What kind of brush was used in Zhang Xu's calligraphy works in the Tang Dynasty?

The calligraphers in the Tang Dynasty basically used purple brush (Xuan pen) made of rabbit hair.

According to the official history books, the famous Xuanbi in China was invented in the Han Dynasty. In the Jin Dynasty, the purple brush made of rabbit hair in Xuanzhou, Anhui Province was famous for its sharp pen tip. Xuanzhou Chen's pen is highly praised by Wang Xizhi and others.

Xuanbi reached its peak in the Tang and Song Dynasties. Xuanzhou became the center of pen-making in China, and Xuanbi gained a growing reputation. At that time, literati took Xuancheng Zihao as the top grade. At this time, Xuan Pen has been perfected in making skills, selecting materials, or carving art of penholder, and Liu Gongquan and others have had a high evaluation of Xuan Pen.