When purchasing fixed carbon (often referred to as recarburizers or carbon additives), the most critical factor is the Fixed Carbon Content, which should typically be above 90% to 98.5% depending on the application. You must also verify the Sulfur content (ideally <0.05% for high-quality steel production) and Nitrogen content to prevent brittleness in the final metal product. Additionally, check the Ash and Volatile Matter levels; lower percentages indicate higher purity and better absorption rates in the furnace.