An electrofax is an electrostatic printer and copier technology where the image is formed directly on the paper, instead of first on a drum (and then transferred to paper) as it would be in xerography. It was used in the United States from the 1950s through the 1980s.
The paper used in this process is coated with a zinc oxide powder, adhered with a resin in order to make it able to hold an electrostatic charge, absorb toner to form an image, and allow the evaporation of toner dispersants. Users of electrofax machines purchased such paper with the coating already applied.
In copiers, the paper, typically fed from a roll, is given a static electric charge, after which it is exposed to light reflected from the original document and focused through a lens; the zinc oxide particles either preserve or discharge the electric charge, depending on the amount of light which reaches them. After exposure, the paper is passed through a toner station where toner, typically carbon black suspended in an organic liquid known as a dispersant, is spread over the paper. Where charged areas remain on the paper, the black toner particles adhere; where the paper is discharged, they do not. A knife then cuts the paper to the proper length (typically letter or legal size). The now free sheet of paper then passes out of the toner station where excess dispersant is wiped off. Typically, the paper is sent to an output tray where any remaining dispersant evaporates, leaving the copies with a faint "kerosene" odor.
In printers and plotters, the paper is typically electrostatically charged by passing it over bar containing hundreds or thousands of charging contacts. The image is formed by either applying or removing a charge from each contact individually as the paper passes over it. This leaves a grid of charged dots on the paper. Toner is then applied as described above.
This technology was first developed at RCA (Radio Corporation of America) in the early 1950s. Later, many office machine companies, including SCM (Smith Corona Marchant), Savin, etc. introduced copiers that utilized it. Versatec was one brand of computer printers and plotters that also had machines that used this process. Electrostatic printers like the Versatec were important stepping-stones to later laser printers. Programs like vtroff, by Tom Ferrin at UCSF, would go on to justify early more expensive laser printers.
Copying machines using electrofax were common from the 1960s through the 1980s. During that time, they were less expensive to manufacture than xerographic copiers, although the paper was slightly more costly than the plain paper used by xerography. As soon as other copier technologies could produce markedly better quality copies and became less expensive, electrofax fell out of favor. Electrofax, by comparison, suffered from a number of drawbacks, including weak blacks in the image (most machines could only produce a dark gray), the dampness and odor of the copies, the need for special paper, and multiple-bottle liquid toner replacement. Likewise, as laser printers became cheaper, followed by inkjet printers, the need for electrofax based printers & plotters faded.