Urushi is obtained by cutting about a dozen horizontal slashes into the trunk of a 10 year old Kiurushi tree. The slashes should run deep enough to sever the bark and bite into the sapwood, thus seeping a grayish-yellow sap. The sap is then filtered, colored and subjected to a relatively low-heat heat-treatment. Colorization is achieved by mixing in powdered iron to make black urushi or powdered rust (ferric oxide, hematite, rust ... all the same thing) to make red urushi, but the natural color of the sap is a medium to dark brown which will vary in consistency and richness depending on the initial state of the sap and the performance of the filtering and heating processes. There are other colorizations as well, but these are rare, as are a few de-colorization techniques designed to produce a totally clear lacquer. The lacquer may now be applied to any number of objects as noted above.

Application of urushi lacquer usually requires rubbing (better finish, but risks skin contact) or brushing (safer than rubbing, but trickier to get a good finish) and once applied, the items are placed in a warm, humid chamber such as a curing room or curing oven for between 12 and 24 hours. This allows the lacquer to cure (polymerize) thus forming a clear (to the base color) hard and waterproof surface.

Application and decoration can be a long process, requiring a few hours a day for several days to carefully apply several well executed layers in order to achieve that deep, rich finish that is the goal. The creation of a single high end art object such as a bowl can sometimes take weeks or months to complete properly. Now you know why that custom saya takes so long to get to you.

Now, here’s the fun part. Urushi lacquer is, as I’ve said, essentially the sap of the Kiurushi tree and is nearly pure Urushiol, this oil itself having derived its name from the tree and its sap. Now, this exactly the same oil secreted by poison ivy, poison oak and poison sumac thus causing those really unpleasant rashes. Keep in mind that the oil secreted by those plants is usually in trace amounts unless you’re cutting old growth, and even then is often encountered in fairly small amounts: generally no more than a half an ounce or so. Urushi as a lacquer is obtained in far greater quantities as typically 9 to 12 fluid ounces of pure sap is required to do a good job on a katana saya. This is a whole lot more exposure to the toxin than you get running through poison ivy infested woods.

Urushiol is a caustic alkaloid toxin, and when in contact with the skin, uncured lacquer can cause anything from bad rashes (ala poison ivy) to severe anaphylaxis and even anaphylactic shock in the worst cases (high exposure combined with high sensitivity).

Even the vapors created by the heat-treating and curing processes can create caustic vapors which can cause extreme rashes from even relatively controlled contact. Such vapors most often affect the skin of the exposed person, but if inhaled or ingested, the vapors can have drastically toxic reactions in the respiratory and digestive tracts and can lead to blindness if in contact with the eyes. Some folks have even died from this.

Now, to clarify something, properly done lacquer jobs exhibit virtually no reactions except in the rarest of circumstances where the person affected is hyper-sensitive to begin with. This is because the curing process itself is an enzyme-catalyzed process resulting in a complex oxidative polymerization. The result is a chain-bonding that not only hardens the lacquer but also renders it inert.

The process is delicate and complex, however, so the curing has to be done thoroughly and completely. In less well-done jobs, the curing may be incomplete, the monomer structure of the urushiol still evident (even trace) and allergic reactions still possible. In these cases, seemingly hardened lacquer may still react toxically, though such reactions are uncommon.

Errata:

Urushiol is also present in the double shell of the cashew nut and it is for this reason that cashews are sold almost exclusively as a shelled product. It is from this source that cashew lacquer is derived and while cashew lacquer is slightly less toxic than that of the Kiurushi tree, that’s not saying a lot. Cashew lacquer is commercially available in a formulation that allows for air-curing, thus alleviating the need for curing chambers and virtually guaranteeing a hypoallergenic, non-toxic product once done. In fact, most folks I know who work with the stuff and understand its toxicity will let it cure for longer than necessary just to make sure.

The skin of the Mango fruit also contains urushiol, though in far less concentration than poison ivy. Some people experience a reaction from touching the fruit’s skin or sap, but can generally eat the meat of the fruit so long as someone else peels it for them. Oddly enough, the leaves of the mango are highly toxic to virtually all cattle.