Thin the paint until it runs like watercolor
Frankenthaler took her oil paint and thinned it heavily with turpentine (the solvent painters use to loosen oil paint) until it stopped behaving like paint and started behaving like watercolor or dye, runny enough to pour and flow. In 1952 she was working in thinned oil. Her stain pictures are often described as acrylic, which is true of her later work, once acrylics became available, but wrong for this one. Mountains and Sea is turpentine-thinned oil and charcoal, full stop.
Next, she laid the canvas flat on the studio floor, the way Pollock did. And then, the move that changes everything, she did it on raw canvas. The full, exact description from the people who own the picture is “oil and charcoal on unsized, unprimed canvas.” Both technical words matter. Unsized means the cloth has no size, the glue-like sealant that is normally brushed into raw fabric to stop it drinking up liquid. Unprimed means it has no primer, no white gesso ground laid on top to give paint a smooth, sealed surface to sit on. So this is bare, thirsty cloth, sealed by nothing.
Pour the thinned color onto that bare cloth and the result is the whole point. On a normal primed canvas the paint would sit on the surface as a film you could see and touch. Here there is nothing to hold it on top, so the color soaks down into the weave of the fabric and stains it, the way wine soaks a tablecloth. The pigment becomes part of the cloth itself rather than a layer resting on it. It is a little like fresco, the old wall-painting method where pigment is laid into wet plaster and fuses with the wall as it dries, becoming the wall instead of a coat on it. To steer the color once it was down, she tilted and lifted the canvas so the pools ran, and worked them with sponges, squeegee-like wipers, and rollers, drawing into and around it all in charcoal.
That technique has a name: the soak-stain. Paint thinned enough to sink into raw, unprimed canvas and dye the cloth, rather than lie on the surface. The scope of it is easy to overshoot, so be exact: Frankenthaler did not invent pouring. Pollock had been pouring enamel for years, and she worked from his precedent openly. What she pioneered is the soak-stain, the pour that sinks in and becomes a stain. Mountains and Sea is the first painting she made this way. She was twenty-three.