Glass decanters get stained from wine, tea and other liquids if the glass is not rinsed well after use. Decanters are fragile, and the neck is often too small to fit your hand down into, which makes them a bit tricky to clean. Fortunately, it's possible to remove the stains in very little time using the appropriate method.
Place a towel in the bottom of your kitchen sink to protect the bottom of the decanter from the sink. Allow the decanter to come to room temperature if it has been holding hot or cold liquids (otherwise, the temperature change when you add water may cause it to crack).
Fill the decanter half full of warm water and 1 tsp of liquid dish detergent. Add 2 tbsp of dry, uncooked rice to the decanter. Hold the decanter firmly and swirl the rice around in the liquid. Repeat this often during a period of 30 minutes.
Pour out the liquid and the rice. Refill the decanter with warm water up to the top. Add 2 tsp of dish detergent.
Push a bottle brush down into the decanter and brush the stains. Pour out the water and rinse the decanter with warm, clean water. If any stains remain, including hard-water deposits, pour white vinegar into the decanter and allow it to sit for 15 to 30 minutes. Use straight vinegar, or dilute it with half water. Rinse the decanter well with clean water.
Dry the decanter with a soft towel as well as you can. Leave the decanter out to finish air drying before you store it away.