These pieces are fired in the American Raku tradition, where the bisque piece is glazed with special glazes and re-fired in a gas kiln to almost 1900 degrees.  It's then removed and placed into a reduction container with combustable material.  When the hot clay piece touches the combustable material it will start it on fire.  The container's lid is placed on it and the the fire continues to burn until the oxygen is used up  this oxygen starvation allows the glazes to "flash" different colors including "copper".   This fast cooling will cause cracks in the glaze that smoke will penetrate, giving the Raku piece that familiar black lines.  What makes Raku Pottery beautiful also so makes it nonfunctional...  it is not food safe or water tight.