How printing works
by Chris Woodford. Last updated: March 16, 2023.
It's amazing that you're sitting reading these words at your computer; back in the 15th century, it would have been just as amazing to be reading
them in a book. That was when printing technology hit the big time
and the invention of the modern printing press made it possible for
books to be reproduced in their hundreds and thousands instead of
being copied out laboriously, one at a time, by hand. Although
newspapers, books, and all kinds of other printed materials are now
shifting online, printing is just as important today as it's ever
been. Look around your room right now and you'll see all kinds of
printed things, from the stickers on your computer to the T-shirt on
your back and the posters on your wall. So how exactly does printing
work? Let's take a closer look!
Photo: Potato printing: This is printing the way most of us learn it. It's an example of relief printing in which the ink is applied to a raised surface (the parts of the potato surface that haven't been cut away) before the paper is pressed onto it.
What is printing?
Printing means reproducing words or images on paper, card,
plastic, fabric, or
another material. It can involve anything from making a single
reproduction of a priceless painting to running off millions of
copies of the latest Harry Potter. Why is it called printing? The
word "printing" ultimately comes a Latin word, premĕre, which
means to press; just about every type of printing involves pressing
one thing against another.
Although there are many different variations, typically printing involves
converting your original words or artwork into a printable form,
called a printing plate, which is covered in ink and then pressed
against pieces of paper, card, fabric, or whatever so they become
faithful reproductions of the original. Some popular forms of
printing, such as photocopying and
inkjet and laser printing, work
by transferring ink to paper using heat or static electricity and we
won't discuss them here; the rest of this article is devoted to
traditional printing with presses and ink.
Photo: A typical old-fashioned, wooden printing press, as used by none other than Benjamin
Franklin around 1730. Photo from Carol M. Highsmith's America Project in the Carol M. Highsmith Archive,
courtesy US Library of
Congress.
Printing is hard, physical work so it's usually done with the help of a
machine called a printing press. The simplest (and oldest) kind of
press is a large table fitted with an overhead screw and lever
mechanism that forces the printing plate firmly against the paper.
Hand-operated presses like this are still occasionally used to produce small volumes of printed materials.
At the other end of the scale, modern presses used to print books,
newspapers, and magazines use cylinder mechanisms rotating at
high-speed to produce thousands of copies an hour.
Animation: How a traditional printing press works. 1) You put the original item you want to print from (typically metal type, black) face up on a table (light gray) and cover it evenly with ink (blue). You put the paper you want to print onto in a wooden frame and slide it along the table under the press. 2) The press consists of two blocks (dark and light gray) held together by a screw mechanism supported by a sturdy wooden frame (brown). 3) When you turn the lever, the lower block, called the platen (4), screws downward and presses the frame, containing the paper, tightly and evenly onto the inked type (5). Finally, you loosen the screw and remove the printed paper from the frame.
Types of printing
The three most common methods of printing are called relief (or letterpress),
gravure (or intaglio), and offset. All three involve transferring
ink from a printing plate to whatever is being printed, but each one
works in a slightly different way. First, we'll compare the three
methods with a quick overview and then we'll look at each one in
much more detail.
- Relief is the most familiar kind of printing. If you've ever made a
potato print or used an old-fashioned typewriter, you've used
relief printing. The basic idea is that you make a reversed,
sticking-up (relief) version of whatever you want to print on the
surface of the printing plate and simply cover it with ink. Because
the printing surface is above the rest of the plate, only this part
(and not the background) picks up any ink. Push the inked plate
against the paper (or whatever you're printing) and a right-way-round printed
copy instantly appears.
- Gravure is the exact opposite of relief printing. Instead of making a raised
printing area on the plate, you dig or scrape an image into it (a
bit like digging a grave, hence the name gravure). When you want to
print from the plate, you coat it with ink so the ink fills up the
places you've dug out. Then you wipe the plate clean so the ink is
removed from the surface but left in the depressions you've carved
out. Finally, you press the plate hard against the paper (or other
material you're printing) so the paper is pushed into the inky
depressions, picking up a pattern only from those places.
