The local grocery store sells pre-made meals in aluminum trays with a plastic cover. Verde enchiladas, mac and cheese, that sort of thing. The label says: “Heating instructions: Aluminum food containers are safe to be used in microwaves made after 1980.”
So what changed in 1980? And why do I get plasma bolts if I stick some aluminum foil in the microwave, where this metal tray (example) is fine to use in the microwave?
Microwaves now often have metal racks. I guess they figured out a way to make an alloy that is OK in a microwave.
Nope, this tray is straight-up aluminum. The label says it is.
Is it safe because the aluminum tray is sitting on the floor of the microwave, so you’re not going to get plasma arcing?
Opposite of that, it should be a on the rotating tray to keep it out of contact with the walls of the microwave. Notice the plastic top also. I don’t remember why but it is supposed to be a problem if the aluminum tray was also covered with aluminum foil. I have seen different reports on the danger to the microwave from metal objects arcing, some claim it’s not a big problem but no one recommends it. This is not the same thing as grape plasma which is encouraged.
It may not list the alloy, but I guarantee it’s not straight aluminum. That shouldn’t matter re: arcing – I expect the shape to be much more important.
Great cites, thank you!
I don’t think it’s clear why the instructions in the OP specified microwave ovens made after 1980.
Who has a 40 year old microwave oven?
But clearly the design or capabilities of microwaves changed around then.
Think of the metal as a reflector. If it is under the food, it won’t stop the food from heating up, and any microwaves reflected by the metal will be reflected back into the food above it. Cover or wrap your food with metal like aluminum foil and it reflects the microwaves back at the electronics potentially causing damage to the oven, but at the very least, preventing your food from being warmed or cooked.
That said, my WAG is that really old microwave ovens heated the interior from multiple directions, thus making metal problematic regardless of its relation to the food.
The 1980 changes may be the result of the 1975 Energy Policy and Conservation Act and the corresponding energy efficiency improvement targets set by the Federal Energy Administration. See Federal Register, Volume 42, No. 136, Friday, July 15, 1977, starting at page 36648.
“The targets are required to be designed so as to identify the maximum improvement in the energy efficiency of appliances determined by FEA to be technologically and economically feasible by 1980.” (Page 36648)
“The microwave oven design options that were selected to demonstrate the technological feasibility of the proposed target include improved oven linings, improved power sources, reduction in the power requirements of features such as lights and timers in the microwave oven, and improved coupling between the magnetron and the food.” (Page 36658)
I was surprised at this statement, because I always thought that aluminum foil was pure aluminum (with a microscopic oxidation layer of aluminum oxide).
But after doing some research, aluminum foil makers are indeed evidently using aluminum alloys to increase strength. Accordingly to the manufacturer, “Reynolds Wrap is made from 8111 alloy aluminum, at the thickest gauge specifications available in the marketplace. Reynolds Wrap® Aluminum Foil is 98.5% aluminum. The balance is primarily iron and silicon.”
I’d be surprised if any practical metal product, of any sort, is made of a pure element, without any deliberate alloying at all (discounting bullion coins and the like as not being practical).
That said, 98.5% is pretty close to pure.
Thanks for the cites. I was rather mystified when a few years ago one of my favorite grocery stores – the one with exceptionally good prepared foods – started packaging all of them in aluminum trays that they claimed were microwave-safe. To this day I have never actually used them in my microwave because of the major hassle it would be if anything bad happened – and the cite mentions some gotchas, like the fact that all or most of the bottom of the aluminum tray should be covered with the food, so microwaving a half-used portion could be bad news. Many of the foods actually do better in the oven anyway, especially things that require browning like lasagna or other casseroles. For the rest, it’s just as easy for peace of mind to dump them into a ceramic casserole dish.
OT, but the other precaution I take (learned the hard way from my previous microwave) is when microwaving popcorn, place the bag (correct side up) on a ceramic dish, not directly on the rotating glass platform. If you put it directly on the glass, there is some non-zero chance it may crack it even though it’s supposed to be heat-resistant Pyrex or equivalent, because the oil in the bag gets really, really hot over a small area. The glass will probably be OK, but why take the chance? The one in my old microwave was fine for the first 10 or 30 bags of popcorn, then one evening it decided to explode.
Lead pipe I believe would be once such product.
It wouldn’t have necessarily surprised me to find that consumer-grade aluminum foil was “only” 98.5% pure, with the balance due to manufacturing impurities. What surprised me was that the manufacturer was intentionally alloying the foil with a small amount of added iron and silicon. For some reason I always thought that aluminum foil was pure aluminum (other than manufacturing impurities, of course).
Lead pipe is, of course, no longer used for drinking water (or much else), due to its toxicity.
Copper pipe, on the other hand, is still commonly used. According to this, copper pipe has a minimum purity of 99.9%.
Although the additional elements are very small percentages virtually all aluminum is intentionally alloyed to provide specific properties.