Silver Birch bark - Betula pendula - from Hunworth Common, Norfolk.  
 
First tar oil is extracted from the bark by the same process used to create charcoal, except we're less interested in the charred contents of the tin, and what leaches out and is collected underneath. 
 
The tar oil is then further boiled down to burn off the unwanted extras, and then finely ground Willow charcoal is added to firm it up. The end result is a marvelous thermoplastic adhesive which dries hard but can be made malleable by heating it up. 
 
The glue is provided in aluminium tins which can easily be heated. Tins are recycled so they may have the odd ding or scratch on the outside. 
 
The small tins (lip balm size) have about 9g of glue in them, the large tins have 25g.