- Offset printing also transfers ink from a printing plate onto paper
(or another material), but instead of the plate pressing directly against the paper, there is an
extra step involved. The inked plate presses onto a soft roller,
transferring the printed image onto it, and then the roller presses
against the printing surface—so instead of the press directly
printing the surface, the printed image is first offset to the
roller and only then transferred across. Offset printing stops the
printing plate from wearing out through repeated impressions on the paper,
and produces consistently higher quality prints.
Photo: The three most common types of printing: Left: Relief—Raised parts of the printing block (gray) transfer the ink (red) to the paper (white rectangle with black outline at the top) when the two are pressed together. Middle: Gravure—Grooves dug into the printing block transfer the ink to the paper when the paper is pressed tightly into them. Right: Offset—A rotating cylinder (blue) transfers ink from the printing plate to the paper without the two ever coming into contact.
Relief printing
For over 500 years now, most high-volume, low-quality printed material has been
produced with letterpress machines, which are more or less
sophisticated versions of the printing press Johannes Gutenberg
invented back in the 15th century. In the simplest kind of
letterpress, known as a flatbed press or
platen press, the paper is
supported on a flat metal plate called the platen, which sits
underneath a second flat plate holding a relief version of the item
to be printed (the printing plate, in other words). The printing
plate is covered with ink (either by hand, with a brush or by an
automated roller) before the paper is pressed tightly against it and
then released. The process can be repeated any number of times.
Photo: The keys in an old-fashioned typewriter produce images on paper by relief printing. When you press a key, these metal type letters flip up and press a piece of inked fabric against the paper. The letters are cast in reverse so, when they hit the paper, the printed impression comes out the right way round. Typewriters like this are now largely obsolete, but great fun to use—if a little noisy—when you can find them!
Flatbed presses are generally the slowest of all printing methods, because
it takes time to keep lifting and inking the printing plate and
loading and removing sheets of paper. That's why most
letterpresses use rotating cylinders in place of one or both of the
flat beds. In one type of machine, known as a flatbed cylinder
press, the printing plate is mounted on a flat bed that shifts
back and forth as a cylinder moves past it, inking it, pressing the
paper against it, and then lifting the printed paper clear again.
That speeds up printing considerably, but loading and removing the
paper is still a slow process. The fastest letterpresses, known as
rotary webfed presses, have curved printing plates wrapped
around spinning metal cylinders, which they press against paper that
feeds automatically from huge rolls called webs. Newspapers are
printed on machines like this, which typically print both sides of
the paper at once and can produce thousands of copies per hour.
Gravure printing
The simplest kind of gravure printing is engraving, in which an artist draws a
picture by scratching lightly on the surface of a copper plate that
has been thoroughly coated with an acid-resistant chemical.
Lines of shiny copper are revealed as the artist scrapes away. The plate is then dipped in acid
so the exposed copper lines are etched
(eaten much deeper into the metal) by the acid, while the rest of
the plate remains unchanged. The acid-resistant chemical is then
washed off leaving a copper printing plate, from which a number of
copies, called etchings, can be printed. Traditional engraving and
etching is quite a laborious process, so it's used mainly by
artists to produce relatively small volumes of (originally) hand-drawn pictures.
A similar but much quicker and more efficient process called
photogravure is used
commercially to produce large volumes of high-quality prints.
Instead of being slowly and painstakingly drawn, the image to be
printed is transferred photographically onto the copper printing
plate ("photo") and then etched into it ("gravure"). Once
the plate has been produced, it's used to make prints on either a
flatbed press (fed with single printed sheets) or a rotary web press
known as a rotogravure machine. Glossy magazines and cardboard
packaging containers are often printed this way.
Offset printing
The most common type of printing today uses a method called offset
lithography (typically shortened to "offset litho"), which is a
whole lot simpler than it sounds. As we've already seen, offset
simply means that the printing plate doesn't directly touch the
final printed surface (the paper or whatever it might be); instead,
an intermediate roller is used to transfer the printed image from
one to the other. But what about lithography?
