least 100 liters s t p preferably 200 400 liters of oxygen per kilogram of tannin solids where other oxidizing agents are utilized equivalent amounts thereof should be added to the medium the degree of oxidation of tannins is readily observable inasmuch as coincident therewith the ph of the medium declines the color of the tannins darkens and the desired solubility is achieved therefore the amount of oxidizing agent and the rate and period of reaction necessary for optimum results are determinable from monitoring these characteristics ordinarily the reaction should be permitted to continue for from about 10 minutes to about 3 hours depending upon the reaction pressure degree of agitation and concentration of the reactants additionally the reaction medium should be maintained at a temperature of at least 50 u00b0 c preferably between about 60 u00b0 and 120 u00b0 c during the course of the oxidative reaction heat should be removed from the reaction medium in order to maintain the temperature within the desired limits finally the reaction should be stopped for example by removing the tannins from the oxidizing environment when the tannins have reached the desired degree of solubility it is also possible during the course of the oxidative reaction to control the rate and degree of reaction through intermittent addition of alkali as noted previously the medium becomes progressively more acidic during the course of oxidation or between incremental additions of alkali regardless of whether the alkali is added in one or several increments however it has been discovered that the desired degree of oxidation can be reached by permitting the ph of the medium after the last increment of alkali to fall to within the range of from about.