This is a great example of Paul Soldner's Clear Crackle Raku Glaze. It is also a pretty nice tea bowl too.
The crackles come from pulling the piece of pottery out of the hot raku kiln and placing it in another container full of burnable materials, in this case torn up news papers and sawdust.
Once the pottery is placed in this reduction chamber, the burnable materials ignite, a lid is placed on the reduction chamber and smoke fills the cracks in the glaze.
The recipe for Soldner's Clear Crackle is 80% Gerstley Borate and 20% Nepheline Syenite.