Is a person allowed to use the same microwave for meat and milk? What about meat and fish?
I'm really surprised how few people have heard of the Star-K's potato test (scroll down to the section on microwaves). Although it's in reference to Chameitz, my Rebbeim have all said it applies to Basar B'Chalav as well.
In it, one takes a potato and bakes it in the microwave. When it begins steaming, open the door and immediately put your hand on the microwave ceiling. If you can hold it there for 15 seconds, it does not absorb, and you can cook meat and milk, or Pesach and Chametz, in the same microwave, provided that they're not in the microwave at the same time (including splatters). If it fails the test, then the microwave is either milchig or fleishig, depending on what you put in.
From page 233 the Laws of Kashrus by Rabbi Binyomin Forst.
D. Microwave ovens
A microwave oven is essentially different than other ovens. Gas and electric ovens produce heat which is transmitted to the food. A microwave oven works in the reverse: the food is heated and in turn heats the area surrounding it. Despite this distinction, a microwave oven shares the problems of other ovens. Foods may come in contact with residue of other foods if the oven surface is not clean. Two foods cooked simultaneously can absorb aroma (reicha) from one another. In addition, the most prevalent issue, a microwave may cause foods to ab- sorb taste through steam zeiah. Foods steam quite quickly in a micro- wave oven (indeed, the moisture in the foods is the first thing to heat) The steam rises and visibly condenses into droplets on the top of the oven. The concern that these droplets will subsequently fall onto the food is genuine. The question of whether a microwave oven effects act- is moot for this purpose. Our concern is with zeiah, regardless of its source. One distinction, however, between a microwave and other ovens is the fact that the wall of the oven generates no heat; rather, all heat is derived from the food itself. Thus, the steam reaching the top of the oven is often no longer hot. Nevertheless, if food is heated for an extended period of time, even the oven top becomes hot from the vapor In many aspects a microwave is worse than other ovens: a) The area is small and confined, thus, the steam is not vented and has no means of escape and is therefore prone to condense on the oven top.se b) The moisture in all foods steams very quickly, thus even solid foods emit zeiah (e.g., a potato baked in a microwave produces droplets of steam) c) Foods splatter on the oven top and sides quite frequently.
Taking these facts into account, it is suggested that the following procedure be followed in using a microwave oven for both meat and dairy foods.
1) Cleanliness: The oven must be kept clean from any food residue. As noted, foods cooked in a microwave oven splatter, boil over and drip One must inspect the floor, walls and top of the oven before use to ascertain that no residue remains from previous use.
2) The foods should be covered: All foods cooked in a microwave emit steam. Thus, meat or dairy foods placed in a microwave that is used for both should be covered. Perhaps parve solid foods (e.g., a potato) can be cooked without a covering
3) The bottom surface: One should not place meat and dairy dishes on the same surface. The microwave oven differs in this aspect from other ovens with regard to the following: F cooked in a microwave boil more quickly and tend to seep and spill Thus, the bottom surface has absorbed taste from both meat and dairy. Other ovens generate heat which quickly dries any food residue on the hot oven grate In addition, the pot is usually placed on a grate which does not permit liquid to collect. We noted earlier that taste cannot be transferred from vessel to vessel without the medium of liquid. In contrast, a microwave oven does not generate its own heat and the dish may rest on the flat oven bottom. As a result the oven bottom may become damp in the process of cooking. Placing a meat dish on this treifsurface may cause treif taste to be absorbed into the dish. The proper method of avoiding this problem is to use one plate under meat dishes and another plate under milk dishes. If possible, glass plates are desirable 38
4) Kashering a microwave oven: A microwave cannot be kashered in the manner of other ovens. Running the oven without food generates no heat to effect libun kal. Some permit kashering a microwave oven by means of first thoroughly wiping and cleaning all inside surfaces, waiting twenty-four hours and then boiling a bowl of water in the oven until the oven fills with steam. The steam can kasher a surface that absorbed taste likewise through steam 39