Salt of Sorrel: Wood Sorrel and the Iron-Mould Stain

Wood sorrel, Oxalis acetosella, is a small woodland plant with three heart-shaped leaves and a white flower veined faintly pink. It tastes sharply sour. From the seventeenth to the nineteenth century it was harvested across Britain, dried, and sold under the apothecary name sal acetosella, salt of sorrel, primarily as a treatment for iron mould on linen. The chemistry is unusual enough to be worth setting out.

Identifying wood sorrel

Wood sorrel grows on shaded hedge banks, in old woodland, and at the foot of mossy trunks. The leaves come in groups of three, each leaf heart-shaped at the tip with the cleft pointing forward; they often fold downwards along the central vein in the heat of the day or before rain. The flower is small and white, five-petalled, with faint pink veins running down each petal from the centre. Pinch a leaf and taste it. The clean, sour bite is oxalic acid, and the concentration is high enough to be useful for what follows.

Two plants are sometimes confused with wood sorrel. The first is common sorrel, Rumex acetosa, which is much larger and grows in open grassland; the leaves are arrow-shaped and a different green entirely. The second is clover, which has three leaves in the same general pattern but rounded rather than heart-shaped, and yields no sourness when chewed.

What iron mould is

Iron mould is the rust-orange staining that appears on white linen left in contact with damp metal: a forgotten pin in a folded shirt, a wet iron set down on a tablecloth, a damp chest with iron fittings closed up with bedlinen inside it. The stain is iron oxide, formed when the iron oxidises in the presence of moisture and the oxide bonds chemically to the cellulose fibres of the linen. The bond is strong. Ordinary detergent does not break it. Bleach lightens it but also damages the surrounding fabric, particularly any colour or fine weave.

This made iron mould a persistent household problem before stainless steel and synthetic fibres. Linens were expensive and were expected to last decades. A single bad stain on a christening shirt or a tablecloth was a serious loss. The household manuals of the period address it more often than they address almost any other laundry problem.

The chemistry

The oxalic acid in wood sorrel does something specific. It reacts with the iron oxide of the stain and converts it to iron oxalate, a different compound entirely, which is soluble in water. Once the iron is in soluble form, the stain rinses out without further treatment. The fibres themselves are not bleached, and the surrounding fabric is left untouched.

The reaction needs sunlight to finish. Ultraviolet light accelerates the conversion of iron oxide to iron oxalate, which is why every household manual that addresses iron mould is specific about laying the treated linen out in direct sun. Indoors, in overcast conditions, the reaction still happens but takes much longer, sometimes half a day for a single stain. A bright May or June morning, with the sun high and the air dry, is the ideal condition for the work, and wood sorrel is at its peak in those same weeks.

Sal acetosella: the apothecary trade

From the seventeenth century onwards, wood sorrel was a commercial crop in the southern English counties and in parts of France. Foragers gathered the leaves in late spring, pressed them, dried the pulp, and sold the product to apothecaries under the name sal acetosella. The "salt" in the name refers not to common salt but to the white crystalline form of the dried extract, which was sold by weight in small glass jars.

Sal acetosella appears in seventeenth-century herbals (Nicholas Culpeper's Complete Herbal of 1653 names it) and is mentioned in household guides through the eighteenth century as the standard treatment for iron mould. It was still being sold by chemists into the nineteenth century, by which time it was competing with mineral oxalic acid extracted from rhubarb and other industrial sources. The mineral form was cheaper and stronger but also more dangerous: pure oxalic acid is toxic in larger quantities, and there were several recorded household poisonings in the Victorian period. The plant-derived form was safer because the concentration was lower.

Wood sorrel juice for iron mould

The traditional preparation is fresh juice from the bruised leaves, used the same day. Dorothy Hartley records the practice in The Land of England (1979), and the method has not changed substantially since the eighteenth century.

Gather a large double handful of fresh wood sorrel leaves and stems on a dry morning. Rinse in cold water and shake dry. Pack into a mortar or a folded clean cloth and bruise the leaves thoroughly until they break down and the juice runs free. Strain the pulp through muslin, pressing to extract as much liquid as possible. The juice should be bright green and noticeably sharp-smelling. Decant into a small glass jar, and use within two days; the juice oxidises and loses strength after that.

To treat a stain: apply the juice directly to the affected area with a small soft cloth or a clean brush, saturating the fabric. Lay the linen flat in direct sunlight, the stain uppermost. Leave for thirty minutes to an hour, checking every fifteen minutes. The stain will begin to fade. Rinse thoroughly in cold water once the stain has lifted, then wash the linen as normal. For an old or stubborn stain, repeat the application before the final rinse. Do not allow the juice to dry completely on silk or very fine linen; test on an inconspicuous corner first.

Sources: Richard Mabey, Flora Britannica; Nicholas Culpeper, Complete Herbal (1653), Internet Archive; Dorothy Hartley, The Land of England (1979); Royal Horticultural Society species page, Oxalis acetosella.