Urushi commonly refers to the Urushi tree (or lacquer tree) and its sap, which is used as lacquer.
The technical name of the sap is ‘urushiol’ and the tree grows in many parts of east Asia including Japan. In fact, the Urushi tree belongs to the same family as mango, cashew nut and pistachio trees.
Collecting Urushi is like collecting gum from rubber tree. The bark of the tree is cut to yield a milky-white sap with sterilizing properties, which dries fast and turns solid in humid air. In this way the tree tries to protect the wound from further damage. The sap is poisonous to the touch until it dries, the creation of lacquerware has long been practiced only by skilled dedicated artisans.
Although we say the urushi “dries”, it is more accurate to say that it “hardens”. The urushiol becomes a blackish lacquer after contact with oxygen at a certain temperature and humidity. Unlike oil finishing, urushi requires heat and humidity to dry and harden (i.e., oxidize and polymerize): in 75%-90% humidity and 15 degrees centigrade or higher. Furo is the Japanese term for 'bath' but also used to denote the drying cabinet used to dry urushi work. The humidity and temperature within the furo is regulated finely according to seasonal and environmental conditions.
Liquid urushi has excellent adhesive properties and can be applied to just about any surface. It is also used as a special kind of glue used to connect objects and attach decorations. Iro-urushi (色漆), literally "color lacquer", was created by adding pigments to clear lacquer. When it solidifies, it becomes a very hard coating that waterproofs and protects the coated object from the effects of mold, mildew and other forms of weathering. It also provides protection against caustic substances such as acids. Only direct and prolonged exposure to sunlight will cause urushi to deteriorate. It is vulnerable to ultraviolet rays like other natural finishes.