Also known as 'burr' wood in British English.
There are two types of burl that I have come across: closed grain and curly grain. These are my own terms as I am yet to come across a specific definition of each. There is however a clearly discernible difference between the two when carved, and this is what I care about most.
The closed grain burl (so-called because there are no ‘eyes’ which break open the bark, and as a result the grain follows the bark) is what you will see the factory made Kuksa cut from if they are of burl wood. I can only presume that this type of burl is more common on northern Birch trees, but I am still trying to discover the truth of the matter.
The kind I use most often is curly grain burl wood, being slightly more common further south in the Czech Republic. Closed grain burl wood is also a very interesting wood and it is best used in ‘natural form’ Kuksa where the shape of the cup goes with the grain.
Closed grain characteristics: few or no knots or ‘eyes’; discernable grain pattern; wavy light and darker shimmer; less dense.
Curly grain characteristics: many knots or ‘eyes’ in an irregular grain; much tighter contrast between light and darker shimmer; more dense.
What you will find with the factory made Kuksa is that they are all machined similarly regardless of the grain, so the integral strength, the water-tightness and the resistance to cracking, the very reasons for which they were used traditionally are ignored.
There is another variable: the burl can come from above ground on the trunk or thick branches, or from the root, near or below ground. This adds another dimension to the visual effect on the carved wood.
All written material: Copyright Ian Tompsett 2010-2011