Wash, peel and cut lengthwise into ¼-inch-thick slices all the potatoes you intend to use.
Have 2 kettles of deep fat and 2 deep-fat thermometers ready. The first kettle of fat should be heated to 230 degrees and kept at that temperature, the second, to 425 degrees.
Soak the potato slices in ice water for 5 minutes. Drain well and dry on paper towels.
Fry a few slices at a time for 5 minutes in the kettle of low-temperature fat. Transfer these to the kettle of hotter fat and cook until golden brown and as puffy as balloons. Drain, salt and serve immediatle. Continue until all potato slices are cooked.