Glass is a matererial of the highest quality, which confers its brilliance on gardens: it shines, glistens, reflects, sparkles. The traditional use of glass in gardens is for protection. In this role it is superior to plastic in its light transmission but less good in its termal insulation, unless two layers of glass are used and there is a vacuum between the glass layers.
Glass also has decorative properties though it is delicate. Owners are rightly cautious where there is a risk of the glass being broken. But if the glass is safe it is a beautiful material for making ornaments.
Glass ornament cannot be made by extractive methods (equivalent to a sculptor chipping at a block of stone). Ornaments can be made in clay and cast in glass but the results tend to be sentimental, as though they were meant to be in some other material. The classic alternatives are blowing the glass, when it is molten, and adotping a constructive method using glass sheets, glass chips or glass blocks.
Coloured glass and slate