What makes glass tranparent?

Answer 1

Most glass is currently made of silica (sand), lime, and soda ash. These have natural impurities for instance iron oxide, this causes a greenish glass. Glassmakers have been adding chemicals to glass since circa Roman times in an attempt to make clear glass, but it in the 15th century that they were able to develop a reasonably dependable formula. In or around the late 1400s, glassmakers in Venice in Italy developed a perfectly clear type of glass they called cristallo. They used quartz sand and potash to make the glass. This glass was prized possession because it was so clear. Today we take its clearity for granted