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Capillary absorption

Liquid moves through a porous material on its own, and it moves outward. It is why a stain grows after you treat it, and why a powder can pull one back out.

In detail

A porous material is a network of fine channels, and liquid moves through them without being pushed: surface tension draws it along, and it travels in every direction from where it landed. Three familiar problems are the same effect. A treated mark that ends up larger than it started has been carried outward by the solution used on it, which is why the trade procedures say to work from the EDGES of a spill towards the centre rather than the reverse. A ring or tidemark forms because the liquid front carries dissolved soil to its outer limit and stops there as it dries, depositing everything it was holding in a line. And a mark that reappears days later has wicked back up from deeper in the structure as the material dried. The same mechanism runs usefully in reverse. A dry absorbent powder placed on an oily deposit offers a drier, finer network than the material beneath it, so the oil moves upward into the powder and is carried away with it — the leather trade's cornstarch treatment for suede is exactly this. Wetting less, and drying faster, are therefore not fastidiousness; they are the direct controls on how far a problem travels.

A poultice shown in three stages over stained stone: the stain absorbed below an intact surface, a damp absorbent layer laid on top, and the stain drawn upward into the pad as it dries.
A poultice shown in three stages over stained stone: the stain absorbed below an intact surface, a damp absorbent layer laid on top, and the stain drawn upward into the pad as it dries.
A droplet resting on top of a non-porous surface beside the same droplet drawn down into a porous one, showing that absorption is what turns a mark into a stain.
A droplet resting on top of a non-porous surface beside the same droplet drawn down into a porous one, showing that absorption is what turns a mark into a stain.

Why

Sources for this page

Sources

  1. Spot Solver: spot and spill removal procedures

    The Carpet and Rug Institute (CRI) · Trade or professional body

    Relied on for: That solution is worked from the edges of a spill towards the centre to prevent spreading, and that a dry absorbent cleaning compound may substitute for wet treatment. (states this directly)

  2. Leather Care and Cleaning Guide

    The Leather Institute, 2011 · Trade or professional body

    Relied on for: That cornstarch placed over an oil mark on suede draws the oil out as it coagulates with it, and that water-based soil is treated with a damp rather than soaked cloth, feathering the edges. (states this directly)

  3. Technical Bulletin: Cleaning and Maintenance of Laminate Floor Coverings in the Home

    EPLF — Association of European Producers of Laminate Flooring, 2022 · Trade or professional body

    Relied on for: That moisture reaching a wood-based panel causes it to swell, which is why laminate is cleaned with a well-wrung cloth. (applied to this case rather than stated directly)

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