1. In a nonstick saucepan, heat and stir sugar, corn syrup, water, and salt over medium heat until sugar has dissolved. Over high heat, using a candy thermometer, cook sugar mixture to a hard crack stage (290 degrees F).
2. Add pecans and butter and cook to 300 degrees F stirring all the time to keep the nuts from burning.
3. Pull off heat at 300 degrees F and stir in baking soda while beating to froth for 30 seconds.
4. Pour at once on to 4 well buttered 15 1/2 by 10 by 1-inch pans. Spread with a spatula as thin as possible. As the brittle cools you may use gloves to hand stretch the brittle which will give it a better eating quality.