Abstract: A precursor yarn or thread is made by passing a central element, e.g., a filament, a tow, or a flat member, through a bath of a binder, such as a low-temperature hot-melt adhesive, and aqueous urethane, or an acrylic material, with which is mixed a quantity of hard-shelled microspheres which expand when heated to a higher temperature. This is then covered by a sheath, e.g, of PVC, polyurethane, polyester, acrylic resin, polycarbonate, polypropylene, or polyethylene in a second bath. When this product is heated to a transition temperature which is characteristic of the microspheres chosen, the microspheres expand, swelling the sheath. Such a precursor could be woven into a fabric and then heated, so that as the yarn expands the fabric mesh becomes tighter, reducing its porosity. This would be useful as a yarn in making papermaker's felts.