There are four traditional arts in China: seal carving, calligraphy, painting, and poetry writing.
Historically, seals (also called zhang or yin zhang) were symbols of power. In 221 BC, after emperor Qin Shi Huang conquered the six Warring States and unified China, he ordered his national seal to be carved using the famous "He Shi Bi" white jade. He further stipulated that this would also serve as his personal seal. His officials also had seals and these were called zhang and yin. Zhang seals were given to officials who received more than 2,000-dan (dan is a unit of dry measure for 100 liters of grains) grains as salary. Yin seals were given to officials who received salaries from 200-dan to 1000-dan grains. The rest of the seals were personal seals. The official seals were about a cubic inch, while personal seals were smaller.
Seals can have a variety of shapes and sizes. They are usually made from stone but wood, bamboo, bone or ceramics can also be used. There are four parts to a seal: grip or handle, body or platform, sides and face. Carvings may be done on the grip portion of the seal or may cover the entire seal. Common motifs are landscapes, figures, birds or flowers. Engravings on the seal face can follow several calligraphy styles and the arrangement of characters should fit the space appropriately. During Han Dynasty, seal carving became a highly refined art form. Seals from this era, together with Tang poetry, Song ci (lyrics/prose), and Yuan qu (another ancient Chinese literary form), are national treasures.
In ancient China, it was customary for an artist to use both his signature and personal seal on finished paintings, poetry, calligraphy, documents and letters. Seals were often stylized carvings of the artists’ names.
Before using, the seal is dipped in red ink paste made of vermilion (a mercury compound). High quality ink paste must possess a bright color and not fade over time. The containers for the paste are made of porcelain. The ink paste should be stirred often to prevent it from hardening. When not in use, ink paste containers are placed inside wood or silk boxes for safekeeping.
Even today, the seals of many famous calligraphers and painters such as Su Dongpo, Huang Tingjian, and Emperor Hui of the Song Dynasty, are still visible on their works.