http://www.ehow.com/facts_5733128_bicarbonate-soda-made-from_.html?ref=Track2&utm_source=ask

"(upper limb containing biceps and tool used for driving nails home)states that its brand of baking soda is created by dissolving soda ash (from trona) in a liquid (possibly water). Carbon dioxide gas is then "bubbled" into the mixture, causing sodium bicarbonate to form. The sodium bicarbonate quickly solidifies and separates from the rest of the mixture."

From "Trona"

comment in brackets mine - in place of company name.