Cut off and discard the stems of the artichokes with a serrated knife and rub the artichoke bottoms with some of the lemon juice. To a kettle of boiling salted water add the remaining lemon juice and the artichokes, boil the artichokes for 15 to 20 minutes, or until the bottoms are tender when pierced with a knife, and plunge them into a bowl of ice and cold water to stop the cooking. Drain the artichokes well, pull off tough leaves. In a small bowl whisk together the oil, the vinegar, the garlic paste, the pimiento, and pepper to taste. Serve the artichokes with the vinaigrette.