Jorge, pickeling and canning salt may be called 'rock salt' but it has gone through several refinements before certificatation as 'food grade'. There is also "brining salt" which is either for brining meats (like corned beef) and fish (sallted cod). I believe that this brining salt may represent another level of purity depending on whether it is used for butchered meats or whole fish. Rock salt itself is the first crop of crystals from brines and as such it is black dirty stuff. It is more like what they use for street salting around here. (Although even that has a lot of additives.) It is not what I associate with food.Originally Posted by Jorge
Rock salt comes in varying degrees of purity but has one thing in common. It comes as large mainly cubic crystals of NaCl. I would therefore not suggest that anyone blindly buy salt with the label "rock salt" on it.
Any of these salts below 'reagent grade' or 'analytical grade' contain enough extraneous material that there may be an effect on photographic properties. The generic rock salts fall into this category.
I have tested this out to my satisfaction with AgX. I really don't know about Pt/Pd and other imaging systems, but at the cost of precious metals, it would be best to use the best halide rather than chance the loss of some Pt or Pd. Don't you think so?
Anyhow, you use what you wish. I'm just trying to put out some precautionary notes which some people may or may not find important or interesting.