[S] Yellow powder.
Sulfur Flour (not the same as "flowers"). Used as a fuel, usually in compositions with nitrate oxidizers; as a component of black powder (serving as both a binder and fuel); in rocket propellants as the oxidizer for zinc. Ours is 99.9% pure and has a very low acid content.
Click here to learn about different types of sulfur. Along with potassium nitrate and charcoal, sulfur (also spelled sulphur) is one of the three components of black powder. Since you need a lot of black powder in fireworks making, buying sulfur in larger quantities or in a black powder kit will save you money.
Our low-acid sulfur is the type used in most commercial fireworks in the US. Low-acid sulfur is safer to work with, in fireworks. To reduce explosion risks in fireworks making, avoid using "flowers of sulfur."