An envelope is a flat, flexible container, made of paper or similar material, that has a single opening and a flap that can be sealed over the opening. The envelope is usually sealed by wetting an area of the flap. Some envelopes are sealed with a metal fastener. Others are sealed with a piece of string that wraps around flat, circular pieces of cardboard attached to the envelope. A recent development in envelopes is a thin strip of plastic, which is removed to reveal an area of the flap with an adhesive that does not need moistening.
Envelopes are almost always rectangular, but they exist in a wide range of sizes. The two main styles used are banker envelopes, which have the opening on the long side, and pocket envelopes, which have the opening on the short side. In the United States, standard sizes range from 3.5 x 6 in (89 × 152 mm) to 10 x 13 in (254 x 330 mm). In Europe, sizes range from 3.2 x 4.5 in (81 x 114 mm) to 11 x 15.75 in (280 x 400 mm). Sizes are somewhat different in the United Kingdom, with the most common being 4.25 x 8.625 in (108 x 219 mm).
Some envelopes have one or more windows cut into the front to allow addresses written on sheets inside to be seen. These windows may be covered with a transparent material.
The earliest ancestor of the envelope was used by the ancient Babylonians five or six thousand years ago. Messages were written on clay tablets, which were baked to harden them. The tablets were then covered with more clay and baked again. The inner tablet could only be revealed by breaking open the outer layer of clay, ensuring the security of the message.
True envelopes did not exist until much later, long after the invention of paper. The oldest form of paper was papyrus, first manufactured by the ancient Egyptians at least as early as 3000 B.C. Papyrus was made from a fibrous material found within the woody stems of an aquatic, grassy plant (Cyperus papyrus). Long strips of this material were placed side by side, then covered with another layer of strips at right angles to the first. The sheet formed by the two layers was dampened, pressed, dried, flattened, then dried again. The resulting papyrus, if properly made, was pure white and free from spots and stains. An excellent writing material, papyrus was used extensively by the ancient Egyptians, Greeks, Romans, and Arabs. It continued to be used until paper made from other plant sources reached the rest of the world from China. Some papyrus was used in Europe as late as the twelfth century.
Early forms of Chinese paper, made from reeds and rice, date back as far as 1200 B.C. A superior kind of paper, similar to modern paper, was first made about the year 105. Attributed to a court official named Ts'ai Lun, this improved paper was made from a mixture of materials, including mulberry and other woody fibers, hemp, rags, and fishing nets. Papermaking spread slowly from East to West, reaching Central Asia by 751 and Baghdad by 793. By the fourteenth century, there were several paper mills throughout Europe, particularly in Spain, Italy, France, and Germany. The development of the printing press in the 1450s greatly increased the demand for paper.
The early history of the paper envelope is not known. Paper may have been used to wrap messages at a very early date in China. They did not appear in Europe until the seventeenth century, when they began to be used in Spain and France. Until that time, messages were simply folded and sealed. Even today, some stationery is designed to be folded and mailed without an envelope.
Cotton and linen rags were the main raw materials used to make paper until the early nineteenth century, when they were replaced by wood. At about the same time, papermaking by hand began to be replaced by papermaking machines. The emerging envelope industry was noted by Karl Marx in his book Das Kapital in 1867. Envelope manufacturers continued to increase the speed of production, from three thousand envelopes per hour at the time of Marx to more than fifty thousand per hour in the late twentieth century. By the late 1990s, nearly two hundred billion envelopes were made in the United States each year.
Most envelopes are made from paper. Some large, strong envelopes are made from synthetic materials, such as polyethylene. Polyethylene is a plastic made from ethylene, which is derived from petroleum.
Paper used for most envelopes is made from wood. Modern technology allows the wood to come from almost any kind of tree. Paper used to make very high quality envelopes, such as those used to enclose formal invitations, may be made partly or completely from cotton or linen. Some envelopes are made from manila, a fiber from the leaves of a plant found in the Philippines that produces a strong, yellowish paper. Most so-called manila envelopes, however, are made of paper derived from wood which only resembles true manila.
The glue applied to envelopes is of two basic types. The glue applied to the flap that is sealed by the consumer is usually a gum. A typical natural gum is gum arabic, derived from a substance produced by the acacia tree. Synthetic gums are often derived from dextrans, which are produced by the fermentation of sugar. The glue that holds the rest of the envelope must be stronger and more permanent. This glue is often derived from starches, which are obtained from corn, wheat, potatoes, rice, and other plants.
The fastener attached to some envelopes is made of aluminum or other metals. The string attached to other envelopes is made of cotton or other fibers. The material covering the windows in some envelopes is usually polystyrene. Polystyrene is a plastic made from styrene, a derivative of petroleum.
Modern envelope manufacturing is highly automated, and almost always results in a reliable product. Although constant testing is not necessary, certain factors are checked to ensure quality. Paper arriving at the factory is inspected to be sure that it has the correct weight. A very small number of sample envelopes are checked to ensure that they have the correct shape and size, and that adhesives have been applied in the correct places. Any printing that appears on the envelope must be in the correct position, of the correct color, and without printing errors. If any windows are cut in the envelope, they must have the correct dimensions and be in the correct position.
Although major changes in envelope design are not expected, innovations are likely in the way paper is made. Manufacturers are constantly looking for ways to make paper that are more efficient, less costly, and result in less pollution. Genetic engineering may result in trees that grow faster and produce wood that is better adapted to producing pulp. A recent trend that is likely to continue is the increasing use of recycled paper as a raw material for making envelopes and other paper products.
Biermann, Christopher J. Essentials of Pulping and Papermaking. New York: Academic Press, 1993.
Ferguson, Kelly, ed. New Trends and Developments in Papermaking. Miller Freeman, 1994.
Keman, Michael. "Pushing the Envelope." Smithsonian (October 1997): 30-31.
Ohio Envelope Manufacturing Company. http://www.ohioenvelope.com/ (September 30, 1998).
— Rose Secrest