What is baking and what are the effects? In their experiment, Davisson and Germer had to bake the nickel mass because it was oxidized.
What is baking and what does it do to the lattice of the metal?
 A: Usually "baking" a vacuum chamber means heating up the whole installation to over 120 C during a day or so, to drive off adsorbed water etc from surfaces in order to achieve a good vacuum.
Davisson and Germer did something else. From their article:

The investigation reported in this paper was begun as the result of an
  accident which occurred in this laboratory in April 1925. At that time
  we were continuing an investigation, first reported in 1921, of the
  distribution-in-angle of electrons scattered by a target of ordinary
  (poly-crystalline) nickel. During the course of this work a liquid-air
  bottle exploded at a time when the target was at a high temperature;
  the experimental tube was broken, and the target heavily oxidized by
  the inrushing air. The oxide was eventually reduced and a layer of the
  target removed by vaporization, but only after prolonged heating at
  various high temperatures in hydrogen and in vacuum.

So that was reducing the heavily oxidized surface in a hydrogen atmosphere in an oven. The high temperature caused crystals to grow to sizes larger than their electron beam.
A: The primary effect of the baking was basically to boil off the oxygen that had built up at the nickel surface.  When the metal is exposed to air, the oxygen reacts with the surface atoms, and this eventually produces a layer of metal oxide that is typically tens or hundreds of atoms thick.  However, the oxygen remains relatively volatile, so by heating the metal, Davisson and Germer got rid of much of the oxygen, producing something closer to a pure nickel crystal surface.
