Generally speaking, bagged salt is "solar salt" that is evaporated and contains less of the impurities that may affect color. Bulk rock salt is mined and will bring with it whatever else is in the ground around and with it.
This is a VERY general statement. To learn more, visit the The Salt Institute's web site.The Sale Institute
Around here (ann arbor, MI), the salt the road commisions use is blue. I think its treated with some kind of anti clumping agent. Sometimes the supplier that I used to get bulk from would have it, I guess when the county suppliers had a surplus.
The salt is blue because its sweetheart broke up with it....
(Sorry. I know that was silly but I couldn't resist any longer.)
But here's a serious question I've had--It seems like the salt that's left on my windsheild from spray coming off the road is stickier than it was years ago. It takes more than a simple wipe with a rag to get it to come off the glass. I don't remember it being that way before. Is it my imagination, or is there something about the way it's treated these days that makes it harder to wipe off? Any expert opinions?
No expert here, but I'll take my stab at it. Maybe they are using a pre-wet system with brine or liquid de-icer that causes this? That's one possible guess LOL. I can think of other potential guesses but that's my turn. Next?
I'm with BRL on this one. I believe it is some type of wetting agent, or sticking agent that is added to the pre wet system. Not only that, but sometimes calcium chloride is mixed with salt, or applied to roads itself.