What is needed is a material that can be used as readily as a cold patch, but which works as well as a hot one. Larry Zanko and his colleagues at the University of Minnesota Duluth, think they know what it is. They are mixing asphalt with ground iron ore that contains magnetite—an iron oxide which, as its name suggests, is magnetic. A phenomenon called ferromagnetic resonance means that when magnetite is zapped with microwaves of an appropriate frequency it gets hot.

Dr Zanko and his colleagues built an experimental repair vehicle equipped with a microwave generator on the end of a hydraulic arm. Using this on asphalt that contained between 1% and 2% magnetite, he found he could heat the material in a patch to 100°C in about ten minutes. At that temperature it could be tamped down to produce a more effective repair. The heat also drives out moisture, further improving adhesion, says Dr Zanko. He and his team are now trying to raise the money needed to develop the technology into a commercial pothole-fixing system.

An even better approach, however, would be to stop potholes forming in the first place, by sealing the cracks that cause them before any damage is done. Etienne Jeoffroy of ETH Zurich, a Swiss university, has been attempting to do just that. He also mixes iron oxide into the asphalt, but in this case it does not start off magnetic. Rather, he uses a magnetic field to heat it.

The process he employs is one also used to treat certain tumours.