Paper is thin material mainly used for writing upon, printing upon or packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets.
The earliest recorded forms of paper were in use in Egypt in around 3500 BC, made from the papyrus plant. True paper is believed to have originated in China in approximately the 2nd century AD, although there is some evidence for it being used before this date. The use of paper spread from China through the Islamic world, and entered production in Europe in the early 12th century. Mechanised production of paper in the early 19th century caused significant cultural changes worldwide, allowing for relatively cheap exchange of information in the form of letters, newspapers and books for the first time.
Paper is a versatile material with many uses. Whilst the most common is for writing and printing upon, it is also widely used as a packaging material, in many cleaning products, and in a number of industrial and construction processes, and occasionally as a food ingredient, particularly in Asian cultures.
In America, archaeological evidence indicates that a similar parchment writing material was invented by the Mayans no later than the 5th century AD. Called amatl, it was in widespread use among Mesoamerican cultures until the Spanish conquest. The parchment is created by boiling and pounding the inner bark of trees, until the material becomes suitable for art and writing.
These materials made from pounded reeds and bark are technically not true paper, which is made from pulp, rags, and fibers of plants and cellulose.
Papermaking is considered to be one of the Four Great Inventions of Ancient China, since the first papermaking process was developed in China during the early 2nd century. During the Shang (1600 BC–1050 BC) and Zhou (1050 BC–256 BC) dynasties of ancient China, documents were ordinarily written on bone or bamboo (on tablets or on bamboo strips sewn and rolled together into scrolls), making them very heavy and awkward to transport. The light material of silk was sometimes used, but was normally too expensive to consider. While the Han Dynasty Chinese court official Cai Lun is widely regarded to have invented the modern method of papermaking (inspired from wasps and bees) from rags and other plant fibers in AD 105, the discovery of specimens bearing written Chinese characters in 2006 at north-east China's Gansu province suggest that paper was in use by the ancient Chinese military more than 100 years before Cai, in 8 BC. Archeologically however, true paper without writing has been excavated in China dating to the reign of Emperor Wu of Han from the 2nd century BC, used for purposes of wrapping or padding protection for delicate bronze mirrors. It was also used for safety, such as the padding of poisonous 'medicine' as mentioned in the official history of the period. Although paper used for writing became widespread by the 3rd century, paper continued to be used for wrapping (and other) purposes.
Toilet paper was used in China by at least the 6th century AD. In AD 589, the Chinese scholar-official Yan Zhitui (531-591 AD) once wrote: "Paper on which there are quotations or commentaries from Five Classics or the names of sages, I dare not use for toilet purposes". An Arab traveler to China once wrote of the curious Chinese tradition of toilet paper in AD 851, writing: "[The Chinese] are not careful about cleanliness, and they do not wash themselves with water when they have done their necessities; but they only wipe themselves with paper". Toilet paper continued to be a valued necessity in China, since it was during the Hongwu Emperor's reign in AD 1393 that the Bureau of Imperial Supplies (Bao Chao Si) manufactured 720,000 sheets of toilet paper for the entire court (produced of the cheap rice–straw paper). For the emperor's family alone, 15,000 special sheets of paper were made, in light yellow tint and even perfumed. Even at the beginning of the 14th century, during the middle of the Yuan Dynasty, the amount of toilet paper manufactured for modern-day Zhejiang province alone amounted to ten million packages holding 1,000 to 10,000 sheets of toilet paper each.
During the Tang Dynasty (AD 618–907) paper was folded and sewn into square bags to preserve the flavor of tea. During the same period, it was written that tea was served from baskets with multi-colored paper cups and paper napkins of different size and shape. During the Chinese Song Dynasty (AD 960–1279) not only did the government produce the world's first known paper-printed money, or banknote (see Jiaozi and Huizi), but paper money bestowed as gifts to deserving government officials were wrapped in special paper envelopes.
Paper spread slowly outside of China; other East Asian cultures, even after seeing paper, could not make it themselves. Instruction in the manufacturing process was required, and the Chinese were reluctant to share their secrets. The paper was thin and translucent, not like modern western paper, and thus only written on one side. The technology was first transferred to Korea in 604 and then imported to Japan by Buddhist priests, around 610, where fibres (called bast) from the mulberry tree were used.
Some historians speculate that paper was a key element in cultural advancement. According to this theory, Chinese culture was less developed than the West in ancient times prior to the Han Dynasty because bamboo, while abundant, was a clumsier writing material than papyrus; Chinese culture advanced during the Han Dynasty and subsequent centuries due to the invention of paper; and Europe advanced during the Renaissance due to the introduction of paper and the printing press.
The manufacture had spread to Damascus by the time of the First Crusade in 1096; but the wars interrupted production, and it split into two centres. Cairo continued with the thicker paper. Iran became the centre of the thinner papers. It was also adopted in India.
