Mineral oil

A release agent for moulds

Soap sticks to rigid moulds, and a thin film of mineral oil helps it let go. Anne-Marie Faiola rubs lengths of PVC pipe inside and out with mineral oil before pouring round bars, and lubricates rigid plastic specialty moulds with it, reaching into the detail with a cotton bud. For a tube without a liner, her Soap Crafting drizzles a tablespoon around the inside of the pipe and turns it to coat the wall, then pushes the set log out with a soup can wrapped in a tea towel.

Anne Watson notes that some makers grease moulds with mineral oil or petroleum jelly the way a cook greases a cake tin, and that others dislike the residue it leaves on the soap.

Why it is not a soap oil

The oils in a soap recipe are triglycerides, fatty acids joined to glycerin, and lye splits them into soap. Mineral oil is a mixture of liquid hydrocarbons, the "paraffin oils" of its EU ingredient entry, with nothing for lye to split. Simmons and Appleton called it "entirely unsaponifiable"; stirred into soap it "merely exists in suspension", and soap analysts count it as unsaponifiable matter. So it has no saponification value, the calculator does not list it, and any that goes into a batch stays as free oil that never becomes soap. Grosso and Watson both say it has no place in the recipe itself.

A film on a mould is not an ingredient, but it still ends up on the surface of the soap. Pour off any excess so it does not pool in the bottom of the mould.

Alternatives

A liner does the job without oil: a plastic liner in a tube, freezer paper or a plastic sheet in a box mould, and silicone moulds need nothing at all. Alicia Grosso prefers tubes with threaded screw caps, which come off most easily, and pushes the log out with a disc of wood on a stick. If a log will not budge, Faiola freezes the whole tube overnight and tries again.

Faiola also uses cyclomethicone, a volatile silicone, in place of mineral oil. Check what you buy: in its EU ingredient entry cyclomethicone is a mixture of cyclic siloxanes with four to six silicon atoms in the ring, and since 31 January 2020 the EU has barred D4 and D5, the four- and five-membered ones, from wash-off cosmetic products at 0.1 % or more.

Handling it

Mineral oil is not very poisonous, but it is dangerous if it gets into the lungs, for example when a child who has swallowed some vomits and breathes it in, and the outcome can then be poor (MedlinePlus). Keep the bottle capped and out of reach of children, and wipe up spills, which leave a floor slippery.