Thou shalt overlay it with pure gold. — It is possible, but scarcely probable, that gilding is intended. Gilding was well known in Egypt long before the time of Moses, and may have been within the artistic powers of some of the Hebrews. But it is a process requiring much apparatus, and less likely to have been practised in the desert than the far simpler one of overlaying with gold plates. Gold plate would also have been regarded as more suitable, because more valuable. It is the Jewish tradition that gold plates were employed.
crown of gold — i.e., a rim or border of gold, carried round the edge of the chest at the top. The object was probably to keep the kapporeth, or mercy-seat, in place.