g, which is weighable. The cost is that five hundred grams of a trial product may be more than the salon can use before it expires, and a large batch of a formula that is still being tested is a large amount of waste if the formula fails. The batch size should be chosen from the smallest line and the realistic use of the product, not from the size of the largest beaker.
A premix is usually the better answer because it keeps the batch small. A small mass of the active is diluted in a weighed vehicle, and a larger mass of that mixture is weighed into the batch. The vehicle has to be compatible with the active and with the formula: water for a water-soluble preservative, an oil or a glycol for an oil-soluble material, and never a vehicle that reacts with the active or grows organisms on the shelf. A premix is a small batch in its own right, so it needs its own label, its own date and its own record, even when it is made in a 30 mL beaker.
Not every material should be premixed. A preservative that is supplied as a ready-made solution is already a premix, and diluting it again may take it below its effective dose. A fragrance is not water-soluble and will not make a useful aqueous premix. A material that is light-sensitive or volatile may lose activity during the extra handling. The formulation book decides whether a premix is suitable for a given ingredient; this book decides how to weigh it, how to record it and how to subtract the vehicle from the batch. If the formulation book cannot say whether a premix is suitable, the batch waits; a guess at the bench is not a substitute for that decision.
- 1. Identify the smallest line in the formula and its target mass.
- 2. Compare the target with the balance's reliable range, about 0.50 g and above.
- 3. If it is below the range, choose a larger batch or a premix.
- 4. Check with the formulation book that the material can be premixed.
- 5. Choose a compatible vehicle and record its grade and lot.
- 6. Weigh the active and the vehicle into a tared, labelled beaker.
- 7. Record the premix strength, the date, the maker and the vehicle mass.
- 8. Subtract the vehicle carried into the batch from the batch's own vehicle line.
Bench example
A 100 g trial toner needs a preservative at 0.10%, which is 0.10 g. The maker's balance reads 0.01 g, and 0.10 g is only ten display steps, so the decision is between a 1000 g batch and a premix. The salon does not need a kilogram of toner, so the maker builds a premix. A 10.00 g quantity of a 10% premix is made from 1.00 g of preservative and 9.00 g of distilled water, stirred until clear and labelled 'preservative premix 10%, 20261025, made by A'. To deliver 0.10 g of active, the maker weighs 1.00 g of the premix: 1.00 x 10 / 100 = 0.10 g. The batch's water line is reduced by the 0.90 g of water that arrived inside the 1.00 g of premix, so the batch still weighs 100.00 g and the preservative is still 0.10%. The record shows the premix strength, the grams used, the water credit and the delivered percentage, and the remaining premix is discarded at the end of the day rather than kept for next month. The premix is not kept for the next batch, and the record says so, because a water-based premix made on Monday is not the same material on Friday.
Caution: A premix does not create accuracy by itself. If the premix is weighed on the same balance at 0.20 g, the small-mass problem has only moved one step back.
Building a premix with the arithmetic shown
A premix is defined by its strength, and the strength is a mass fraction just like any other percentage. Ten grams of active made up to 100 g with vehicle is a 10% premix; one gram of active with nine grams of vehicle is also 10%, because the total is ten grams. Write the strength as a percentage and as grams of active per gram of premix, so there is no ambiguity. A 10% premix contains 0.10 g of active in every gram, which is the number used at the bench when the batch needs a small mass. Write both forms on the label, because the percentage and the grams per gram answer different questions at the bench.
The arithmetic has three steps. First, find the active mass the batch needs: batch grams multiplied by target percent, divided by 100. Second, divide that active mass by the premix strength as a decimal, which gives the premix mass to weigh. Third, subtract the vehicle carried in that premix mass from the batch's vehicle line, so the batch mass does not change. Check the result backwards: premix grams multiplied by premix strength gives the active mass, and active mass divided by batch mass, multiplied by 100, gives the delivered percentage. Do the three steps on paper before opening the premix jar, and initial each line, so a mis-typed strength is caught before anything is weighed.
Worked case one: a 200 g batch at 0.10% needs 200 x 0.10 / 100 = 0.20 g of active. With a 10% premix, the premix mass is 0.20 / 0.10 = 2.00 g. That 2.00 g contains 1.80 g of water, so the water line drops by 1.80 g. Worked case two: a 200 g batch needs 0.10 g of active, which is 0.05%, and the same 10% premix gives 0.10 / 0.10 = 1.00 g of premix, carrying 0.90 g of water. Both cases are correct; the difference is the target percentage, and writing the percentage beside the grams is what keeps them apart.
Round the premix mass to what the balance reads, and record the unrounded figure beside it. If the premix mass itself is below about 0.50 g, the premix is too weak for the job and a stronger one is needed, or the batch is too small. A 1% premix would need 20.00 g to deliver 0.20 g, which is easy to weigh but adds a lot of vehicle; a 10% premix needs 2.00 g, which is comfortable. Choose a strength that puts the weighed mass in the middle of the balance's range, not at the bottom of it. If the weighed premix mass lands below about 0.50 g, stop and choose a stronger premix rather than trusting a small number on the balance.