Photo: A modern offset printing press used to produce small runs of a weekly newspaper. Note the final printed copy on the top roller and the offset cylinder in the middle just underneath it. Photo by Senior Airman Dilia DeGrego courtesy of US Air Force
and DVIDS.
Lithography literally means "stone-writing," a method of printing from the surface
of stones that was invented in 1798 by German actor and playwright Alois Senefelder. He took a large
stone and drew a design on it with a wax crayon. Then he dipped the
stone in water so the parts of the stone not covered in crayon
became wet. Next, he dipped the design in ink, so the ink stuck only
to the waxed parts of the stone and not the wet parts. So now he
had an inked "printed plate" (or printing stone, if you prefer)
that he could press against paper to make a copy. Lithography avoids
the need to make a traditional printing plate, as you need for both
relief and gravure printing.
Photo: A small offset printing press. Note the paper sheets feeding in from the left and the rollers that transfer the paper and copy the image. Photo by J. Pond courtesy of
US Department of Defense and
and
National Archives.
Modern offset lithography printing presses use an updated version of the same
basic idea in which the stone is replaced with a thin metal printing
plate. First, the image to be printed is transferred
photographically to the plate. The parts of the plate from which the
image is printed are coated with lacquer (clear varnish), so they attract ink, while
the rest of the plate is coated with gum, so it attracts water. The
metal plates are curved around a printing cylinder and press against
a series of rollers, which dampen them with water and then brush
them with ink. Only the lacquered parts of the plate (those that will
print) pick up ink. The inked plate presses against a soft rubber
(offset) cylinder, known as the blanket cylinder, and transfers its
image across. The blanket cylinder then presses against the paper
and makes the final print. High-speed offset lithography presses are
web-fed (from paper cylinders) and can produce something like 20km
(~12 miles) of printed material in an hour!
Other types of printing
Relief, gravure, and offset are used to print the overwhelming majority of
books, magazines, posters, headed stationery, and other printed
materials that surround us, but several other methods are used for
printing other things. T-shirt designs, for example, are usually
produced with a process called silk-screen printing (sometimes
called serigraphy). This involves covering the article to be printed
(something like a blank cotton shirt) with a mesh-screen and a
stencil, then wiping ink over the mesh with a brush. Ink transfers
through the mesh to the fabric below except where it's blocked
from doing so by the pattern on the stencil. Collotype (also called
photographic gelatin) is a less commonplace technique in which a
gelatin-coated printing plate is made from a high-quality original
using a kind of photographic method. It produces finely detailed
reproductions and is still used for making high-quality prints of
paintings.
Black and white, grayscale, and color printing
Photo: Halftones: Here's the same photo up above as a newspaper might print it using different sized areas of black ink. If you squint, or look from a distance, you can see that it looks like it's been printed with many different shades of gray, even though it's really using only one color of ink (black). In practice, newspapers use much smaller dots than this—we've exaggerated greatly so you can see how it works. Photo by Senior Airman Dilia DeGrego courtesy of US Air Force, with simulated halftone treatment by explainthatstuff.
Traditionally, printing presses used a single color of ink (black) to
produce basic black-and-white text, but printing photographs and artworks
was much more difficult because they really needed to be printed either with
many colors or many shades of gray. That problem was solved when people discovered how to simulate shades of gray using
what's called the halftone method. It's a simple way of converting photographs and drawings into images made from tiny
black dots of differing sizes to give the impression they're made from many different shades of gray.
In other words, it's a way of making a convincing gray-scale image using only black ink, and it relies on fooling your eyes
through an optical illusion.
To print in full color, you need to use at least four different inks: three
primary ink colors and black. Most people know that you can produce
light of any color by adding together different amounts of
red, green, and blue light; that's how a
television or LCD computer screen works.
