Rev. & Tax. Code, § 6245

If a purchaser gives a certificate with respect to the purchase of fungible goods, or purchases those goods for resale in the regular course of business, and thereafter commingles these goods with other fungible goods not so purchased but of such similarity that the identity of the constituent goods in the commingled mass can not be determined, sales from the mass of commingled goods shall be deemed to be sales of the goods so purchased until a quantity of commingled goods equal to the quantity of purchased goods so commingled has been sold. Goods removed from the commingled mass for consumption shall be deemed to be consumption of goods not so purchased until a quantity of commingled goods equal to the quantity of goods not so purchased has been consumed.