jerrywh has it right. Potassium ferrocyanide (called 'yellow prussiate of potash' or some such on your can of nice pure Sea Salt) is only moderately poisonous. However, Our Gov't decided while it was too poisonous to use for heat treating, it is OK to add the sodium version to your salt as an anti-caking agent.
I suspect Kasenit also had some carbon added in the form of powdered wood (real wood, not the petroleum coke used in your BarBQ briquetts). Nicholson used to coat their files with "cyanide loaf", mix of potassium ferrocyanide, flour & bone black, boiled together in salt water. Then Our Gov't decided that was dangerous. They really did!!!
I think it was quite a few years before the salt guys paid off Our Gov't enough to get it added as an anti-caking agent to Sea Salt. Yummy. Of course they don't call it "cyanide" on the label or it'd never sell. Rather, they use the more poetic 19th Century term "Yellow Prussiate of Soda" As far as your body is concerned it is the Prussiate that matters, not whether it is of Potash or Soda. Won't kill you but I've heard that some folks are sensitive to a little bit of poison.
Don't believe this cranky old metallurgist? Go peruse the fine print ingredients of sea salt (Morton, for one) in your local grocery store.
