 Avoiding steel-based alloys (which were too hard and rust-prone) they developed an alloy based on aluminum and bronze, and (in 1923) patented a metal ball made of two welded-together hemispheres.A year later, in 1924, they filed a patent for a ball that was cast in a single piece -- "La Boule intégrale".Louis Tarchier and Jean Blanc are generally credited with developing, around 1925, the process by which virtually all metal boules are manufactured today -- steel blanks are pressed into hollow hemispheres which are then soldered together and machined to make a hollow steel boule.