- 1. Decide the premix strength and write it as a percentage.
- 2. Calculate the active mass the batch needs.
- 3. Divide the active mass by the premix strength as a decimal.
- 4. Round the premix mass to 0.01 g and note the unrounded value.
- 5. Weigh the premix into a tared beaker and record the actual mass.
- 6. Calculate the vehicle carried in and reduce the batch vehicle line.
- 7. Recalculate the delivered active percentage from the actual mass.
- 8. Label the premix with name, strength, date, maker and vehicle.
Bench example
A maker needs 0.20 g of a preservative in a 200 g batch, which is 0.10%. A 10% premix is prepared from 5.00 g of preservative and 45.00 g of distilled water, giving 50.00 g of premix. The premix mass for the batch is 0.20 / 0.10 = 2.00 g, and the maker weighs 2.00 g. The water carried in is 2.00 x 0.90 = 1.80 g, so the batch water line is reduced from 180.00 g to 178.20 g. The check is 2.00 x 10 / 100 = 0.20 g of active, and 0.20 / 200 x 100 = 0.10%, which matches the target. A second batch of 500 g at 0.05% needs 0.25 g of active; from the same premix that is 2.50 g, carrying 2.25 g of water. The maker writes both calculations on their own cards, with the premix lot and the date, and discards the unused premix at the end of the day because it is a water-based mixture with no preservative of its own. The maker checks the water credit twice, because a missing 1.80 g is invisible in the jar but changes every percentage in the batch.
Caution: Do not count the premix water twice. If the water line is not reduced, the batch is heavier than the formula and every percentage falls.
Recording and storing a premix
A premix is a batch and gets a batch record. The record carries the premix name, the strength, the active mass, the vehicle mass, the total mass, the date, the maker, the vehicle lot and the active lot. It also carries the calculation that produced the strength, because the next maker needs to know whether 10% means ten grams of active in a hundred grams of premix or ten grams of a commercial solution in a hundred grams of vehicle. Those are different mixtures, and a label that says only '10%' does not distinguish them. Write the vehicle name as well, because a 10% premix in water and a 10% premix in glycerin behave differently in the same formula.
Store a premix in a clean, labelled, closed container, and give it a short life. A water-based premix without its own preservative is a growth medium, and it should be discarded at the end of the day or, at most, kept for the next batch under refrigeration with a date and a discard time. An oil-based premix has a longer life but still needs a label and a date, and it should be checked for smell and colour before use. Never top up a premix jar with a fresh batch: empty it, clean it and make a new one, because a jar that is topped up has no reliable strength and no reliable date.
Record the premix used in the batch record, not only the active mass. Write 'preservative premix 10%, lot P20261025-01, 2.00 g, delivering 0.20 g active' so the chain from premix to batch is visible. If a later investigation asks which preservative lot went into which jar, the premix record is the link. Without it, the batch record shows 2.00 g of a material that does not appear in the formula, and the trace stops there. The premix record and the batch record should point at each other by identifier, so a reader can move from one to the other without guessing.
Do not make a premix in advance for a batch that may not happen. A premix made on Monday for a Thursday batch is a small container of material with an unknown microbial load by Thursday. Make it on the day, use it, and discard the remainder. The exception is a premix that the supplier sells as a stable stock, which comes with its own shelf life and its own instructions; that is a raw material, not a bench premix, and it should be recorded as one. If a premix must be kept, write the discard date on the jar and check it before every use, exactly as you would for a batch.
- 1. Give the premix its own record and its own identifier.
- 2. Write the strength as grams of active per hundred grams of premix.
- 3. Record the active lot, the vehicle lot and the total mass.
- 4. Label the container with the name, strength, date and maker.
- 5. Store it closed, cool and dark, with a discard date.
- 6. Discard a water-based premix at the end of the day.
- 7. Never top up a premix jar with a new mix.
- 8. Write the premix identifier and mass in the batch record.
Bench example
A maker prepares a 20% premix of a thickener in glycerin for a gel. The record shows 4.00 g of thickener and 16.00 g of glycerin, total 20.00 g, strength 20%, made on 20261025 at 09:20 by A. The jar is labelled 'thickener premix 20% in glycerin, P20261025-01, 20261025, A'. For a 400 g batch that needs 0.30% thickener, the active mass is 1.20 g, and the premix mass is 1.20 / 0.20 = 6.00 g. The glycerin carried in is 4.80 g, so the batch glycerin line is reduced by that amount. The batch record says 'P20261025-01, 6.00 g, delivering 1.20 g thickener'. The unused premix is kept for two days in a closed jar in a cupboard, then discarded because the gel batch was cancelled. A second maker, reading the record a month later, can see exactly what the 6.00 g was and why the glycerin line is lower than the formula. The record for the cancelled batch says why the premix was discarded, so the waste is documented rather than unexplained. The date and the reason for the discard are both on the record.
Caution: A premix with no label and no record is an unknown material. If you cannot say what strength it is, do not put it in a batch.
This is a Nandu Nai textbook. It is a private brand book, not a government licence, and it does not give the right to give medical treatment.