Knoll,it is sinew.The bow is made from Hickory with sinew backing, and heat treated belly. The glue is Tb3 for the sinew so I could paint directly over it with Acrylic paint ,and then seal it with shellac.I used white Rabbit fur for the silencers.It has 2 inches of induced reflex. Could have gotten more using hide glue. Natives such as the Hupa ,Wintu Karok,Yurok used Juniper mostly with hide Glue.It has been said that they used 1 third sinew,and 2 thirds Juniper for the thickness of the bows.They strung their bows backwards ,and then sinewed them. Their bows had more like 4 to 5 inches of reflex, and had a lot of spring.They were real craftsmen when it came to making bows,and arrows, but their method of shooting takes a lot of practice, and I am not very good at it yet.