One of the best way to keep your drink at its best taste for a longer time is to use a sphere of ice used for keeping straight spirits, and short cocktails like the old fashioned, cold for as long as possible. Being one large piece of ice with a smaller overall surface area than several pieces of ice, the ball melts more slowly and keeps the drink cooler, and less diluted, for longer. It also looks really cool.
Popularized by the Japanese who hand-carve them using ice picks or insanely sharp sushi knives , a variety of other products have popped up as alternatives for those who value having ten fingers. Silicone moulds are perhaps the cheapest and easiest method but invariably leave an unsightly seam along the ball’s circumference. The best products I’ve seen though are ice-ball molds – large hunks of metal that seemingly defy all logic by using just thermal capacity and gravity to form a perfectly round sphere in under a minute.
Here’s is the instruction using which you can make an ice ball:
1. Take a round balloon of any size and fill it with water from the sink. You can fill it up as much as you like, but keep in mind that the bigger it is, the longer it will take to freeze. You should also be sure that the balloon will have enough space in your freezer.
2. Let all the air out of the balloon. If an air bubble remains inside the balloon, it will cause a large dip in the ice ball. Tie the balloon once the air has been released.
3. Put the balloon in the freezer. If you have a round bowl that is about the size of the balloon, you should put the balloon inside it. This will help the balloon keep a round shape during the freezing process. You can freeze it without putting it in a bowl, but the ice will end up being flat on one side.
4. Let it freeze for at least one day. A large ice ball could take up to three days. A balloon the size of a softball will be ready in one day and a balloon the size of a basketball will take three.
Reference
http://www.taisin-ss.co.jp/icemold/english/index.html