Tosefta su Tohorot 9:12

The airspace above an earthenware vessel is impure and its outer side is pure, [while] the airspace above all [other] vessels is pure and their outer side is impure (Hul. 24b:11), and consequently it is said (Hul. 1:6), "What is pure in an earthenware vessel is impure in all [other] vessels, and what is impure [Vienna Man., "pure"] in all [other] vessels is pure [Vienna Man., "impure"] in an earthenware vessel." Flat wooden vessels are pure, and unformed [wooden vessels] are impure, [while] flat metal vessels are impure, and unformed [metal vessels] are pure, and consequently it is said (Hul. 1:6), "[What is] pure in a wooden vessel is impure in a metal vessel, [and what is] pure in a metal vessel is impure in a wood vessel." And one who brings chests and ovens, basins, and earthenware [vessels] from outside the Land [of Israel]: Before they are fired, [vessels are deemed] impure because they were [produced in] the lands of the [Gentile] nations, except that [they are deemed] pure if they are earthenware vessels, [but] once they have been fired, [vessels are deemed] impure if they are earthenware vessels, and [all other vessels are deemed] pure because they were [produced in] the lands of the [Gentile] nations, and consequently it is said, "Whenever [vessels are deemed] impure by virtue of [being produced in] the lands of the [Gentile] nations, [they are deemed ] pure by virtue of being earthenware vessels, and whenever [vessels are deemed] impure by virtue of being earthenware vessels, they are [deemed] pure by virtue of [being produced in] the lands of the [Gentile] nations." Olives are impure when dry and pure when wet (per MS Vienna, but see Toh. 9:1), [but olive] pits are impure when wet and pure when dry, and consequently it is said, "[What is] pure when dry is impure when wet, [and what is] pure when wet is impure when dry." Bitter almonds: The small ones are liable [for tithing], and the large ones are exempt. And sweet [almonds]: The large ones are liable and the small ones are exempt, and consequently it is said (Hul. 1:6), "[What is] liable in bitter almonds is exempt in sweet [almonds], [and what is] liable in sweet [almonds] is exempt in bitter [almonds]. "