Comfrey (also known as knitbone and boneset) is a large leafy plant (see illustration) that favours wet places such as riverbanks and ditches. It's native to Europe, and widespread in the British Isles. The whole plant has been used medicinally since ancient times. The root is still used to speed up the healing of broken bones, and is believed to be one of the plants used by Indian bone setters.
The peeled comfrey roots (see photo) resemble bones. The structure of the root is highly mucilaginous.
Method1. The roots were washed and peeled.
2. The roots were finely grated.
3. The roots were pounded with a pestle and mortar.
4. The pounded root material was placed in a spherical mold (7cm diameter) and left to set for three weeks.
5. A leathery cover was made for the ball, in the style of an Irish sliotar or American baseball.
6. The ball was thrown against a wall to determine its bounciness.
Findings1. The mucilaginous root material set well. It was found that in a warm or sunny location the material would swell and force open the spherical mold. It was therefore kept in a cool, well ventilated area out of direct sunlight.
2. The ball weighs 164g (between 5 and 6 ounces).
3. The ball bounces really well. It's not as bouncy as a ball made out of rubber, but it's a well bouncing ball.
4. The comfrey root has kept its shape perfectly so far, and not broken up inside the leathery cover.
Conclusions1. Comfrey root is an excellent material for making a bouncing ball.
2. Ancient peoples would have had everything they needed to make a bouncing ball out of comfrey root.
3. The ball greatly resembles the sliotar, used in the ancient Irish game of hurling. These are currently made of cork, but were made of various different materials historically. There's no record of them being made of comfrey root, but rather rope, horsehair, wood and even hollow bronze.
4. Experiments are needed to compare the comfrey root ball with a modern sliotar (currently made of cork in a leather cover). There were experiments with rubber sliotars in the early 2000s, but rubber was found to be too bouncy. Perhaps the comfrey root ball, with less bounciness, might prove perfect. The comfrey root ball is of a similar size (7cm diameter) to a sliotar, and only slightly heavier (164g compared with 110-120g). Before modern improvements, the sliotar weighed around 200g.
5. Perhaps refinements to the process could create an even bouncier ball.