"Krobo powder glass beads" are made in vertical molds fashioned out of a special, locally dug clay. Most molds have a number of depressions, designed to hold one bead each, and each of these depressions, in turn, has a small central depression to hold the stem of a cassava leaf. The mold is filled with finely ground glass that can be built up in layers in order to form sequences and patterns of different shapes and colours. The technique could be described as being somewhat similar to creating a sand "painting" or to filling a bottle with different-coloured sands and is called the "vertical-mold dry powder glass technique". When cassava leaf stems are used, these will burn away during firing and leave the bead perforation. Certain powder glass bead variants, however, receive their perforations after firing, by piercing the still hot and pliable glass with a hand-made, pointed metal tool. Firing takes place in clay kilns until the glass fuses.