Together with the invention of the practical fountain pen and the mass produced pencil of the same period, and in conjunction with the advent of the steam driven rotary printing press, wood based paper caused a major transformation of the 19th century economy and society in industrialized countries. With the introduction of cheaper paper, schoolbooks, fiction, non-fiction, and newspapers became gradually available by 1900. Cheap wood based paper also meant that keeping personal diaries or writing letters became possible and so, by 1850, the clerk, or writer, ceased to be a high-status job.
The original wood-based paper was acidic due to the use of alum and more prone to disintegrate over time, through processes known as slow fires. Documents written on more expensive rag paper were more stable. Mass-market paperback books still use these cheaper mechanical papers (see below), but book publishers can now use acid-free paper for hardback and trade paperback books.
Papers made from chemical wood-based pulps are also unhelpfully known as woodfree papers.
The Kraft process is the most commonly practiced strategy for pulp manufacturing and produces especially strong, unbleached papers that can be used directly for bags and boxes but are often processed further, e.g. to make corrugated cardboard.
There are three main classifications of recycled fibre:.
Recycled Papers can be made from 100% recycled materials or blended with virgin pulp. Recycled papers are (generally) not as strong nor as bright as papers made from virgin pulp.
Pressing the sheet removes the water by force. Once the water is forced from the sheet, felt (not to be confused with the traditional felt) is used to collect the water. When making paper by hand, a blotter sheet is used.
Drying involves using air and or heat to remove water from the paper sheet. In the earliest days of papermaking this was done by hanging the paper sheets like laundry. In more modern times, various forms of heated drying mechanisms are used. On the paper machine, the most common is the steam-heated can dryer. These dryers can heat to temperatures above 200°F (93°C) and are used in long sequences of more than 40 cans. The heat produced by these can easily dry the paper to less than 6% moisture.
Paper at this point is uncoated. Coated paper has a thin layer of material such as china clay applied to one or both sides in order to create a surface more suitable for high-resolution halftone screens. (Uncoated papers are rarely suitable for screens above 150 lpi.) Coated or uncoated papers may have their surfaces polished by calendering. Coated papers are divided into matt, semi-matt or silk, and gloss. Gloss papers give the highest optical density in the printed image.
The paper is then fed onto reels if it is to be used on web printing presses, or cut into sheets for other printing processes or other purposes. The fibres in the paper basically run in the machine direction. Sheets are usually cut "long-grain", i.e. with the grain parallel to the longer dimension of the sheet.
All paper produced by Fourdrinier-type machines is wove paper, i.e. the wire mesh that transports the web leaves a pattern that has the same density along the paper grain and across the grain. Textured finishes, watermarks and wire patterns imitating hand-made laid paper can be created by the use of appropriate rollers in the later stages of the machine.
Wove paper does not exhibit "laidlines", which are small regular lines left behind on paper when it was handmade in a mould made from rows of metal wires or bamboo. Laidlines are very close together. They run perpendicular to the "chainlines", which are further apart. Handmade paper similarly exhibits "deckle edges", or rough and feathery borders.
Paper can be produced with a wide variety of properties, depending on its intended use.
The thickness of paper is often measured by the caliper, which is typically given in thousandths of an inch. Paper may be between and thick.
Paper is often characterized by weight. In the United States, the weight assigned to a paper is the weight of a ream, 500 sheets, of varying "basic sizes", before the paper is cut into the size it is sold to end customers. For example, a ream of 20 lb, 8½ x 11" paper weighs 5 pounds, because it has been cut from a larger sheet into four pieces. In the United States, printing paper is generally 20 lb, 24 lb, or 32 lb at most. Cover stock is generally 68 lb, and 110 lb or more is considered card stock.
The 8.5" x 11" size stems from the original size of a vat that was used to make paper. At the time, paper was made from passing a fiber and water slurry through a screen at the bottom of a box. The box was 17" deep and 44" wide. That sheet, folded in half in the long direction, then twice in the opposite direction, made a sheet of paper that was exactly 8.5" x 11".
In Europe, and other regions using the ISO 216 paper sizing system, the weight is expressed in grammes per square metre (g/m2 or usually just g) of the paper. Printing paper is generally between 60 g and 120 g. Anything heavier than 160 g is considered card. The weight of a ream therefore depends on the dimensions of the paper and its thickness.
The sizing system in Europe is based on common width to height ratios for different paper sizes. The largest standard size paper is A0 (A zero). Two sheets of A1, placed upright side by side fit exactly into one sheet of A0 laid on its side. Similarly, two sheets of A2 fit into one sheet of A1 and so forth. Common sizes used in the office and the home are A4 and A3 (A3 is the size of two A4 sheets).
The density of paper ranges from 250 kg/m3 (16 lb/ft3) for tissue paper to 1500 kg/m3 (94 lb/ft3) for some speciality paper. Printing paper is about 800 kg/m3 (50 lb/ft3).
Some types of paper include:
With increasing environmental concerns about synthetic coatings (such as PFOA) and the higher prices of hydrocarbon based petrochemicals, there is a focus on zein (corn protein) as a coating for paper in high grease applications such as popcorn bags.