The glass consisted of pieces that arrived at recycling facilities in the form of particles less than 3 mm in size, which made them too small to be sorted and recycled. The ash was generated at the facilities by burning un-recyclable items such as staples and bits of aluminum and ceramic, along with certain types of paper and plastic. Importantly, neither the glass particles nor the ash required any grinding before being used in the experimental bricks. Both materials served partially as a flux, which lowered the melting point of the silica (sand) in the mixture.