When wood is heated at high temperature, the ash will be pure white or white brown. But, wood is soaked in sea water and heated, the ash shows black color. In order to measure sodium concentration in tsunami soaked wood, ash color change was investigated. In this report, effect of temperature degree and sodium concentration into ash color, development of measure method of sodium concentration from ash color, and mechanism of ash color changing were described. As a result, ash color showed widest change in 600 degree C heating. And when the sodium concentration was below 1.0%, sodium concentration and ash lightness L* revealed a correlation. Using this correlation, it is possible to develop measuring system of sodium concentration in wood from ash color. Through observation of the black ash by microscope, the black ash was revealed to charcoal. The mechanism of black ash forming was assumed that, sodium was melt in high temperature kiln, small piece of remained wood was covered by melt sodium, oxygen was shut down from wood piece, and wood piece was changed into charcoal.
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