Bunka yarn is a silk-like rayon thread manufactured in Japan and used primarily for a form of Japanese embroidery called Bunka shishu.

Bunka yarn can be pulled through a specialised embroidery needle or punch-needle tool for Bunka embroidery. You work from the front of the fabric (unlike many embroidery forms which work from the back).

Because the yarn unravels and has a curly/looped structure, it creates textured effects — often used for detailed pictures (fish, landscapes, people) that resemble paintings. 

Also you’ll find bunka yarn marketed in miniature/craft contexts (for doll hair, edging, small-scale embellishments) because of the unravelling curly effect