tl;dr black powder and gunpowder must be under pressure to properly deflagrate, and they’re unassuming
They are cheap, easy to build and inconspicuous. And as the explosions this week at the Boston Marathon show, pressure cooker bombs can be devastatingly effective weapons.
But why would someone place a bomb inside a common kitchen implement? As we explained two years ago in an article about the increased use of pressure cookers (for cooking, not bombs):
“It works using basic principles of science: Water normally boils at 212 degrees Fahrenheit — and doesn’t get hotter. But under pressure, it boils at about 242 F and stays there. Raising the boiling point lets the food cook at the higher temperature, which cuts cooking time by two-thirds or more.”
The same principle that increases the boiling point inside a pressure cooker also can be used to amplify the force of an explosive.
As NPR science reporter Geoffrey Brumfiel notes, “If you seal a pressure cooker, the steam builds up in the vessel. It helps to raise the temperature and cook food, but sometimes when the cooker fails, you get a very energetic release.”