Why leather deteriorates
Leather is one of the organic materials that reacts fastest to changes in air humidity. Above 65% relative humidity it grows mould and its fibres swell, while below roughly 22% it shrinks permanently and loses flexibility. Heat and ultraviolet radiation break leather down photochemically, so it turns brittle and fades. Vegetable-tanned leather is most sensitive to polluted air and can suffer from so-called red rot: the surface reddens, turns powdery, and the leather loses strength until it crumbles to dust. Soiled pieces left untouched for a long time attract moths and carpet beetles (a gnawed surface, small holes, droppings).
Leather and metal: why a blade rusts in its sheath
Conservators recommend not storing weapons in leather sheaths or with leather straps, because leather reacts with metal and damages it. The fats and oils that leather contains from tanning create a green, waxy coating on brass and copper fittings, known as copper soaps. Iron and copper in contact with leather leave rust stains and chemically weaken the leather, while aluminium develops dark stains on it. For a sword with a leather sheath, sources therefore advise separating the metal from the leather and storing the blade in a drier place. Steel actively corrodes above 65% humidity; if salts have settled on it, even 20 to 30% is enough to cause rust. Sweat and fingerprints also carry salts, so wipe the blade with a soft cloth and a little non-drying oil before sheathing or storing it, and use clean nitrile gloves for extended handling. For long-term storage, keep the blade out of the sheath.
After every training session
Let sweaty or damp leather dry loosely in the air, not next to a heater, near air conditioning, or in direct sunlight, because rapid swings in heat and humidity deform leather. Remove dust with a soft brush; dust acts as an abrasive and every brushing removes a little of the surface, so clean only when it is actually needed.
Cleaning
Sources recommend only dry methods: careful vacuuming through a fine plastic mesh with reduced suction, a soft brush, and, on a sound, unbroken leather grain, even a soft drawing eraser. Avoid direct water. Conservators reject saddle soap: it contains oils that cannot be removed from leather and instead work deeper in, its high water content damages old leather, and it is also suspected of causing surface cracking. Entrust heavily damaged or richly decorated pieces to a conservator.
Oiling: when it helps and when it harms
Here sources diverge depending on whether you use the leather or preserve it. For collection pieces, conservators clearly advise against oiling: oils oxidise, leather stiffens and darkens, grease attracts dust, and research has for almost a hundred years disproved the idea that fats protect leather. The documented damage includes white fatty bloom, often irreversible darkening, stickiness, a musty smell, and moisture retention that promotes mould. The same source does, however, allow treatment for items in active personal use, such as work boots, gloves, or saddles, and wherever temporary flexibility is needed. A wrap or strap you train with daily can therefore be treated sparingly; a historical sheath or a collector's piece should not be. Raw or semi-tanned leather, such as that covering some shields, should never receive fat.
White coating: mould, or fatty bloom?
Fatty bloom looks like tufts, snowballs, or little broccoli florets, feels soapy or waxy to the touch, has sharp edges, and melts under a hairdryer's warm air. Mould is fuzzy, branching, and tends to be white, grey, green, or black. Sources describe two simple tests: move the leather into humidity below 50%, and active mould will shrink back while bloom remains; or dab a hidden spot with a cotton swab soaked in lighter fluid, and if the coating vanishes within 15 minutes, it is wax, not mould. Bloom can be swept with a soft brush into a vacuum nozzle held just above the surface, but it cannot be removed completely and usually returns. Solvents can damage finishes and colours, and heat can shrink leather.
Mould
Wrap an affected piece in a plastic bag so the spores do not spread. Dry wet leather outside on a clear day first, or freeze it; only dry leather is cleaned. Remove mould with a HEPA-filtered vacuum, twice, with the second pass running perpendicular to the first and the nozzle held just above the surface; protect decorations with a nylon mesh over the nozzle. Then wash the tools and surrounding surfaces, not the leather, with water and a cleaning agent, and disinfect them with a bleach solution diluted tenfold, left to act for 10 minutes. Leather still carries spores after cleaning, so keep handling it with gloves and keep it dry.
Storage and transport
The Canadian Conservation Institute recommends 10 to 20 °C and 50 to 60% relative humidity without fluctuation; the National Park Service recommends 40 to 60% with a daily change of up to 5%. Both call for darkness or UV protection, air circulation, and distance from exterior walls and heaters. Support sheaths and shaped pieces from inside and outside so they do not flatten or crack, do not place anything on top of them, and wrap them in acid-free, unbuffered paper or cotton. Leather should not touch metal shelving, clasps, or labels; where metal is part of the object, insert a polyester film between them. Choose cabinets made of metal with baked enamel, since wood releases harmful vapours. Check storage for insects twice a year, in spring and autumn, and keep a newly acquired piece separate from the rest for a while before storing it with them.
Inspection before training
Examine the wrap, the strap, and the sheath. Sources consider reddening and powdering of the surface, stiffening, cracks, loss of flexibility from over-drying, and insect-gnawed patches to be signs of weakened leather. Leather with such signs no longer has its original strength, so do not rely on it for a strap or wrap that holds the weapon in the hand. On a sheath, check for green coating on the fittings and rust on the blade where it touches the leather.
Common mistakes
A blade left in a leather sheath long-term; drying next to a heater; oiling collector's pieces; saddle soap and soaking; metal directly against leather.
What this applies to
To leather sheaths for swords, sabres, and knives, leather wraps on the handles of Japanese and European weapons, hanging straps and belts, shields covered in leather or rawhide, and leather training weapons.