Kelp was burned and the iodine extracted from the ashes in the 18th and 19th century, but now it is not good enough for common use. Most iodine now is extracted from seawater. Chlorine is reacted with seawater to change the iodide to iodine. Air is blown over the water to evaporate the iodine. Then the iodine is reduced with sulfur dioxide to hydriodic acid. Then the hydriodic acid is oxidized with chlorine again to make iodine. This process is repeated until the iodine is pure. 