Urushi no ki (Latin: Rhus-verniciflua or also Toxicodendron vernicifluum), a tree belonging to the family of anacardiaceae (sumac family), is the source of the natural lacquer known under its Japanese name urushi 20 that is obtained in Japan, China, and Korea. After ten years of growth an urushi no ki tree reaches a height between ten and fifteen meters with a trunk circumference of about half a meter, and it produces about 200 grams of lacquer.
Raw lacquer is necessary for gluing projects. The standard lacquer usually used for those projects is ki urushi which in Japan is also used for kint- sugi. One can find that ki urushi is available under its trade name Seshime Urushi.
In former times the neatly cut tree branches were immersed in water during the winter in order to win seshime urushi – the branch lacquer – or also known as eda urushi. Seshime urushi is mainly used for base coating and glue lacquer which can be used in combination with other natural glues such as nikawa (animal glue).
The best-known lacquer is the black lacquer (kuro urushi) that impresses by its fantastic gloss and its glaze-like depth. In the past, carbon in the form of lamp soot or fine charcoal dust has been used for its manufacturing. Later, acetic iron oxide (iron content between 0.5 % and 2 %) was added to the refined lacquer in order to achieve a blackening with a certain optical effect. A lacquer that obtained its color from iron oxide is also referred to as roiro urushi or kuro roiro urushi.
By mixing cinnabar, suki hakushita urushi (translucent lacquer), and Perilla oil, red lacquer (aka urushi) is produced that is also called shu urushi. The shade of color varies according to the fineness of the pigments from light to dark.
In comparison to shu urushi, bengara urushi is a red lacquer although it is more brownish in color. Bengara urushi turns into a translucent lacquer by adding iron-III-oxide (red iron oxide, indian red).
Very starchy rice as it is used in sushi is the best choice. It is cooked until it is very mushy or is ground to flour and is then further processed. Since 500 years, wheat flour is also used as a gluing thickener in the base and composition mass for the natural lacquer technique but is also used as glue in combination with other materials and urushi.
Warm glues have a gluten base, are water-soluble, and not heat-resistant. Gluten glues can discolor when they come in contact with metals which also lowers their adhesiveness.
The skin glue is more elastic than the bone glue and is thus better for use in the kintsugi technique.
For over 500 years diatomaceous earth (jinoko) has been mined in Wajima. Diatomaceous earth is kieselguhr – also known as triple or celite – a mineral wathering product from silicon dioxide containing husks from fossil diatoms.
The second filler material used is tonoko – a mixture of melleable (clay and china clay) and non-kneadable minerals. The non-malleable portion is about 75% and consists mainly of quartz, feldspath, and mica. Other micro elements influence the color of the clay which can range from white (shiro tonoko) to red (aka tonoko). Tonoko is mined in Yamashina, in the vicinity of Kyoto.
For kintsugi powdered gold (kinpun, honkin) is used. It is available in different grain sizes, forms, and colors where the natural color of pure gold is preferred.