Kitchen countertops are generally cold, but sometimes hot ĥametz foods or boiling pots from the stove are placed on them, and if some sauce spills on the countertop, it is absorbed at the level of a “kli rishon removed from the flame” (see 10:8 above).
In order to kosher a countertop, one must first clean it well, paying special attention to crevices and making sure that no food remains stuck in them.
Marble countertops should ideally be koshered by pouring boiling water over them while placing a scalding hot stone or piece of metal on them. By doing so, the water is brought to a boil and reaches a koshering level of a kli rishon removed from the flame. However, it is difficult to bring metal to such a heat in private homes, and doing so could damage the countertop. Therefore, the general practice is to suffice with pouring boiling water on the countertop. In this case, one should make sure not to use the countertop for Pesaĥ foods until twenty-four hours have elapsed since the last time hot ĥametz foods were on the countertops. Instead of pouring hot water, one may also cover the countertops entirely with oilcloth or thick foil in order to separate between the countertops and the Pesaĥ utensils.
Those who are stringent do both – they pour boiling water on the countertop and then cover it with linoleum or thick foil.
Fragile countertops, on which boiling pots are never placed, can be koshered by merely cleaning and pouring boiling water on them.
Some think that it is possible to kosher marble countertops by burning alcohol on them, but the koshering strength of alcoholic spirits is less than that of boiling water, and therefore boiling water should be poured on the countertop. Cleaning countertops with a steam cleaner is as effective as pouring boiling water on them, but where hagala in a kli rishon is required, it is ineffective.
. SA 451:20 states that one can kosher the tables on which pots are placed by pouring boiling water on them (and does not rule leniently based on the principle that koshering method is determined by main usage, which in this case is with cold food). MB 114 ad loc. cites Rabbeinu Yeruĥam that one must kosher such a table according to its most intense usage, i.e., a kli rishon removed from the flame. Therefore, one must pour boiling water over a scorching stone on the table, effectively koshering it on the level of a kli rishon removed from the flame. However, as we saw in 10:9 above, in extenuating circumstances one may kosher a utensil based on its main usage; in this case merely washing the table would suffice.
Practically, even one relying on the lenient opinions should at least pour boiling water on the countertop or cover it with an oilcloth. Those who act stringently both pour boiling water and cover the table, preferring not to rely solely on a covering that may slide around. It is worth mentioning the concern raised by some that cheaper countertops made of compressed stone have the status of earthenware, on which hagala is ineffective. However, even earthenware can be koshered by merely washing it, as long as the majority of its usage is with cold items. Additionally, it seems more likely that compressed stone countertops do not have the same status as earthenware vessels; see Hagalat Kelim 13:1. Nevertheless, there are better grounds for covering this type of countertop. See also Sidur Pesaĥ Ke-hilkhato 8:2 and Piskei Teshuvot 451:44.
See Hagalat Kelim 13:433 regarding koshering a countertop with alcohol. We do not condemn one who uses this method, as long as he cleaned the countertop well, since be-di’avad cleaning a countertop, whose primary usage is cold, with cold water is effective. Cleaning with the alcohol alone, however, is ineffective.