Ancient glass had many impurities, making it difficult to produce clear glassware. However, by adding a decolorizing agent such as antimony or manganese, by suppressing the coloration of iron and others, and by adjusting the temperature, colorless or light green glass could be produced. Initially, glass was developed as a substitute for lapis lazuli, turquoise, and onyx, but by this time, people favored glass containers where food or items stored inside were visible on the outside.