  Sintered powders and wires of superconducting $MgB_2$ have been fabricated
under a variety of conditions in order to determine details of the diffusion of
the $Mg$ into $B$ and to study the types of defects that arise during growth.
For samples prepared by exposure of boron to $Mg$ vapor at $950^{\circ}C$, the
conversion of particles of less than $100\mu m$ size particles to $MgB_2$ is
complete in about $2 h$. The lattice parameters of the $MgB_2$ phase determined
from X-ray are independent of the starting stoichiometry and the time of
reaction. Wire segments of $MgB_2$ with very little porosity have been produced
by reacting $141 \mu m$ diameter boron fibers in an atmosphere of excess $Mg$
vapor at $950^{\circ}C$. Defects in the reacted fibers are predominantly the
voids left as the boron is converted to $MgB_2$.
