context: The oldest known type of glass is alkaline lime glass, which is made from a mixture of soda, sand and lime. In Egypt, glazed stone beads were made into it for about 4000 BC. [8] It was discovered early on that glass could be colored by adding various ores. Such colored glass was used for ceramic glazing and imitation gemstones. [8]
question: How long has the glass been made?
answers.text: 4000 BC