I would be a little leary about Kosher salt for photographic use. It is my understanding that Kosher means approved and blessed by a Rabbi for human consumption in accordance with certain principles that do not necessarily preclude the presence of iododes or other naturally occurring substances, so long as they are not harmful to humans. I'm pretty sure a thorough chemical analysis is not necessary, though I do not know the actual rules. The Law is much older than the chemistry. A better choice would be canning salt IMO.