Press molding is a handbuilding technique in which plastic clay, generally a slab, is pressed by hand into or over a mold, taking on the mold's shape and any surface texture or detail carved into it. Unlike slip casting, which pours liquid slip into a mold, press molding shapes clay while it is still in its ordinary plastic, workable state.
A rolled slab of clay is pressed into a concave mold (a press mold) or draped over a convex one (a hump mold), and worked by hand or with a rib to conform closely to the mold's surface, including any texture, relief detail, or lettering cut into it, which transfers directly onto the clay. The clay is left in or on the mold until it firms up enough to hold its shape unsupported, then removed. A two-piece mold, one half for each side of a form, can be pressed separately and the two halves joined together while still soft, the standard handbuilding join (scored and slipped) used the same way as elsewhere in handbuilding.
A plaster mold used for press molding needs to be fully dry before use, since a still-wet mold will not draw water from the pressed clay the way a properly dried one does, making the clay hard to release cleanly. Press molding is a common way to reproduce a detailed surface, a relief pattern, a maker's mark, or a specific found texture, repeatably across many pieces from the same mold, without the mold needing to survive contact with liquid slip the way a slip-casting mold does. Because the clay stays in its plastic state throughout rather than shrinking as it dries against a mold wall the way cast slip does, a press-molded piece releases from its mold sooner than a comparably sized slip-cast piece would.
A dry plaster mold's own porosity is normally enough on its own to release the clay cleanly, without any release agent. A very fine, sparing dusting of cornstarch or talc is sometimes used anyway on molds with especially fine detail, where the clay might otherwise cling in the deepest part of a carved line; too heavy a dusting works against the goal, since excess powder fills exactly the fine detail it was meant to help release, leaving a softer, blurrier impression than the mold is actually capable of.