The inside and the mouth pieces are coated with bees wax.  The outside finish is a mixture of bees wax and pure turpertine.

The second picture shows the back.  The membrane is kitchen plastic wrap applied to a ring of bees wax.

What are they carved from?  Sweet potatoes.

They are carved from fresh sweet potatoes, forming the faces and hollowing out the inside and making holes for the mouth- piece, the diaphragm, and air exit hole at the same time.  When they dry they shrink about 1/3 in each direction, accenting the cuts and becoming hard and woodlike.

These were made separately, but then I discovered they fit together held side by side.  They sound great played together, each having a separate tone.  One is held in each hand with a thumb near the membrane and exit hole.  The tone can be changed by the thumb partially blocking the hole, and the position of the thumb near or on the membrane changing the buzz.

An added bonus, between the sweet potato and the bees wax, they have a very nice smell.