Stir in the salt, sugar, water, and corn syrup, and bring the mixture to a boil. Boil gently over medium heat, without stirring, until the mixture reaches hard-crack stage (300°F on an instant-read or candy thermometer). The syrup will bubble without seeming to change much for awhile, but be patient; all of a sudden it will darken, and at that point you need to take its temperature and see if it's ready, about 10 to 12 minutes. Pay attention; too long on the heat, and the syrup will burn.