How hot can a fridge magnet get?

By Fridge Magnets Editorial

Ceramic ferrite works to about 250 C. Neodymium N42 stops at about 80 C, and above that the loss is permanent. A fridge door never approaches either figure. An oven front, a hob splashback and a car in summer sun can, and the print and the adhesive usually give up before the magnet does.

A fridge door is the dullest thermal environment in the house, which is exactly why this question only gets interesting once a magnet leaves it. The appliance exists to be cold, its outer skin sits near room temperature, and a magnet on it will outlast the fridge.

The material sets the ceiling. Ceramic ferrite has a maximum working temperature of about 250 C, and neodymium N42 has one of about 80 C. Everything after that is working out which surfaces in a house sit between those two figures, and the answer is fewer than people expect.

What heat does inside the material

A permanent magnet holds its strength because the magnetic domains inside it point the same way. Heat is atomic motion, and enough of it knocks domains out of alignment.

Some of that is reversible. Warm a magnet and it holds slightly less; cool it and the strength comes back. That is a temporary loss and it happens to every magnet, all the time, in both directions.

Past the maximum working temperature, some of the domains that go out of alignment stay out. That is irreversible loss, and the magnet cools down permanently weaker than it went in. It looks identical, it has no visible damage, and a 2,500 g disc that came off a hot surface might now be a 2,000 g disc with no way of telling except by measuring it. Higher again is the Curie point, where the material loses its magnetism completely, but nothing in a kitchen gets near that.

Note: a grade suffix such as N42H or N42SH marks a high-temperature version of the same magnet. It costs more, it holds the same or slightly less at room temperature, and on a fridge door it is a waste of money. On a hob splashback it is the only version worth buying.

What a kitchen actually reaches

Nobody publishes surface temperatures for domestic kitchen fittings, and we have measured none of them, so this section deals in what can be reasoned rather than in figures we would have to invent.

The fridge door and its side panels stay near room temperature. Nothing to think about. The magnets on them are unaffected by anything except sunlight and the print fading, and an N42 disc there loses roughly 1 percent of its strength per decade, which is the subject of how long magnets last rather than a heat problem.

An oven front while the oven is running is the first surface where the answer changes. The outer door of a modern oven is designed to stay touchable, but the trim above the door and the panel around the controls run hotter, and the vent that exhausts across the top of the door is hotter still. Ferrite has 250 C of headroom and will cope. Neodymium has 80 C, which is not a comfortable margin against a surface you would not rest your palm on.

A hob splashback and an extractor hood are the same argument with more variation. A steel hood directly over a gas burner receives the whole plume. A tiled splashback two feet to the side does not. The magnet on the hood is where the problem lives, and it should be ferrite.

Anything on top of a fridge under a window is a distance-and-sun question rather than an appliance question, and it belongs with the car case below.

Cars and sun-facing glass

A car parked in the sun is the harshest place a fridge magnet routinely ends up, because a closed car in summer sun runs far above the outside air temperature and a dark dashboard runs above the cabin. This is where promotional magnets on a car body panel and souvenir magnets forgotten in a glovebox get their real test.

The ferrite in a sheet magnet or a photo magnet is not the weak part. The vinyl binder in a flexible sheet magnet softens, and a sheet magnet on a hot car panel will curl at the corners and lose contact along the edges, which is how the vehicle door magnets you see peeling off get that way. Flexible sheet magnets has more on the material.

A neodymium magnet left in a car in summer is a genuine gamble. It will not visibly fail. It will just come out of the season holding less than it did going in, and the loss will be attributed to a cheap magnet rather than to the parking space.

Direct sun through a window is the same problem in a kitchen. A magnet on a south-facing door in a conservatory kitchen sits in a hot spot for several hours a day, every day, and the accumulated effect on a neodymium disc is worth avoiding by putting a ferrite one there instead. The other casualty is the picture, which fades in exactly the way any print in a window fades.

The print and the glue fail first

This is the part people miss. In almost every real case the magnet outlives the thing attached to it.

A pressed photo magnet is a clear mylar cover, a photo print, a crimped steel shell and a flat ferrite magnet, about 4 mm thick overall. The ferrite is good to 250 C. The mylar and the print are not, and neither is whatever holds the layers together. Heat that leaves the magnet untouched will yellow the cover, lift a corner of the print, or soften the crimp seal enough to let moisture in later.

Flexible sheet magnets are ferrite powder held in vinyl. The vinyl is the limit and it is nowhere near 250 C. A laminated print on the face of one delaminates in heat, and the sheet takes a permanent curl.

Cast resin souvenirs are glued to their magnets, and adhesive is almost always the first thing in the stack to give way. A souvenir that has spent a summer on a sun-facing door will often part company with its magnet in autumn, leaving a bare disc still stuck to the fridge and a decorative body on the floor.

Adhesive-backed magnets on car panels and shop windows fail in the same order. Glue, then print, then the magnet, and the magnet usually never gets its turn.

The buying rule falls out of that table. On anything warm, choose ferrite and accept the weaker hold, since a 25 by 5 mm C8 disc still manages 1,100 g on bare steel and that is more than enough for paper. Save neodymium for the cold surfaces, where its 2,500 g at 12 by 3 mm is not going anywhere. Neodymium against ferrite sets the two materials against each other on the rest of the specifications.