Decanters, brouilles and absinthe fountains were invented for one particular purpose: to enable you to pour the water into your absinthe glass as gently as possible – ideally, drop by drop – especially at the begining, when the absinthe starts to louche. There are two main reasons for pouring the water carefully:
1. The louche is even more beautiful.
2. The different aromas of your absinthe develop more slowly, and can become much more complex and interesting. (Chemistry helps to understand this phenomenon: each essential oil precipitates at a different dilution, and pouring the water slowly enables the aromas to develop one after the other).
However, it is all about louching your absinthe without “drowning” it by letting the water flow as slowly as you should savour your drink.
Marcel Pagnol illustrated this in his novel “Le temps des secrets” – 1960 (The time of secrets):