Cornflour or talc to draw out an oil mark

Does it work?

Supported

Two independent trade bodies working on unrelated materials — a hide and a concrete paving unit — specifying the same first step for the same class of deposit, with the same instruction not to wipe. That is corroboration rather than repetition, and it is why this sits at supported rather than at practitioner practice. It is one of the few household techniques in this register that the trade sources put FIRST rather than tolerate.

This is a verdict on whether the technique is effective. Whether it is safe on a particular surface is a separate question, answered below.

Why it works

A fine powder presents an enormous surface area of small pores. Where those pores are in contact with a liquid oil, capillary action draws the oil into them, and the oil is then held in the powder rather than in the substrate. Removing the powder removes what it holds. The mechanism is physical rather than chemical and is not in dispute; the leather institute's wording — rubbed gently until it coagulates with the oil — describes the same process from the practitioner's side. What is not established anywhere read here is how much of a given deposit any particular powder recovers, so the record describes the direction of the effect rather than its completeness.

Background

This one is better evidenced than most of what circulates alongside it, and two independent trade sources in this register name it explicitly. The leather institute's instruction for an oil or grease mark on suede is to place cornstarch over the area and rub it gently with a finger until it coagulates with the oil, repeating as necessary, and only if the mark survives to move to a cloth dampened with odourless mineral spirits. Absorbent first, solvent last. A paving manufacturer gives the same sequence for a different material: oil penetrates paving readily, spillages are lifted promptly with an absorbent such as paper towel, cat litter, cornflour or talc, and the stain MUST NOT BE WIPED, because wiping drives it in and spreads it. Two trades, two materials, one method. The reason it works is capillary action rather than chemistry — a fine, dry, high-surface-area solid in contact with a liquid oil draws it up and holds it, and nothing is dissolved or driven deeper. Two limits follow. It addresses oils and not dyes. And it works far better on a fresh deposit than an aged one, because absorbed oil is no longer available at the surface.

Conditions

• Fresh deposits, before absorption has finished
• The mark is not wiped at any point
• Repeated with fresh powder as necessary

What it does not do

• It addresses oils and greases and does nothing for a dye, tannin or ink mark.
• It is far less effective once the oil has been absorbed, which on a porous surface can be minutes.
• It rarely finishes the job on its own — both trade sources treat it as the first step in a sequence rather than the whole method.

What to use instead

• Absorbent powder treatment — Lifting an oily mark out of a porous surface with a powder rather than dissolving it with a liquid. The gentlest route on anything water would ruin.
• Solvent treatment — The branch that starts before water rather than after it — and it carries a warning the aqueous branch does not.
• White spirit — The petroleum solvent the leather and carpet trades name for oil marks. One substance under three names, and it attacks plastic, rubber and coatings.
• Poultice — A paste left on a stain for a day or two to draw it back out of a porous material. The only method here measured in hours rather than seconds.