Colored inks work in a different way by subtracting
color: they absorb some of the light that falls on them and reflect
the rest into our eyes—so the color they appear is effectively
subtracted from the original, incoming light. If you have an inkjet
printer with replaceable cartridges, you'll know that you can print
any color on white paper using the three colors cyan (a kind of
turquoise blue), magenta (a reddish purple), and yellow.
Theoretically, you can produce black with equal amounts of cyan,
magenta, and yellow, but in practice you need a fourth ink as well to
produce a deep convincing black. That's why full-color printing is often
referred to as the four-color process,
sometimes as CMYK printing (Cyan, Magenta, Yellow, and K meaning "key," a printer's word that usually means black),
and sometimes (since each color has to be printed separately) as color-separation printing.
Just like with black-and-white printing, the halftone process can also be
used to create varying shades of color.
Photo: Color printing: With black, magenta, cyan, and yellow ink, you can print any color you like.
Why is color printing more expensive?
Printing in color costs much more than printing in black-and-white, for
various reasons. First, and most obviously, there are four inks
involved instead of just one and each is printed by its own
printing plate, so the cost of making the printing plates alone is
several times greater. Second, color printing presses need to be able
to print the four inks on the page one after another, in perfect
alignment, so they need to be considerably more sophisticated and
precise. Third, it takes extra time and effort for the person
operating the printer to check that the colors have been aligned and
reproduced successfully, so there's more human effort involved.
Finally, because color printing is often used for reproducing
photographs, heavier, glossier, and more expensive paper is usually
needed to do it justice.
Sometimes designers get around the cost of color printing by using different
colored papers and inks. So, instead of printing black ink on white
paper, they might print black ink on red paper or red ink on yellow
paper. That achieves a colorful effect but keeps the cost down by
still using only a single color of ink. Another option is to
use spot-color printing, where a single,
specially mixed color is applied to a black-and-white document—though
that is labor intensive and can work out even more expensive than four-color
printing. Another alternative is to use two- or three-color printing, in which pages are printed
with black and one or two other colors. If you were using just cyan
and magenta inks, for example, you could create a whole range of reds
and blues and print quite colorful pages without the expense of
four-color printing. Another option is to print some pages with the
four-color process and other pages with only black-and-white. Books
that contain photographs are often made this way, with the art pages
printed through the four-color process on glossy paper that's bound
inside text pages printed with the black-and-white process on
ordinary paper.
Who invented printing?
If your immediate answer was "Johannes Gutenberg," you're only
half-right. As this whistle-stop tour through printing history will
show you, the celebrated German appeared only halfway through the story—one of many people who made
printing what it is today.
Artwork: A drawing of Ottmar Mergenthaler's revolutionary Linotype typesetting machine, taken from his original
US patent #543,497: Linotype machine,
courtesy of US Patent and Trademark Office.
- ~3000-1000BCE: Ancient Babylonia: People use signet stones (stones with designs cut into
their surface) dipped in pigment (paint) to print their signatures
in an early example of gravure printing.
- ~30BCE–500CE: Ancient Rome: Slaves laboriously copy out manuscripts by hand.
- 105CE: The Chinese invent the first paper, based on tree bark.
- ~500CE: The Chinese perfect printing from a single wooden block into which
designs are slowly and laboriously engraved.
- ~751CE: A book called Mugujeonggwang Daedaranigyeong (The Great Dharani Sutra) is printed with wooden blocks. Today, it's believed to be the oldest surviving printed text in the world.
- ~900CE: Wooden block printing is further developed in Goryeo (a kingdom of Korea that lasted from the 10th-14th centuries).
- ~1040CE (11th century): A Chinese printer called Bi Sheng invents the idea of printing with movable type. He makes lots of small clay blocks,
each containing a separate letter or character in relief, and rearranges them in a printing frame so he can print many different things. Unfortunately, because the Chinese language can use thousands of different characters, the idea doesn't immediately catch on; printers prefer to carry on using carved wooden blocks.
- 11th-13th centuries: In Goryeo, scholars produce the Tripitaka Koreana, a collection of Buddhist scriptures carved onto some 81,000 wooden printing blocks.
