Combine the water and clam juice in a 4-gallon stockpot fitted with a strainer insert. (This may also be done in a 3 quart stockpot without a strainer insert simply remove clams, potatoes, corn, etc., using a large strainer or strain in a large colander placed in the sink.)
Add the crab boil, onions, celery, garlic, lemon, salt and black pepper. Add the sweet corn and potatoes. Bring to a boil, reduce the heat to a simmer, and add the sausage. Cook, covered, until the potatoes are almost fork-tender, about 10 minutes.
While the boil is heating, clean the clams by scrubbing thoroughly and rinsing under cold running water. Fill the sink with cold water and swish clams back and forth to dislodge any sand and grit that may be caught in the necks. Rinse and repeat several times.
When the potatoes are almost fork-tender, add the clams, increase the heat to high, cover, and cook until the shells pop open, 6 to 8 minutes. Discard any clams that do not open.
Remove the strainer insert from the pot and drain the boil. Serve the clam boil on paper bags or newspapers. Pass the melted butter and bread.