LETTER SEALS

What is a seal?

A seal can mean a wax seal bearing an impressed figure, or an embossed figure in paper, with the purpose of authenticating a document, but the term can also mean any device for making such impressions or embossments, essentially being a mould that has the mirror image of the figure in counter-relief, such as mounted on rings known as signet rings. This article is concerned with devices and methods for making such imprints.
If the imprint is made as a relief resulting from the greater pressure on the paper where the high parts of the seal touch, the seal is known as a dry seal; in all other cases a liquid or liquified medium (such as ink or wax) is used, usually in another color than the paper's.
Sigillography is the term used for the study of seals.

Impression

Wax seal on an envelope

Pine resin seal on vellum tag or tail of an English deed dated 1638.
The use of seals, in wax, in lacquer or embossed on paper, to authenticate documents, is a practice as old as writing itself. Seals of this nature were applied directly to the face of the document or attached to the document by cords in the owner's, or to a narrow strip of the document sliced and folded down as a tail but not detached from the document. This helped maintain authenticity by not allowing the reuse of the seal. If a forger tried to remove the seal in the first case, it would break. In the other cases, although the forger could remove the seal intact by ripping the cords from the paper, he would still have to separate the cords to attach it to another document, which would destroy the seal as well because the cords had knots tied in them inside the wax seal. Most governments still attach seals to letters patent. While many instruments required seals for validity (i.e. the deed or covenant) it is rather uncommon for private citizens to use seals anymore.

Seals were also applied to letters and parcels to indicate whether or not the item had been opened since the seal was applied. Seals were used both to seal the item to prevent tampering, as well as to provide proof that the item was actually from the sender and is not a forgery. To seal a letter, for example, a letter writer would compose the letter, fold it over, pour wax over the joint formed by the top of the page of paper, and then impress a ring, metal stamp, or other device. Governments would often send letters to citizens under the governmental seal for their eyes only. These were called letters secret. Seals are no longer commonly used in this way, except for ceremonial purposes.

Ancient Near East

Ring stone from Zafar showing a torah shrine
Seals were used in the earliest civilizations and are of considerable interest in archaeology. In ancient Mesopotamia seals were engraved on cylinders, which could be rolled to create an impression on clay e.g., as a label on a consignment of trade goods. From Ancient Egypt seals in the form of signet-rings of kings have been found.
Recently, seals have come to light in South Arabia datable to the Himyarite age. One example shows a name written in Aramaic (Yitsḥaq bar Ḥanina) and engraved in reverse so as to be visible in the impression.
In the Indus Valley Civilization, rectangular seals were used to label trade goods and also had other purposes.

Ancient Aegean
From the beginning of the 3rd millennium BC until the Dark Ages, seals of various kinds were in production in the Aegean islands and mainland Greece. In the Early Minoan Age these were formed of soft stone and ivory and show particular characteristic forms. By the Middle Minoan Age a new set for seal forms, motifs and materials appear. Hard stone requires new rotary carving techniques. The Late Bronze Age is the time par excellence of the lense-shaped seal and the seal ring.

East Asia
A Baiwen name seal, read up-down-right-left: Ye Hao Min Yin (lit. "Seal of Ye Haomin")
Known as 印章 (Pinyin: yin4zhang1) in China, dojang or ingam in Korea and inkan or hanko in Japan, ink seals have been used in East Asia as a form of written identification since the invention of writing. Even in modern times, seals are still commonly used instead of handwritten signatures to authenticate official documents or financial transactions. Both individuals and organizations have official seals, and they often have multiple seals in different sizes and styles for different situations. East Asian seals usually bear the name of the person or organization represented, but they can also bear a poem or a personal motto. Sometimes both types of seals, or one large seal that bears a name and a motto, are used to authenticate official documents. Seals are so important in East Asia that foreigners who frequently conduct business there also commission the engraving of a personal seal.
East Asian seals are carved from a variety of hard materials, including wood, soapstone, seaglass and jade. East Asian seals are traditionally used with a red oil-based paste consisting of finely ground cinnabar, which contrasts with the black ink traditionally used for the ink brush. Red chemical inks are more commonly used in modern times for sealing documents. Seal engraving is considered a form of calligraphy in East Asia. Like ink brush calligraphy, there are several styles of engraving. Some engraving styles emulate calligraphy styles, but many styles are so highly stylized that the characters represented on the seal are difficult for untrained readers to identify. Seal engravers are considered artists, and in the past, several famous calligraphers also became famous as engravers. Some seals, carved by famous engravers, or owned by famous artists or political leaders, have become valuable as works of art and history.
Because seals are commissioned by individuals and carved by artists, every seal is unique, and engravers often personalize the seals they create. The material of seal and the style of the engraving are typically matched to the personality of the owner. Seals can be traditional or modern, conservative or expressive. Seals are sometimes carved with a figure on the owner's zodiac animal on the top of the seal. Seals are also sometimes carved with images or calligraphy on the sides.
Although it is a utilitarian instrument of daily business in East Asia, Westerners and other non-Asians seldom see Asian seals except on Asian paintings and works of calligraphy. All traditional paintings in China, Japan, Korea, Vietnam, and the rest of East Asia are watercolor paintings on silk, paper, or some other surface that the red ink from seals can adhere to. East Asian paintings often bear multiple seals, including one or two seals from the artist, and the seals from the owners of the painting.
East Asian seals are the predecessors to block printing.

http://en.wikipedia.org/wiki/Seal_(device)