Blending and Heating Raw Materials

The activated clay is blended with the other raw materials and dry-ground, usually in batch or continuous ball mills, to a mean size approaching 15 gm. Typical recipes (in wt.%) are  

The furnace is allowed to cool to 500 °C when air is admitted in controlled amounts. The oxygen reacts with excess sulfur to form sulfur dioxide, which exothermically oxidizes the di- and triatomic polysulfide ions to 82 and free radicals, leaving sodium sulfoxides and sulfur as by-products. When oxidation is complete, the furnace cools and is unloaded, a full kiln cycle takes between 3 and 4 weeks. The raw ultramarine product typically contains 75 wt.% blue ultramarine, 23 wt.% sodium sulfoxides, and 2 wt.% free (uncombined) sulfur with some iron sulfide. 

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