Burr Puzzles - Chinese Cross
The term Burr probably refers to the similarity of shape of certain grass seeds bearing the same name. The name Chinese Cross most likely arose due to the fact that early commercial models were produced in Asia. The 6 piece Burr first appeared in toy catalogues in 1803 but the puzzles were probably around long before that.
Discussions in books started appearing in the 1900’s, most notably Edwin Wyatt’s “Puzzles in Wood,” first published in 1928 but still available in various forms today, as is the sequel “Wonders in Wood.” The lure of solving 6 piece Burrs led to some very creative Mathematics and a particularly sophisticated and painstakingly constructed cataloguing system by the Dutch mathematician J.H. de Boer. He made and catalogued all 2906 pieces that could possibly make a Burr. These were filed in boxes along with the one solid locking piece. The possibilities are greatly expanded by allowing non-solid Burrs and doing away with the solid locking piece. Higher level Burrs require a number of moves before the first piece can be removed. The highest level unique six piece Burr is 10, meaning that 10 moves are required to remove the first piece.
The complete solution to Burr Puzzles was undertaken in the early 1990’s using a number of high speed computers. The run time was 2.5 years. We now know that there are roughly 35.65 billion ways to assemble Burr Puzzles.
Burr Puzzles range from the relatively simple, solid with a locking piece and 2 or 3 identical pieces, to exceptionally difficult, higher level puzzles requiring a great deal of patience and spatial awareness. When solving Burrs it is useful to memorise the symmetry of the completed puzzle. By noting that perpendicular pairs pass through the centre of the other two perpendicular pairs, most combinations can be eliminated early.