If one is in doubt as to whether a certain vessel is dairy, meat, or Pareve, what is the law?
If one is unsure as to a vessel’s dairy or meat status, and 12 months have not passed since its last use, then it is required to be Koshered if one desires to use the vessel for dairy or meat foods. If one does not desire to Kosher the vessel, or it is unable to be Koshered, it may only be used for Pareve foods. If Pareve food is cooked in this vessel [i.e. a pot] then if it does not contain a Davar Charif, the food may be eaten together with dairy or meat in the same meal. If it contains a Davar Charif, it may not be eaten within a meat or dairy meal.
If 12 months have passed: Some Poskim rule that after 12 months the taste absorbed in the walls of a vessel becomes nullified, as stated in the previous Halacha. Accordingly, in a case of doubt as to the status of the vessel, there is room to permit designating it for meat or milk so long as 12 months have passed. Nonetheless, it is best to Kosher the utensils beforehand, [especially if it is made of Kosherable material]. If it is made of non-Kosherable material [i.e. porcelain] then it should be immersed three times in the Hagala waters.
Glass vessels: If one is unsure as to a glass vessel’s dairy or meat status, one may designate it for whatever food one desires, meat or dairy. Nonetheless, if the vessel is a Pyrex pot or baking pan, it is proper to have Hagala performed to it three times prior to use.
 See Chaim Sheol 2:38 regarding one who is unsure if a new vessel bought from a gentile was previously used for non-Kosher; Admur 451:5 regarding one who is unsure of a vessels Chametz status
The reason: As it is a case of doubt to which we rule stringently. Now, although if the pot is no longer Ben Yomo it is a case of Rabbinical doubt, nevertheless, since this doubt is due to ignorance, it is not considered a doubt at all and one cannot apply the rule of Safek Derabanan Lihakel. [See Michaber Y.D. 98:3, Shach 98:9 and Taz 98:6 regarding Daas Shotim that it is not considered a Safek even by a Rabbinical case] Furthermore, one may come to use the pot with a Davar Charif and the opposite dish, which according to many Poskim makes it Biblically considered like the taste of the pot even when not Ben Yomo. [See Panim Meiros 1:64; P”M 96 M”Z 1 based on Rashba 496; Shivas Tziyon 32; Rav Akiva Eiger, brought in Pischeiy Teshuvah 95:4]
 See Rama 95:2
 Chacham Tzevi 75, 76 and 80 [regarding Bedieved even with Charif, based on Michaber Y.D. 135 that 12 months suffice for Yayin Nesech vessel, although, Lechatchilah is Machmir], brought in Pischeiy Teshuvah Yorah Deah 122:3 and Shaareiy Teshuvah Orach Chayim 451:1; See Sefer Hakashrus p. 176
Other opinions: Some Poskim rule a vessel remains forbidden forever due to its absorbed taste, even after 12 months. [Rashba 575; Chaim Sheol 2:38; Panim Meiros 1:31; Teshuvas Rav Akiva Eiger 43; See Pischeiy Teshuvah ibid; P”M 103]
 Conclusion of Chaim Sheol 2:38 regarding a porcelain vessel bought from a gentile and one is unsure if it was used for non-Kosher that possibly one may be lenient to a Sfek Sfeika; Seemingly, all the more so would this allowance apply by meat and milk, as it is not considered Derech Bishul and hence is a mere Rabbinical prohibition
 Conclusion of Pischeiy Teshuvah and Shaareiy Teshuvah ibid
 See the section on “Glass vessels”, and that the main opinion follows as rule the Sephardim that is does not absorb. Hence, in a case of doubt certainly even Ashkenazim may be lenient, as writes Chaim Sheol ibid
 See the above Halacha regarding glass, that glass cookware is more stringent than regular glass, and hence it would be proper for Ashkenazim to Kosher it if they are unsure of its status.