Veganbaking.net - The Hows and Whys of Vegan Baking
Veganbaking.net - The Hows and Whys of Vegan Baking
  
A double boiler takes advantage of the insulating property of air in between boiling water and the item you're heating. Keep in mind that the upper pot sits just above the almost boiling water and is not immersed in it. This allows for a very gentle and even warming of your ingredient. This is best used for things like chocolate or fondant.