The "extension" in seal cutting is mainly used for boundary paragraphs.
Boundary section is an important part of seal cutting, which usually records the seal cutting interpretation, printer, event record, seal script description, time, author and other information, and is another form of calligraphy.
The extension process is mainly alum, covering the printing surface, wetting, scraping through the covering paper, inking and so on. Although the technique is different, the principle is the same.
As a part of seal cutting works, rubbings obtained by "rubbings" make the works more complete, and also fully reflect the beauty of seal cutting.