Ointments and their bases
An ointment is a greasy semisolid you spread on skin. Almost everything about how it behaves comes from its ointment base, of which there are four classic families. Hydrocarbon (oleaginous) bases like white petrolatum are pure grease: they seal in water, stay on for a long time, but feel heavy and are hard to wash off. Absorption bases add a water-in-oil emulsifier so they can take up some watery fluid while staying greasy. Water-removable (emulsion) bases are oil-in-water systems that wash off with water and feel light — these are the everyday vanishing creams. Water-soluble bases (polyethylene glycols) contain no oil at all and rinse away completely.
Creams, gels and pastes
A cream is really just a thick, spreadable emulsion — the same oil-and-water science as a milky lotion, only with more internal phase and higher consistency. An oil-in-water cream feels light and washes off; a water-in-oil cream feels richer and more occlusive. A gel takes a different route: a polymer network (or clay) traps a large volume of liquid into a clear, jelly-like solid that holds its shape yet shears thin under a fingertip. Gels feel cool and non-greasy, which patients love on the face and scalp. A paste is an ointment stiffened with a very high fraction of powder — so stiff it stays exactly where placed, ideal as a protective barrier (think zinc oxide paste on a nappy rash).
Water-rich semisolids face the same microbial risk as any aqueous form, so they carry a preservative; and because they sit on skin, a humectant such as glycerin is often added to hold moisture and stop the product drying out in the tube.
Levigation: no grit allowed
When you blend an insoluble powder into an ointment or paste, the product must feel smooth — gritty cream is unacceptable on broken skin. Levigation is the technique: wet the powder with a small amount of a compatible liquid (a levigating agent like mineral oil or glycerin) to form a smooth paste, then fold that into the base. The thin film of liquid lets you break down aggregates by shear without scratchy lumps surviving.
- Choose a levigating liquid that mixes with the base — mineral oil for oily bases, glycerin or propylene glycol for water-washable ones.
- Triturate the powder with a few drops of liquid on a tile or in a mortar until a smooth, lump-free paste forms.
- Incorporate the smooth paste into the remaining base by geometric dilution, mixing until uniform.