Switching from one treatment type to another — what is the full re-test list?

Six items minimum: dispersion, viscosity, stability, skin feel, colour where a pigment is involved, and whatever functional claim the product makes.

Dispersion and viscosity move first, because the agent sets how the powder wets into the system. Expect the process window — shear time, temperature — to need adjustment as much as the formula.

Stability is not optional: 40°C for one to three months, freeze-thaw cycling, and a centrifuge test. Incompatibility between the new layer and the system shows up as oil separation, layering or coarsening during storage, not on day one.

Skin feel and colour are sensory and optical, so they need actual panels and actual drawdowns. Functional testing follows the product: in-vitro SPF and PA for sunscreen, oil absorption and sebum control for an oil-control product.

The step most often skipped is regulatory. A different treatment changes the INCI list, and on a notified product that may mean a variation. Treat a treatment change as a raw-material change and run the full change-control route.