Actually, I think what they call pure silver is 999 parts out of 1000 silver, so it could be only 74 parts out of 1000 different from my .925 sterling. Some sterling is 950/1000 silver, etc....
I found this Society of American Silversmith's internet quote inserted as a suggestion in a post from our forum from December 2011 in a closed thread entitled Cleaning Silver.... it was closed because people couldn't play nicely and I guess I'll give the suggestion a try. However, I am a bit perplexed -- at the end of the post, it says if it still does not work after a three attempts, have the sterling professionally refinished. I am not referring to silver plate but rather sterling, so what would the professional be doing, covering over the corrosion with a layer of sterling silver? My silversmith quixotically referred to taking the machine to it and it getting very hot. He wouldn't exactly explain what went on behind the curtain to warrant the $20 fee. I just don't want to put it in ammonia and wind up with a bottomless sterling salt cellar because somehow the ammonia ate through the bottom.
Salt Shaker Corrosion
Those crusty corrosion marks on and in your salt shaker can be a real annoyance. One way to avoid this problem from the very start is to empty the shaker after a dinner party and thoroughly wash it; this way the salt doesn't have time to do its damage. Heavily gold plating the interior is the only other way to preserve the finish because gold is impervious to the effects of salt. It is still wise to clean out the shaker at least twice a year and inspect the plate to make sure it has not been abraded by the salt.
There is a simple way to remove the corrosion yourself. Do this in a well-ventilated area and with nitrile gloves since you will be using ammonia.(Silver dips will not perform as well as ammonia.) If you are removing corrosion from a salt shaker, pour ammonia into a container, place the shaker inside, and cover the container. Let the shaker sit for ten minutes, then remove from the container and inspect. If the black corrosion spots remain, place the shaker back in and let stand for another ten minutes and inspect again. If the corrosion is not gone after 30 minutes, have the shaker professionaly refinished.