Sand in a Box: Fire and Metal

Four new elements

● Lithium and Potassium — metals that can be unstable.
● Aluminium — a new lightweight solid, but with a relatively low melting point.
● Thermite — burns hot and can be a bit explosive.

Bug Fixes and Improvements:

● "Metals" category is now properly defined.
● Added missing translations and images.
● Various improvements to particle definitions.
● Flammable property now works correctly, and some materials burn as they should!