Phyllida Barlow was one of the most important British sculptors of her generation, whose work often transformed quotidian materials—plywood, cardboard, cement, fabric, plastic—into what she called “very impractical and very illogical” pieces of unexpected beauty. Her death this week at 78 has left a sudden void in the art world, especially among the many artists whom she taught, mentored and befriended over decades as a teacher and a generous guiding light in her field.