There are two ways to make warmed sake. The first way is used in professional establishments such as restaurants. The second method is recommended for very busy restaurants or to people who want to enjoy warmed sake at home.
By Hot Water Bath
The first method involves the use of a Tokkuri (pottery carafe). This Tokkuri has to be placed up to its neck in a hot water bath. It is best to use a thermometer to determine the precise temperature (104 °F ~ 131 °F). At this point, it cannot be put into a pot or in an oven because boiling water will make the sake too hot, ruining its original taste.
If sake is to be heated by convection in a microwave, the temperature in a thin-trunked Tokkuri would be uneven, so a thicker-trunked Tokkuri would be more suitable for heating sake in a Microwave.
Cover the top of the bottle and place in the microwave for a minute. This will ensure that the sake will have an even temperature and that the upper portion will not be the only warm part.