Xerography is a dry printing process that uses static electricity and light to transfer images onto paper. The word comes from the Greek for “dry writing,” and it’s the technology behind every office photocopier and laser printer you’ve ever used. Invented in 1938, it replaced messy, chemical-based copying methods and became one of the most commercially important technologies of the 20th century.
How Xerography Works: Six Steps
Every xerographic copy, whether from a 1960s office copier or a modern laser printer, follows the same six-step cycle. The entire process happens in seconds, but each stage relies on a precise interaction between light, electrical charge, and heat.
1. Charging
The process starts with a drum coated in a special material called a photoconductor. This material has an unusual property: it acts as an insulator in the dark but becomes electrically conductive when exposed to light. Before anything else happens, the drum’s surface receives a uniform electrical charge. A device called a corona wire ionizes nearby air molecules, and those charged molecules coat the drum evenly, giving it a static charge across the entire surface.
2. Exposure
Next, a bright light (or in laser printers, a laser beam) projects an image of the document onto the charged drum. Wherever light hits the drum, the photoconductor becomes conductive and the charge drains away to a grounded metal layer beneath the surface. The dark areas of the original document, like the black ink of printed text, block the light, so those corresponding areas on the drum stay charged. What remains is an invisible pattern of electrical charge that mirrors the original document. This is called the latent image.
3. Development
The latent image is invisible, just a pattern of charge on a drum. To make it visible, fine plastic powder called toner is brought close to the drum. Toner particles carry their own electrical charge, which is opposite to the charge on the drum’s image areas. The charged regions of the drum attract and hold toner particles, while the discharged regions repel them. The result is a visible powder image sitting on the drum’s surface, matching the original document.
4. Transfer
A sheet of paper passes against the drum, and a second corona wire on the back side of the paper gives it a charge strong enough to pull the toner off the drum and onto the paper’s surface. At this point, the toner is just sitting loosely on the paper. If you touched it, it would smudge right off.
5. Fusing
The paper passes through a pair of heated rollers that melt the toner particles and press them into the paper fibers. Fuser temperatures typically range from 175°C to 215°C (347°F to 419°F), depending on the type of paper or media being used. Coated paper stocks require higher temperatures, while transparencies use the lower end of that range. This is the step that makes the image permanent and gives freshly printed pages their characteristic warmth.
6. Cleaning
Not all the toner transfers perfectly to the paper. Residual particles left on the drum would interfere with the next copy, so scraper blades or rotating brushes wipe the drum clean. An erase lamp floods the drum with light to eliminate any remaining latent image, and a second corona unit with opposite polarity neutralizes lingering surface charges. The drum is now back to its starting state, ready for the next cycle.
How Toner Actually Works
Toner is not ink. It’s a fine powder made of thermoplastic resin mixed with pigment, usually carbon black for standard black printing. The particles are tiny, typically 5 to 10 micrometers in diameter, and they pick up an electrical charge through friction when mixed with larger particles called carrier beads inside the printer’s development unit. This charging process is somewhat statistical in nature, meaning individual toner particles end up with slightly different charge levels. Most particles get the correct charge polarity, but a small fraction end up with the wrong sign, which is one reason why xerographic prints aren’t perfectly clean at the microscopic level.
Toner particles charge up until the electric field at their surface reaches roughly 10 volts per micrometer. This value varies by material but stays within a narrow range, giving engineers a predictable parameter to design around. The specific charge a toner acquires depends on the chemical makeup of its surface, which is why toner formulations are carefully controlled mixtures rather than simple powders.
The Invention of Xerography
Chester Carlson, a patent attorney and part-time inventor, came up with the idea for xerography in the mid-1930s. He was frustrated by the slow, expensive process of copying patent documents by hand and saw an opportunity in the physics of photoconductivity and electrostatics. After filing a patent application in October 1937, he set up a small laboratory in Astoria, Queens, and hired a German refugee physicist named Otto Kornei as his assistant.
On October 22, 1938, the two men created the first xerographic image. They coated a zinc plate with sulfur (a photoconductor), then wrote “10-22-38 Astoria” in India ink on a glass microscope slide. After charging the plate, exposing it through the slide, and dusting it with powder, they found a near-perfect duplicate of the inscription transferred onto the plate. It was a crude setup, but the principle worked exactly as Carlson had predicted.
Turning that lab demonstration into a commercial product took another two decades. More than 20 companies, including IBM and General Electric, rejected Carlson’s invention before a small photographic paper company called Haloid took interest. Haloid eventually renamed itself Xerox, and in 1959 it released the Xerox 914, the first automatic plain-paper copier. The machine could produce 100,000 copies per month and was fast, economical, and simple enough for any office worker to operate. It became one of the most successful commercial products in history, fundamentally changing how offices handled documents.
Xerography vs. Inkjet Printing
Xerography and inkjet printing achieve the same end result through completely different physics. Inkjet printers spray tiny droplets of liquid ink directly onto paper. Xerographic devices (laser printers and copiers) never use liquid at all. They rely entirely on dry powder and electrostatics.
This distinction has practical consequences. Xerographic prints are water-resistant as soon as they leave the machine because the toner has been melted into the paper fibers. Inkjet prints can smear when wet unless special coated paper or pigment-based inks are used. Xerographic machines also tend to print faster at high volumes, which is why office environments with heavy printing needs almost always use laser printers. Inkjet printers, on the other hand, handle photographic color better and cost less upfront, making them more common in homes.
Ozone and Air Quality Concerns
The corona wires that charge the drum also produce a small amount of ozone as a byproduct, because the high voltage ionizes oxygen molecules in the surrounding air. Ozone at low concentrations can irritate the eyes, nose, throat, and lungs, and may cause headaches.
Occupational safety limits set by OSHA cap ozone at 200 micrograms per cubic meter (0.10 parts per million) as a ceiling that should never be exceeded. The American Conference of Governmental Industrial Hygienists recommends a stricter maximum of 100 micrograms per cubic meter. Modern copiers and printers include ozone filters and use lower-emission charging methods (like charge rollers instead of corona wires) to keep levels well below these thresholds. Laser printers also release small amounts of volatile organic compounds from toner heated during the fusing step, with toluene and styrene among the most common. Placing copiers and printers in well-ventilated areas reduces exposure to both ozone and these compounds.
Where Xerography Is Used Today
Xerography extends well beyond the office copier. Digital printing presses use the same electrostatic principles to produce short-run books, marketing materials, and packaging. Large-format xerographic printers handle architectural drawings and engineering blueprints. Multi-function office devices that scan, copy, fax, and print all rely on the same six-step cycle Carlson demonstrated in 1938, just executed with far more precision and speed. The core physics have remained remarkably stable for over 80 years, even as the surrounding electronics have gone fully digital.