Key takeaways
- Check pH, viscosity, appearance and temperature at defined stages.
- A failed in-process check is cheaper than a failed finished batch.
- The check points and limits belong in the batch record, not in a memory.
In-process checks are measurements taken while a batch is being made, at defined stages, against defined limits. Their value is timing: a pH that is wrong after emulsification can be adjusted in the tank, while the same pH discovered after filling means 20,000 unusable units.
1. The check points
Typical check points are after the water phase is heated, after emulsification, after cooling and before filling. Each point has a purpose: the first confirms the phases are correct, the second confirms the emulsion formed, the third confirms the formula is at specification, the fourth confirms the fill is safe.
2. pH
pH is measured at the cooling stage, when the batch is below 40 degrees, with a calibrated meter. A pH outside the limit at that point is adjustable with a small amount of acid or base. A pH outside the limit after filling is a rework decision.
3. Viscosity
Viscosity is measured at a defined temperature and a defined spindle speed, because both change the reading. A batch that is too thin at the cooling stage can be adjusted with a thickener; one that is too thick after filling is a problem.
4. Appearance and colour
A visual check against a retained standard catches colour shifts, specks and phase separation before the batch is packed. A draw-down on a glass plate under a lamp is the standard method. A photograph under uncontrolled light is not a standard.
5. Temperature
Temperature is recorded at each stage because it drives the emulsion and the preservative addition. A preservative added at 60 degrees may be partly deactivated, which is invisible at release and fatal at month six.
6. Homogeneity
A sample from the top and the bottom of the tank should match on viscosity and appearance. A difference indicates incomplete mixing, and incomplete mixing produces a batch that varies from unit to unit. This check is quick and it catches a common failure.
7. Fill weight
Weigh the first, the tenth and the last unit of a run. A drift across the run indicates a filler problem rather than a formula problem. The weight should be recorded against the declared net weight, not against the target.
8. The limits
Each check needs a limit and an action: continue, adjust, hold or reject. A check without an action is a number in a book. The action should be defined in advance so that the operator does not have to decide under pressure.
9. Who performs it
The operator performs the routine checks and the quality team confirms the critical ones. A single person performing and approving the same check is a control gap, and it is what an audit looks for first.
10. Records
Record the value, the time, the analyst and the action taken. A batch record with in-process values is a document that can be reviewed; one with only a signature is a document that cannot. The record is the product’s evidence of control.
11. Where it fits
Process control runs across the clay mask range; the deviation path continues in the CAPA entry.
Reference: Batch records and in-process control data, 2025
