In Mountains and Sea, Frankenthaler poured paint directly onto the unprimed surface of a canvas, allowing the color to soak into its support, rather than painting on top of an already sealed canvas as was customary. This highly intuitive process, known as "stain painting," became the hallmark of her style and enabled her to create color-filled canvases that seemed to float on air. Hailed by art critic Clement Greenberg as a breakthrough moment in the history of modern art, Frankenthaler's innovation was critical to the development of a new group of artists known as the Color-Field painters. As fellow artist Morris Louis was to declare, Frankenthaler's art "became a bridge between Pollock and what was possible."

Unlike other Color-Field painters of her generation who produced non-objective work, Frankenthaler continued to base her work in nature--both observed and imagined. In this dialogue with nature, her art should rightly be seen as a continuation of the American landscape tradition, reinterpreted in the context of mid-20th century abstraction. In her art, the process of pouring paint onto and over the canvas, allowing the flowing pigment to create its own shapes and edges, became a literal metaphor for experiences of nature. As a consequence, the works maintain powerful allusive qualities, even the impression of infinite space.

Frankenthaler's paintings are perhaps best loved for their exquisite color sensibility and the emotional responses the colors engender. In Spiritualist, 1973, the almost aggressive contrast of the pink, blue, green and yellow gives the painting a forceful visual edge, a superb example of the way Color-Field painting can both push and pull, please and provoke, the viewer in the hands of one of its major practitioners.