| http://www.w3.org/ns/prov#value | - Further, though inorganic materials such as Si, Fe, Ca and the like tend to be mixed in as impurities by being contained in the coloring material and the like, in general, it is advantageous that the amount of polyvalent inorganic materials is small irrespective of cation, nonion and anion, and the concentrations of Si, Fe and Ca are preferably 20 ppm or less, respectively.
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