פירוש על עבודה זרה 150:18

There is a dispute if the Jew uses the vessel before it was kashered. According to one opinion, the contents cooked or prepared in the vessel are prohibited. This accords with the opinion that if a forbidden element imparts a worsened flavor the food is permitted. The non-Jew’s vessel will emit the flavor embedded in the vessel but this flavor is old and will not improve the taste of the dish. Nevertheless, this opinion holds, the forbidden element causes a prohibition.
According to the other opinion, the food is permitted because he holds that if the forbidden element imparts a worsened flavor, the dish is permitted.