1. Remove all the crab meat from the shells and set it aside.
2. Put half of the shells into a large pot with 4 quarts of water over high heat. Add onion 1 stalk of chopped celery, and garlic, then bring mixture to a boil. Continue to boil for 1 hour, stirring occasionally (The white part of the shells will start to become transparent), then strain stock. Discard the shells, onion, celery and garlic, keeping only the stock.
3. Measure 3 quarts (12 cups) of the stock into a large sauce pan or cooking pot. If you don't have enough stock, add enough water to make 3 quarts.
4. Add potatoes, bring mixture to a boil, then add 1/2 of the crab and the remaining ingredients to the pot and bring it back to boiling. Reduce heat and simmer for 4 hours, uncovered until it reduces by about half and starts to thicken. Add the remaining crab and simmer for another hour until the soup is very thick.