Rinse the decanter with plain water to remove any remaining liquid. Pour water into the decanter and swish it around. Pour out the water.
Place several drops of dish detergent in the decanter. Fill the decanter one-quarter full with water along with one-eighth cup of uncooked rice.
Cover the top of the decanter with your hand and shake. Swish and shake the decanter to create a soapy solution and begin scrubbing the inside of the decanter with the rice as you swish.
Allow the solution to sit in the decanter for a day or so. Periodically swish the decanter to scrub the inside with the rice.
Pour out the soapy solution and rice. Scrub the inside neck of the decanter with a bottle brush, water and a small amount of detergent. Thoroughly rinse the decanter, inside and out, with running water until clean.