Good suggestion, mudbug. Also, check any large grocery stores in your area in the spice section with the other salts. My stores carry the stuff year round, and I've also seen rock salt in automotive stores for use in melting ice on driveways (remember folks, this is Texas, not Oregon).
However, sarah, there is nothing magical about rock salt. It is salt, albeit non-food-grade, and is in large solid rocks, instead of ground. If your ice cream is not firming with table salt, then you're not using enough (and it takes A LOT to substitute for the mass of rock salt).
Test the salinity by using a ten-minute rule: After ten minutes of churning, if the ice cream is beginning to firm, then you've got the salt ratio about right. If it is not beginning to thicken, then you need to add more salt and recheck in another ten minutes. If your ice cream is getting crusty around the edges of the churning bowl, then you have too much salt -- just remove about half of the water and add more ice to refill the bucket.
There are few things in this world more delicious than homemade ice creams and yogurts! Good luck!
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