Chronologically, silicate glazes are the most ancient variety of vitreous materials. Their technology is
inseparably linked with the origin of ceramic crafts in Mesopotamia and Predynastic Egypt in 4,000–5,000 B.C.
Since the first potteries were originally baked in bonfires at 600–800ºC, they had residual porosity making long
storage of liquids impossible. To protect the porous ceramics against external impacts, fusible glazes made from a
powdered mixture of quartz sand (SiO2) and wood ash containing Na2CO3, K2CO3, and small amounts of CaCO3
were originally used. The slurry mixture applied to a fired ceramic object fused at about 800ºC to form a vitreous
shiny coating preventing the penetration of moisture into the surface. Another glazing process was developed in
parallel. It involved preliminary fusion of all components together to obtain a vitreous semifinished product (frit),
which was then ground and placed on the ceramics to be further fused. The glazes initially played a leak-proofing
role, and similar primitive glazing process (slip glazing) has been preserved to this day in handicraft workshops.
Natural impurities of copper and iron oxides made the glazes dark blue or pale blue. Table 1 shows compositions of
two types of Ancient Egyptian dark blue (1) and pale greenish-blue (2) glazes.