This might help:
From America's Test Kitchen Website:
Crisping Chicken Skin
From Season 10: Chicken Classics, Reinvented
First, the combination of baking powder and table salt will draw moisture from the skin of the chicken, helping the skin to dry out. The drier skin will become crispy faster because the skin cannot go above the boiling point of water (212 degrees) until all the water has evaporated from the surface. The temperature of the skin needs to rise above 300 degrees before it will start to brown and crisp.
Second, the baking powder is composed of an alkali (sodium bicarbonate) and an acid (monocalcium phosphate) in solid form. As the baking powder absorbs the moisture from the skin, the acid and alkali will react. The calcium ions from the acid can be absorbed into the skin and activate enzymes called calpains, which will start to break down the proteins within the skin. The alkaline baking soda and broken-down proteins will undergo browning reactions faster, thus creating a browner, more flavorful skin.
Here's the link (it may not work unless you are a member of ATK):
http://www.americastestkitchen.com/science/detail.php?docid=23098&extcode=M**ASCA00
