Decolourising glass removes green casts
Sometimes contaminations creep into the glass during the production process. It only takes a slight impurity in a raw material or additive which will act as an unwanted colouring element in the batch. This is where decolourising comes in.
What this means: The glass is purified during the production process already. If this is not done, the precious glass might end up with a green cast in the end. Not everyone likes such a shade, for example in their window panes.
Decolourising glass is actually a colouring process, as the method uses the principle of complementary colours. That means: One colour balances out another colour shade to neutralise it to the human eye.
So another colouring substance is simply added to the batch. Glass decolourising is therefore a bit of a myth, really. Counterdyeing or neutralisation would be more suitable terms.