sodium hydroxide the brightness before bleaching in examples 9 13 was rather lower than that obtained in comparative example 2 however the relationship between the initial ph and the brightness after bleaching shown in fig2 reveals that the brightness is higher than that obtained in comparative example 2 this suggests that metal ions and extractives detrimental to hydrogen peroxide bleaching were removed by impregnation with dtpa and as a result the hydrogen peroxide bleaching reaction efficiently proceeded the mechanism by which the brightness after bleaching is improved by impregnation with sodium hydroxide or impregnation with a chelating agent according to the first aspect of the present invention is unclear but extractives such as flavonoids are known to be detrimental to bleaching of woods having low bleachability such as douglas fir and representative known compounds thereof include dihydroquercetin and quercetin this indicates that the bleachability with hydrogen peroxide was improved as a result of extraction of these substances by impregnation with sodium hydroxide flavonoids are known to form complexes with metal ions to cause coloration thus it is concluded that the impregnation of chips with a chelating agent dtpa had the effect of extracting flavonoids by the alkalinity of dtpa forming complexes of dtpa with metal ions contained in the chips and inhibiting the complexation of flavonoids with metal ions to suppress the decomposition of hydrogen peroxide and to improve the bleaching efficiency chips of 100 douglas fir with low bleachability were defibrated by primary refining at a concentration of 40 solids by weight and a temperature of 133 u00b0 c this was diluted with warm water at a.