A waxed field coat or vest is a tool in many ways like a fine 12-bore double. It’s functional, yes, but it’s also symbolic inasmuch as it embodies the heritage, the tradition of fieldsports. And while it protects the wearer from moisture, like the 12-bore, it must itself be protected from H2O – basically from rain and the like.
Waxed cotton is not ordinary cotton fabric. It consists of extremely long staple fiber. This is one of several features that allow it to hold wax properly. Yet, this staple length means that waxed cotton fibers have a proportionally high response factor to mercerization.
Mercerization involves placing cotton under tension and first treating it with aqueous sodium hydroxide followed by rinsing. The hydroxide wash disrupts intermolecular hydrogen bonding, thereby allowing cellulosic chains that constitute cotton fibrils to disentangle and align themselves in response to the applied tension. Rinsing reinstates intermolecular hydrogen bonding, and even allows it to exist to a greater degree than before such that those same cellulosic chains become locked in a more linear, crystalline array. Because the process increases fibril surface area on the one hand and decreases fibril entanglement and spur count on the other, mercerized cotton thread is not only easier to dye, softer, and more lustrous, it is also stronger and less likely to catch.
Moisture, meaning water, can however reverse the mercerization process. This is something that we all know qualitatively from experience: Cotton garments shrink, fray, and pill with repeated washing. Slowly but surely, water catalyzes the breaking of intermolecular hydrogen bonds and the reforming of new ones. Unfortunately, while the first set locks cellulosic chains in a linear, crystalline array achieved while cotton fibre is under tension, the second freezes them in a more disordered state reflective of being able to move around relative to each other.
Just as the extremely long staple fibers of waxed cotton have a high response factor to mercerization, benefitting greatly from the process, they suffer a more pronounced degradation concomitant with cross-chain hydrogen bond reformation. Thus, a once nicely tailored waxed cotton field coat, which was comparatively impervious to thorns, now becomes prone to warping and tearing as a consequence of regular water adsorption. All of this can be avoided, though, if there exists a barrier protecting the cotton fibrils from water. Reproofing wax is just that. It keeps water from adsorbing microscopically, which in turn allows the garment to macroscopically keep it away from the wearer.
The moral: Don’t skimp on reproofing! A jacket that is worn only for sunny day dog-walking still loses wax via evaporation. But given how it’s used, reproofing once every several years is probably sufficient. In contrast, if it’s a field coat worn by a hunter working grouse coverts every weekend rain or shine, not only does it lose even more wax via evaporation, it loses it through abrasion, too. This garment must be reproofed at least once a season at a minimum. It will very likely require attention several times, however, between opening and closing days. Only you can decide where your waxed cotton garments fall on the spectrum. Just keep in mind the principles when you make the judgment call.
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