Contemporary Nihonga has been the mainstay of New York's Dillon Gallery. Key artists from the "golden age of post war Nihonga" from 1985 to 1993 based at Tokyo University of the Arts have produced global artists whose training in Nihonga has served as a foundation. Takashi Murakami, Hiroshi Senju, Norihiko Saito, Chen Wenguang, Keizaburo Okamura and Makoto Fujimura are the leading artists exhibiting globally, all coming out of the distinguished Doctorate level curriculum at Tokyo University of the Arts. Most of these artists are represented by Dillon Gallery. "Nihonga" are typically executed on "washi" (Japanese paper) or "eginu" (silk), using brushes. The paintings can be either monochrome or polychrome. If monochrome, typically "sumi" (Chinese ink) made from soot mixed with a glue from fishbone or animal hide is used. If polychrome, the pigments are derived from natural ingredients: minerals, shells, corals, and even semi-precious stones like malachite, azurite and cinnabar. The raw materials are powdered into 16 gradations from fine to sandy grain textures. A hide glue solution, called "nikawa", is used as a binder for these powdered pigments. In both cases, water is used; hence "nihonga" is actually a water-based medium. "Gofun" (powdered calcium carbonate that is made from cured oyster, clam or scallop shells) is an important material used in "nihonga". Different kinds of "gofun" are utilized as a ground, for under-painting, and as a fine white top color. Initially, "nihonga" were produced for hanging scrolls ("kakemono"), hand scrolls ("emakimono"), sliding doors ("fusuma") or folding screens ("byōbu"). However, most are now produced on paper stretched onto wood panels, suitable for framing. Nihonga paintings do not need to be put under glass. They are archival for thousands of years. In monochrome "Nihonga", the technique depends on the modulation of ink tones from darker through lighter to obtain a variety of shadings from near white, through grey tones to black and occasionally into greenish tones to represent trees, water, mountains or foliage. In polychrome "Nihonga", great emphasis is placed on the presence or absence of outlines; typically outlines are not used for depictions of birds or plants.