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Mordant technology is often required for dyeing by natural dyes. The major problem with natural dyes is deepness of black color. So we analyzed the characteristic chromaticness with four kinds of natural dyes and three metals to improve it. We focused on vanadium, which is a transition element in a same period with iron or copper. Effective methods were round for the improvements of the dyeings with tetravalent vanadium compounds, vanadyl sulfate and tannin. Especially, tannic acid and gallic acid were hypochromically and fastly dyed on wool for the first time in the history of dyeing. [ABSTRACT FROM AUTHOR]