Adam VoorhesToner is one of those everyday products we all take for granted. When the printer runs low you pop a new cartridge in—out of sight, out of mind. Well, we got to wondering what’s actually in that cartridge … so we busted one open. Bad idea! (More on that later.) But we’re all cleaned up now and back with answers.
Turns out toner is mostly powdered plastic—and that’s key to the whole technology. Plastic has two handy properties: You can move it around like magic with static electricity, and then you can melt it onto the paper for crisp, smudge-proof images. This technique of printing with powder instead of ink is called xerography (xeros is Greek for “dry”), and it works the same whether you’re printing or copying. In fact, Gary Starkweather invented the laser printer at Xerox in 1969, in a famous bit of rogue engineering, by modding one of the company’s office copiers. (He had to work in secret after his boss ordered him to drop the idea.)
See, a photocopier has a rotating drum that’s coated with a semiconductor like selenium; that coating converts light into electricity, just like in a solar cell. By bouncing bright light off a hard copy (… or select parts of your anatomy) and onto the drum, it creates a ghostly reflection of the original in static charges for toner to stick to. Starkweather realized you could use the same rig to print digital files by scanning a laser directly on the drum. The only difference is in how the electrostatic image is generated.
Aside from plastic, early printer toners in the 1970s contained little more than soot and rust. The latter—iron oxide—made it magnetic, for better control in the imaging process. That wouldn’t work for color printing, which came along in 1994; the dark oxide would have turned the colors brown. But manufacturers have come up with other additives and refinements to improve speed and image quality. Actual formulations are custom-designed for specific machines, so ingredient lists can vary. But here’s the basic recipe for most newer printers.
Color toners are 85 to 95 percent plastic, milled into a superfine powder; the smaller the grains, the better the image resolution. Because plastic doesn’t conduct electricity, the particles can hold a static charge—and like socks in a dryer, they’ll cling to anything with an opposite charge. Laser printers use that clinginess to get the toner onto the imaging drum, and from there onto a sheet of paper. The page then goes through hot fuser rolls that melt the plastic and smoosh it into the paper fibers. A variety of polymers can be used, but polyester, the stuff of disco suits and soda bottles, is the top choice nowadays. It’s pricier than the old standby, styrene acrylate, but it makes for bright colors, smells less toxic, and has a lower melting point, which saves energy and lets the machine run faster. Just handle those cartridges with care: Toner spills are a mess, and inhaling tiny airborne particles can do a number on your lungs. Oh, and don’t wash your pants in hot water; that low melting point will turn your cotton Dockers into a polyester blend.
The first xerographic copiers in the early '60s used radiant heat, like toaster ovens, to melt the toner onto the page; unfortunately, the boss’s memos sometimes caught on fire. (Xerox’s flagship model came with a small extinguisher.) Fuser rolls fixed that problem but caused a new one: Toner would stick to the rollers and smudge the next page. The solution? Add polypropylene wax for lubrication. It’s a polymer like polyester, but its long carbon strands have fewer chemical gewgaws hanging off them, so the molecules can easily slip and slide past one another.
Polyester is clear. To make it look black, manufacturers stir in this grimy stuff—essentially high-purity soot. Made by burning tar or creosote, carbon black is mainly used to toughen up rubber products; it’s why tires are black. It’s also a class II carcinogen, but once the melted plastic hardens on your copies it’s safely sealed in place. Chemically, it’s a jumble of carbon atoms over which float clouds of shared electrons. Because these electrons have lots of room to move, they can absorb light energy at all visible wavelengths. The result: No light reflects back to your retina, an absence your brain calls “black.” (If you think about it, you can’t actually see these words. You’re inferring their shape from the white space around them.)
Along with black, color printers have separate cartridges for yellow, magenta, and cyan toners, and these four can be overlaid to make any other hue. The yellow comes from this benzimidazolone compound. Like all organic pigments, it has alternating single and double bonds that again leave electrons free to absorb light—but not all of it. Here, short-wavelength violet light is trapped while longer-wavelength yellow passes through, bouncing off the page and into your eyeballs.
Compounds of quinacridone produce a range of intense reddish hues, depending on their exact makeup and arrangement. They’re super durable, which is why they’re favored in exterior paints—think cherry-red sports cars. In Red 122 (2,9-dimethyl-quinacridone), the flat molecules are stacked up like dinner plates in a neat crystal structure; that shifts the reflected color toward the blue end of the spectrum, yielding magenta.
Copper phthalocyanine produces cyan, a rather alarming hue midway between green and blue. Surgical gowns are made in this color because it’s complementary to crimson (blood spatters on cyan look black). This common pigment is also used as a thin-film semiconductor in solar cells. It could even power quantum computers someday, since its electrons can hang out in a state of superposition for long periods.
Microscopic glass beads (SiO2) on the surface of the toner particles provide a silky, almost liquid flow. That’s essential to spread toner over the page at the maniacal speeds of modern office printers. It’s especially needed in polyester toners, which are more prone to caking. Fun project: Make your own fumed silica by vaporizing beach sand in a 3,000-degree-Celsius electric arc.
As the toner leaves the cartridge, it brushes against a metering blade, which gives it a static charge. Scientists call that triboelectrification, and it’s what actually makes those socks cling in the dryer or a balloon stick to the wall after you rub it on your sweater. You’re literally scraping electrons from one material onto another (tribo- means to rub, same root as in diatribe). Here, putting a negative bias on the toner makes it cling to the imaging drum, and added bits of iron, chromium, or zinc help boost and hold the charge. Pro tip: If you ever do spill toner, don’t try to vacuum it up. Without special gear, all that agitation can spark a violent, albeit colorful, dust explosion.
Lee Simmons (@actual_self) is an editor at WIRED.
A shorter version of this story by Lee Simmons and Kaitlin Duffey ran in the March 2013 issue of WIRED magazine. Special thanks to John Cooper, Toner Research Services.
