Light colored and infusible aluminium minerals when moistened with a drop of cobalt nitrate and heated intensely before the blowpipe assume a dark blue color. Zinc silicates will also yield a blue color under similar conditions. Blowpipe Test with Cobalt Nitrate Procedure:
1. Oxide Coating on Charcoal. When an antimony mineral is heated in the oxidizing flame on charcoal, a heavy white coating of antimony oxide settles on the charcoal at a short distance from the mineral. The coating is readily volatile when heated.
2. Open Tube Test. When metallic antimony or a compound of antimony with sulphur is heart in the open tube, a white powdery sublimate of antimony oxide, Sb2O3, forms in a ring on the inner wall of the tube. It is another oxide of antimony, Sb2O4. It is non-volatile and is usually more conspicuous than the first.
3. Sublimate on Plaster Tablet. Mixed with a mixture of potassium iodide and sulphur yields in the oxidizing flame an orange to red sublimate of SBI3.