levigation
Levigation is the trick of adding a little liquid to a gritty powder and grinding the two together into a smooth, lump-free paste — the same idea as wetting cocoa powder with a splash of milk before stirring it into the rest, so it does not stay clumpy. It is a hands-on compounding technique you would see at a pharmacy mortar and pestle.
Technically, levigation is the process of wetting a solid with a small amount of a suitable liquid (the levigating agent) and then triturating, or grinding, the mixture to reduce particle size and produce a smooth, uniform dispersion. The thin film of liquid coats the particles, reduces friction and prevents them clumping, so vigorous grinding can break aggregates down and spread the powder evenly through the vehicle.
It is the standard way to work an insoluble powder smoothly into an ointment, paste or suspension without leaving a gritty, scratchy texture. The levigating agent must be miscible with, or at least compatible with, the final base — mineral oil for an oily base, glycerol or a small amount of the vehicle for an aqueous one — and is used in just enough quantity to wet the powder, not to dilute the product.
To compound a zinc oxide ointment, the pharmacist first levigates the zinc oxide powder with a little mineral oil into a smooth slurry, then incorporates that into the soft-paraffin base to avoid a gritty product.
Wetting and grinding a powder first prevents a gritty texture in the finished semisolid.
Match the levigating agent to the base: a small amount of mineral oil for a greasy ointment, glycerol or some of the vehicle itself for an aqueous one — using just enough to wet the powder, not to thin the product.