One thing that religious Jews know is that every kosher kitchen needs two sets of dishes. One set for meat foods (Yiddish: fleishik, פֿליישיק; Hebrew: basari, בשרי), and another set for dairy foods (Yiddish: milkhik, מילכיק; Hebrew: halavi, חלבי.)
Food that is neither meat nor dairy is considered neutral (Yiddish פאַרעוו ; Hebrew פרווה) and may be eaten with either type of food, on either set of dishes.
But although this is common knowledge, is it absolutely correct? Do observant Jews always need two sets of dishes? Could it be permissible to clean meat dishes and then make them ritually useful for milk/dairy dishes? Apparently, yes!
Rabbi Haim Ovadia , based on the teshuvot of Rabbi Ovadia Yosef, writes
The requirement to make dishes Kosher was originally mentioned in the Torah regarding dishes looted from non-Jews at war. It is based on the notion that the flavor of the non-Kosher food was absorbed in the dishes. If they will be used without Koshering, the flavor will be exuded into the food and will make it non-Kosher. There is a simple rule which governs the process of Koshering dishes: פולטו כך כבולעו – It exudes the same way it absorbs.
The meaning of this rule is that if a certain dish is used with boiling liquids, the flavor should be extracted from it by using boiling water. If it is used directly on the fire, it has to be heated, empty, to a higher temperature. There is an extensive discussion in the Halakhic literature regarding the fine details of this requirement, but they all depend on the assumption that the dish absorbed flavor from the non-Kosher food. This factor must be examined in order to determine how to make dishes kosher.
There could be several circumstances when koshering would be necessary, for example: for Pesah, in case of a mixture of dairy and meat, when staying at a hotel room or at an AirBnB.
When discussing material quality of dishes in the modern kitchen, we have to take into consideration the tremendous progress achieved in the field since the industrial revolution. Some materials, such as clay, were used in antiquity but are not used today, while others, such as stainless ateel, Teflon, Bakelite (poly-oxy-benzyl-methyl-englycol-anhydride), plastic, and Pyrex, are new inventions.
The Halakhic literature recognizes two systems for determining koshering method. One is the absorbent/non-absorbent divide, and the other is by material: wood/metal/clay and so on. Since modern materials differ from their namesakes in antiquity, it is understood that when we want to define the status of a certain dish, we should use the general halakhic yardstick of absorption and not the material label for the dish.
What we need to know about physical changes in the absorption of dishes
Brian Yehuda Tebbit writes about rabbinic opinions which hold that separate sets are not really necessary unless the keilim (utensils, plates, etc.) involve: adobe, other non-glazed forms of earthenware, rough cast metals, iron cast and non-stick cookware, or wood plates.
Do we really need two sets of stainless steel pots? No.
According to studies carried out by qualified people known to Rabbi Dov Lior and Rabbi David Bar Hayim, the rate of absorption of particles into steel is so minuscule that it is on the atomic level and therefore certainly not halachically significant.
The upshot of this is that you need use only one steel pot for both milk and meat cooking (just use hot water and soap and possibly wait a day in between). If travelling, or with non-observant family, you may feel free to use their steel pots for cooking.
In this lecture, Rabbi David Bar Hayim notes that stainless steel only requires a good thorough cleaning, without requiring separate milk and meat sets.
Do we need a separate set for Passover?
According to the ruling of the Shulkhan Aruch glass need not be changed for Pesach and glass dishes can be used for milk and meat interchangeably. This view is based on the concept “hachush me’id ” – an observation of the empirical reality, which shows that glass simply is non-porous and non-absorbent.
On the other hand, Rabbi Moses Isserles, the רמ״א (Rama), rules that because glass is produced from sand, it has the equivalent status of a kli cheres (earthenware vessel) and it is therefore absorbent. This ruling is made even though this ignores empirical, observable reality. As a result, Ashkenazim tend to go through the trouble and expense of having a set of glass for milk, meat, and also a set for Pesach.