Trim the stems of the artichokes so that they will sit upright. Bring a large pot of salted water to a boil. Add the lemon juice, drop in the artichokes, and cook until a few leaves pull out with only slight resistance, 20 to 30 minutes. (You will cook the artichokes again later in the recipe; do not overcook them now.) Drain the artichokes, invert them, and cool complet