In calcination why is the ore heated in the absence of air in a reverberatory furnace, does the presence of oxygen affect any thing?
Most of the ore will be oxidized if thermally treated under air. That's why it's usual to heat it under controled atmosphere or even in vacuum.
Yes in calcination, with limited supply carbonates are reduced to oxides and to remove organic matter or to remove water of crystallization or evaporation of volatile components such as arsenic or antimony vapors.
If the oxygen component is more it would actually be then a roasting process where sulfides are converted to oxides since they are easier to reduce.