PURPOSE: To solidify a waste oil which is liquid at ordinary temp. to obtain a solid to be discarded as a kitchen refuse or reused as a crude soap, by using a solution comprising an alkali for partially saponifying the oil and a saponification accelerator for solidifying the oil within a few minutes after initiation of the saponification.

CONSTITUTION: An oil which is liquid at ordinary temp. is saponified with an alkali, i.e., caustic soda or caustic potash, in an amount of 1/6 to 1/3 of that necessary for complete saponification, in the presence of a saponification accelerator for solidifying the oil within about one to three minutes after initiation of the saponification. The alkali is preferably caustic soda from the standpoint of the hardness of the solid saponification product. Examples of the accelerator include diethylene glycol monoethyl ether, diethylene glycol monoisobutyl ether, diethylene glycol monobutyl ether, propylene glycol, and 2-phenoxyethanol polypropylene glycol. Also effective are combinations of any of these with ethyl alcohol, isopropyl alcohol, n-propyl alcohol, etc.