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Buying new vessels for Pesach rather than Kashering the Chametz vessels:
There is no Mitzvah Min Hamuvchar involved in purchasing new vessels for Pesach rather than to Kasher one’s Chametz vessels, with exception to knives and all other vessels that are difficult to fully clean, in which case it is a Mitzvah Min Hamuvchar to by new one’s rather than to Kasher them. [However, some Poskim write that it is better to have a separate set of Pesach vessels than to Kasher the Chametz vessels for Pesach. Nevertheless, some Gedolei Yisrael were particular to Kasher at least one vessel for Pesach in order so the laws of Kashering not be forgotten.]
Kashering new vessels:
New vessels which are bought from a gentile do not need to be Kashered. Hence all vessels which appear new may be bought from a gentile store or company. However, vessels sold by a gentile individual from his home are not to be purchased.
The law today-Must new pots or pans be Kashered or contain a Hechsher?
In the manufacturing process of cookware, it is common for oil-based primer and/or polish to used. Occasionally, this polish/primer is made from non-Kosher animal fats, and hence enters the cookware into the question of whether it is permitted to be used without Kashering.
The ruling: Many Poskim rule that based on the current way that the non-Kosher oil is applied, there is no need to Kasher new pots, pans, or aluminum cookware, even if one knows for certain that a non-kosher-based oil was used as a primer or polish during the manufacturing process. However, other Poskim rule stringently that the pot requires Kashering. Some say the pot requires Hagala, while others suffice with Iruiy Keli Rishon. [The above reflects the current known method of pot manufacturing. In the event that cookware manufactures change how the oil-based polish is used, then this will also affect the law, whether for leniency or stringency.]
The custom: The widespread custom is not to require new cookware to be Kashered. However, in certain areas, such as Eretz Yisrael, many are accustomed to be stringent. It is for this reason that there exists cookware, and aluminum pans, with Rabbinical certification. The Hashgacha of Rav Landau in Bnei Brak is accustomed to Kasher all new cookware and cutlery.
Cast iron cookware: Cast iron cookware which is preseasoned, may only be used without Kashering if one verifies that a Kosher [non-animal based] oil was used for the seasoning. If one is unsure, and certainly if he knows for certain that it was seasoned using non-Kosher oil, then the pot must be koshered prior to use.
 See Admur 451:14 regarding knives that it is a Mitzvah Min Hamuvchar to buy new knifes being that knives are difficult to clean. This implies that other vessels contain no Hiddur in not being Kashered for Pesach;
 Yesod Veshoresh Haavoda Hagalas Keilim that G-d fearing Jews should not use the Kashered vessels as they are difficult to clean; Yerushas Hapleita 20; Meiam Loez Bo; Piskeiy Teshuvos 451:2;
 This is to be done in order to avoid entering into Halachic questions regarding Kashering the vessels, which not all are experts in. Likewise, some vessels are difficult to properly clean, and hence Kashering them is to be avoided. [Yesod Veshoresh Haavoda ibid]
 Piskeiy Teshuvos ibid
 Admur 451:5
 The reason: As if they appear new we do not suspect that perhaps the gentile used it to cook in one time. [Admur ibid]
 Minchas Yitzchak 4:112; Tzitz Eliezer 12:55; Mishneh Halachos 7:112; Yabia Omer 1:7; Rivivos Ephraim 6:212; The laws of Pesach [Blumenkrantz] p. 3-73; Rav Shmuel Furst in name of Rav Moshe Feinstein; See Koveitz Mibeis Levi 1:32; Avnei Yashpei 2:58 in name of Rav Elyashiv; Halichos Shlomo Moadim 2
 As the heat of the manufacturing process is so great that it would burn any possible non-Kosher fat. [The pot is commonly entered into an oven that is 500 degrees Celsius.] Furthermore, the fat is not edible even in its original state, as it is mixed with other items, and is hence deemed Nosein Taam Lepegam. [Poskim ibid]
 Mishmeres Shalom Yoreh Deah 121:12; Chazon Ish Moed 44; Har Tzevi Y.D. 110; Teshuvos Vehanhagos 1:422; Moadim Uzmanim 4:282; Kinyan Torah 4:92; See Seridei Eish 2:35; Rav Eli Landau Shlita informed me, based on directives of his brother, the Chief Rabbi of Bnei Brak and head of the Landau Hashgacha, that all new cookware must be Kashered, including spoons, forks, cups, and especially pots and pans. The reason for this is because they use hot non-Kosher animal oils to compress and polish the items, and some specifically use lard for this process.
 Har Tzevi ibid; Chazon Ish ibid; Rav Wozner, Rav Elyashiv hold even for those who are stringent, it suffices to do Hagala on the inside; Rav Landau ibid; Regarding if Libun or Hagala is required when an item absorbs non-Kosher liquid directly, see Shach 121:8 in name of Rameh Mipuno 96; Rav Akiva Eiger 121; Aruch Hashulchan 121:11
Teflon: Rav Landau related to me that Teflon coated pans that have suspicion of having a non-Kosher oil primer placed under the Teflon coating, it requires Libun Kal, and hence is not Kasherable.
 Tzitz Eliezer ibid regarding those who desire to be stringent
 Minchas Yitzchak ibid
 Mishneh Halachos 7:112; The laws of Pesach [Blumenkrantz] p. 3-73
 Piskeiy Teshuvos 451:4; Kashrus Manual of the Eida Hachareidis;
 The reason: The seasoning that is added to a cast iron pot is very different than the polishing/premier added to regular pots, as a) The seasoning is not entered into a libun level oven which cures the pot and burns any of the Issur, and b) The seasoning is edible for a human and is not Pagum. [See Kinyan Torah ibid]
How to Kasher a preseasoned cast iron pot: There are several points that require clarification when Kashering such a pot, such as a) Must one first remove all the oil seasoning that is absorbed inside the metal of the pot, through scrubbing it with soap and detergent? b) Does Hagala suffice, or is Libun required. See Shach 121:8 and Rav Akiva Eiger there [concludes with Libun]; P”M 451 M”Z 16 [concludes with Hagala]. In order to Kasher according to all opinions, the pot is to be Kashered with Libun, and placed through a self clean oven cycle.