Powder is an explosive, powdery substance. It explodes violently when contacted by Hot. Powder is easily the most versatile and complicated material in the game, with no fewer than 9 separate Parameters having an effect on it's behaviour. Carefully tuning these parameters allows you to create things like sand, dirt, mud, clay, salt, icing sugar, and whatever other fine-grained substance you can imagine, as well as giving very fine control over how Powder explodes.
The default Powder settings create an explosion that is very violent but short-lived, making it good at cutting holes in things but not very good at pushing things like a cannon ball. Setting powderExplosionCoefficient and powderExtinguishProbability to lower numbers will make Powder last longer, push farther, and is less likely to "blast through" the walls containing it. Setting powderExtinguishCoefficient to 0 makes an angry swarm of bees. The explosive power of Powder is also strongly affected by maxSpeed and standardDistance.
Powder combined with Elastic or Brittle + Elastic creates a strange material. It sits still until touched, then it will begin vibrating rapidly and can even tear itself apart. This is caused by an interaction with the standardDistance parameter, and is responsible for the Powder-Snow Reaction.
Adding Jet or Fuel to Powder allows you to see Fire in the explosion, increasing its visual value.
With powderExplosionCoefficient set to 0 and with some tuning of powderLightProbability and powderExtinguishProbability, you can perfectly re-create Fuel but without the Fire animation.
Powder with powderExplosionCoefficient set to a low number and powderExtinguishProbability set to 0 creates a strange fluid that resembles plasma. It could also represent atoms vibrating, and powderExplosionCoefficient would roughly approximate temperature.
Powder's physical density is directly controlled by standardDistance and powderSpringCoefficient. Changing the density of the powder strongly changes it's explosive and bulk characteristics, such as whether it floats or sinks in Water and how it tumbles down a hill.
With some tuning, Powder can have many different behaviors.