漆と抗菌

Urushi and bacteria: what was actually measured

Lacquerware is often sold as naturally antibacterial. Unusually for a claim of that kind, there is a public measurement behind it — and it is a strong one. What follows is that measurement, including the two lacquers that scored lower and the question its own authors say is still open.

What was tested

Kobayashi and Machida, at the Iwate Industrial Research Institute, tested eleven urushi films in a study published in the institute’s report for 2009, with the testing itself carried out by the Japan Food Research Laboratories. The eleven covered eight lacquers — raw Chinese, raw Japanese from Joboji at three different tapping periods, refined, fast-drying, a pigmented one and an iron-black one — applied by three different methods, brushed, polished roiro, and heat-cured. After the test the untreated control carried 2.1 x 10^7 E. coli and 9.2 x 10^5 S. aureus per piece. Most of the urushi films came back below the detection limit of 10.

What made a difference, and what did not

What the study set out to find was whether any of that variation mattered, and the answer was mostly no. Japanese and Chinese lacquer did not differ. Nor did the tapping period of the Japanese lacquer, nor refining, nor the fast-drying treatment, nor the method of application — brushed, polished and heat-cured pieces all performed. The two that came in lower were the two with something added: the lake-pigmented film at an activity value of 5.3, and the iron-black at 3.8. The authors note that the black is made by reacting iron hydroxide with urushiol, and suggest the chemical change to the urushiol may be why, while saying plainly that the details are unknown. Both were still far above the standard’s threshold.

What the test does not cover

The limits are stated by the authors rather than inferred here. One piece was weathered first, in a xenon weatherometer for 120 hours, and its antibacterial activity did not drop — but they point out that ultraviolet weathering strips the surface away and keeps exposing fresh lacquer underneath, so it does not tell you about a bowl that has simply been used and kept in a cupboard for years. That, they write, needs separate work. The test is also a film test on flat coupons under controlled conditions for 24 hours, which is what JIS Z 2801 is; it is not a measurement of a bowl in a kitchen, and nothing here says a lacquer piece needs less washing than any other dish.

What follows from it

• ✓The property belongs to the cured film. Origin, tapping period, refining, fast-drying treatment, brushed or polished or heat-cured — none of it changed the result.
• ●Additives lowered it. The pigmented film scored 5.3 and the iron-black 3.8, against above 6.3 for the plain lacquers. Still past the standard’s threshold of 2.0, but not the same figure.
• ✗It is not a licence to wash it less. A 24-hour film test on flat coupons is not a statement about a used bowl, and the study says nothing of the kind.
• ✗Years of ordinary use are still unmeasured. The weathering test kept exposing fresh lacquer, so it answers a different question from the one about a bowl kept in a cupboard for a decade. The authors say so.

What it is worth to a buyer

The practical reading is narrow and worth having anyway. The antibacterial behaviour is a property of the cured urushi film itself, not of a finish or a treatment applied over it, and it survived every ordinary variation the study threw at it — origin, refining, and how the piece was finished. What it did not survive intact was additives. So on this one property, the plain lacquers and the decorated ones are not quite the same object, which is the sort of distinction this site exists to point at, and it is not on any listing.

Where to look

There is no shop link on this page. The finding above separates plain lacquer from pigmented and iron-black on one measured property, and no listing states which of those you are looking at in terms this study would recognise — so a recommendation here would be guesswork wearing the study’s authority. What is worth carrying into a purchase is elsewhere on this site: whether the piece is urushi at all rather than a urethane coating, since the material field on a listing uses the same word for both.