You can also make pickles in stone crocks. The kind of pickles these make are a salt brine type fermented cucumber pickles. The lids use an air lock scheme that prevents any air from entering the vessel, but allows fermenting gasses to escape, just like that used for beer and wine fermenation. It may be that they use a 'starter' that will make natural lactic acid. Its very similar to making cabbage sour kraut. You can also use a big dish towel draped over the opening of the vessel, but it can allow a bit of 'unfriendly' bacteria in. My mom used a dish towel and then placed a large piece of plastic wrap over that, and held it in place with a big rubber band. She also put a big dish and weight on the surface to help keep the veggies under the brine. Usually once it starts 'working' the lactic acid will kill off any other bad things. To make any kind of pickles, you need to start with absolutely fresh (like within a couple of hours of being picked) cucumbers. I make a half sour and don't use any of these fermentation locks, as its just put out at room temperature for 2-3 days, then goes in the fridge. I add a small amount of vinegar to slow down any further fermentation, and they keep will in the fridge for up to a year. What your seeing is a food grade vessel for the whole operation. These can also be the 5 gallon tubs used by donut makers for their fillings.

Here is a link that might be useful: Beer and wine making supplies

or you can buy a package of Mrs. Wages Pickle mix and they'll be ready in 24 hours. I did her pickled veggies and they came out great!