In 1855, Robert Arthur discovered that by heating the gold foil impurities could be driven off, and the gold could be made to adhere to itself, a property known as cohesion. He passed each portion of foil through a flame before inserting it in the cavity, and devised methodical routines for filing cavities of different shapes with metal of consistent density, by using plugging instruments with small working ends, and so exerting high pressures to compact the gold, and weld it into a solid mass. 

Digital facsimile from the U.S. National Library of Medicine at this link.