Slow Mo Guys Capture Billions of Sparks from Burning Wire Wool on Bike Wheel Using High-Speed Camera

Gavin Free and Daniel Gruchy returned to an old favorite, this time scaling the entire idea far beyond earlier versions. They loaded fine steel wool into small wire cages, attached those cages to a bicycle wheel, and spun the wheel at full speed after lighting the wool with a torch. Earlier attempts had relied on hand spinning or a simple fan; the bike wheel pushed the speed and volume of sparks into a different range, with the stated goal of filling frames with more sparks than the camera sensor held pixels.

Steel wool catches fire all too easily because the fibers that make up the stuff are incredibly thin. You’re looking at strands that are only tens of micrometres across – which basically means that almost every single iron atom is right up against the surface and primed to grab onto oxygen in the air the moment it gets the chance. When a torch lights things off, the heat gets trapped inside each tiny fibre, rather than spreading out into a bigger mass. That causes the oxidation reaction to go into overdrive, the iron starts to melt into tiny droplets, and those droplets break free in the form of bright orange sparks, and they just keep on coming. The chemistry going on here is the same basic chemical reaction as rust, but rather than taking weeks or months to show up, this one’s been speeded up to the point where you can see it lighting up in real time. A byproduct of that speed-up is the bright light and heat you can see.

Getting the thing mounted proved to be a major headache at first. The centrifugal force of the spinning wheel kept sending the cages flying off the edge, and that was only the beginning. One of the attachment points failed completely, and to make matters worse, the two of them had to go in and rebuild the entire setup from start. This time, they used tighter wire ties, spent their time getting the wheel perfectly centered, and even added some extra protection, in the form of microwave oven doors that they had taken out to use as shielding.

Even then, they had to put on fire-resistant suits, masks, and other protective gear. They were able to cover the people involved, but despite this, the spinning wheel suffered significant damage, with the gear cables melting, the frame becoming scarred, and the lens glass receiving permanent markings from stray sparks. The wheel was also going crazy, roaring and shaking all over the place, giving the impression that the entire operation was on the verge of losing control.

Footage from the Freefly Ember S2.5K at 500 frames per second shows the result in detail. The sparks leave the cages in the form of bright small dots, which spread out into these expanding rings as they move through the air. Sparks farther away begin to slow down, but those closer to the spinning wheel maintain a steady rate, creating fleeting illusions of faster loops.

The spray of sparks becomes so heavy at places that you lose all feeling of dark space, leaving just this sea of light to fill the frame. Sparks bounce off the bike frame and scatter all over the place, forming small glowy arcs that like tiny galaxies or sparkling filings. Sometimes the wheel simply vanishes against the cloud of sparks, leaving you with a dark shape against a background of increasing light.

Later versions managed to control the sparks by spinning them out through a six-foot cardboard circle. The sparks are emitted in a controlled ring, which you can see expanding or shrinking depending on where the camera is positioned, and it generates these really cool portal-like shapes when someone walks or leaps through it. Handheld images of the sparks reveal these tiny outlines on the edges, with individual sparks bouncing off clothing or flesh.

One of the clips even shows the sparks climbing all the way to the roof of a barn before reaching out to touch the moon. Secondary sparks occur on the ground as larger fragments break off. The sparks just keep pouring, no matter where you look, transforming a normal photography trick into a continuous, dazzling field that lasts long after you’ve given over the camera.