Sulfur is a raw material that can be refined into Heavy Explosive Materials at the Refinery. It can be obtained by mining Sulfur piles on a Sulfur Field or by collecting them from a Sulfur Mine which has to be fueled in order to work.
Sulfur can be mined manually with a Sledge Hammer or in bulk with a Harvester. A sulfur field starts with 30 nodes and 167 more in its reserve. Each node yields 154 sulfur by default meaning one field contains about 30,000 sulfur. Once fully depleted, the field replenishes after 6-10 hours. The state of the field can be viewed at the large rock central of the field, by interacting with it (default
E) or by hovering your mouse over the field's icon on the map.