- 12th-14th centuries: The technology of papermaking is transferred from eastern to western countries.
- Late 1300s: Block printing is first used in Europe.
- 1377: In Goryeo, a two-volume book called Jikji (an anthology of Zen Buddhist teachings) is printed with metal type almost 80 years before Gutenberg. Only one copy survives, currently preserved in the National Museum of Korea in Seoul.
- 1450: Johannes Gutenberg develops the first modern printing press using movable metal type (with each small printing letter or character cast in relief out of metal). Note that he didn't invent either the printing press or movable type: his innovation was to bring these things together in a powerful new way that caught on and spread rapidly through the world.
- 1456: Gutenberg prints the famous Gutenberg Bible.
- ~1500: Thousands of printers have adopted Gutenberg's method and millions of books are being printed.
- 1798: German actor and writer Alois Senefelder invents lithography, the water-and-grease printing process used in modern offset printing.
- 1800: English nobleman Charles, Earl of Stanhope makes the first iron printing press.
- 1803: Brothers Henry and Sealy Fourdrinier invent the modern papermaking machine, based on a series of huge rollers arranged in a row.
- 1814: Long before electric power becomes widely available,
Friedrich König
invents the steam-driven printing press to speed up the laborious printing process. It's a type of flatbed cylinder press, in which
the cylinder is powered by a steam engine.
- 1846: Richard March Hoe develops the rotary press for newspaper printing and later perfects it so it can print on both sides of the paper at up to 20,000 pages per hour.
- 1863: William Bullock invents the web-feb rotary press for printing newspapers from giant rolls of paper.
- 1868: Christopher Latham Sholes develops a machine for printing personal letters and other writing—the typewriter with its QWERTY keyboard.
- 1880s: American brothers Max and Louis Levy develop halftone printing.
- 1886: Ottmar Mergenthaler invents the
Linotype machine, an automated way of creating "hot metal" type in a printing plate by casting a whole line of a book or magazine at a time. Typesetting, as this innovation is
known, allows newspapers to be printed more quickly and efficiently than ever before.
Photo: A vintage Linotype machine viewed from the side.
The person operating the machine enters text on the keyboard (at the bottom).
The machine then creates a mold of each line of text that's filled with molten metal to form a
line of type that can be used for printing.
Photo by Carol M. Highsmith, from Gates Frontiers Fund Colorado Collection within the Carol M. Highsmith Archive,
Library of Congress, Prints and Photographs Division.
- 1887: American inventor Tolbert Lanston develops a rival typesetting system called Monotype, which sets each letter or character as a
separate piece of type instead of a whole line.
- 1905: Ira Rubel develops offset printing.
- 1912: Walter Hess of Switzerland is one of the first people to experiment with
lenticular printing (putting lenses over printed paper to make a 3D effect).
- 1938: Chester Carlson invents the basic principle of the photocopier (a way of reproducing documents almost instantly using static electricity), though another decade passes before the first commercial copier goes on sale and the invention isn't taken up widely until marketed by Xerox in the 1960s and 1970s.
- 1949: Phototypesetting machines are invented, which produce type by photographic methods instead of using "hot metal." First is the Lumitype, invented at ITT by Frenchmen, René Higonnet and Louis Moyroud. and later developed by the American Lithomat corporation, which markets the machine as the Lumitype Photon.
- 1967: Gary Starkweather of Xerox gets the idea to develop a laser printer, and is finally granted a patent 10 years later. The machine he produces is very large and cumbersome by today's standards.
- 1980s: Scott Crump of Stratasys pioneers the modern approach to 3D printing called fused deposition modeling (bulding up a 3D object from hot plastic, layer by layer).
- 1984: Steve Jobs launches the Apple Macintosh computer (loosely based on the earlier Xerox Alto), which, partnered with a compact and (relatively) affordable laser printer, begins a revolution in desktop publishing.
- 1989: Tim Berners-Lee writes a proposal for an online publishing system called the World Wide Web, which makes it possible to publish documents instantly and view them anywhere else in the world, seconds later, using the
Internet.
