Each ball (with a leather strap in the middle) is "hand dipped" in Synthetic Rubber (latex and plastic free) 4 times. This Synthetic Rubber dries into a hard yet flexible form and is often used in to coat instruments. This dipping process is very time-consuming and must be done at just the right temperature, or it bubbles and drips. Because the balls are completely dipped in Synthetic Rubber, they can be cleaned; and as there is no exposed hole in the center of the ball, nothing can be trapped inside during use. 

NOTE: Yellow Ball gag: Straps are PVC!