Remove the outer leaves of the artichokes and leave about 1 1/2-in. of stem. Trim off the hard part of the leaves with a sharp knife. Open the leaves and remove the beard from the center. Place in cold water and lemon juice, to avoid discoloration. Drain the artichokes and press them upside down until the leaves open up completely. Salt and pepper the inside of the artichokes, then place them still upside down in a deep terracotta or iron pot with 1/2 litre of olive oil. Fry the artichokes for 18 minutes over a medium-high flame, with the pot covered. Remove from frying pan and place on paper towels to drain excess fat and cool. Re-heat the same oil, this time making sure it is very hot. Place artichokes into the oil stem up and cook till they open up like flowers. Remove from oil, pat them dry from oil drippings, and serve hot.