Use of Charcoal in Blowpiping
Small charcoal blocks, that should best be about 4 inches long, 1inch wide and ½ inch thick, are employed in a number of blowpipe tests. They are used as a support upon which various reactions are accomplished. For instance, metals like lead, silver, copper, etc., may be reduced from their minerals by means of the blowpipe flame, the experiment being performed upon charcoal. Characteristic oxide coatings also may be obtained upon the surface of a charcoal block (see Fig. 204). The charcoal should be of a fine and uniform grain. If should not be so soft as to readily soil the fingers, nor should it be so hard as not to be easily cut and scraped by a knife. The following table gives a list of the elements which yield characteristic oxide coating when their minerals are heated in the oxidizing flame on charcoal. In some cases more characteristic coatings are obtained when the assay has had some chemical reagent added to it. The most important reagent is either hydriodic acid or the so-called bismuth flux, which consists of a mixture of potassium iodide and sulfur. When these reagents are used colored iodide coatings may result.
Composition of coating. | Color and character of coating on charcoal | Remarks |
Arsenious Oxide AS2O3 | White and volatile, depositing at some distance from the mineral. | Usually accompanied by garlic odor. |
Antimony Oxides | White and volatile, depositing close to the mineral. | Heavier than arsenic oxide. |
Selenium Oxide. SeO2. | Volatile white, tinged with red on outside; to gray near assay. | Accompanied by a peculiar odor. Coating touched with R.F. gives blue flame. |
Tellurium Oxide. TeO2. | Dense white; volatile. On outside gray to brownish. | In R.:F coating gives bluish green flame color. |
Zinc Oxide. ZnO. | Yellow when hot, white when cold. Nonvolatile in the oxidizing flame. Deposits very close to mineral. | Inf coating is moistened with cobalt nitrate and heated intensely, it turns green. |
Tin Oxide. SnO2. | Faint yellow when hot, white when cold. Nonvolatile in the oxidizing flame. |
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Molybdenum Oxide. PbO. | Pale Yellow hen hot, white when cold. Sometimes crystalline. Volatile n the oxidizing flame. | If the coating is touched for a moment by a reducing flame, it becomes dark blue. |
Lead Oxide. | Yellow near the mineral and white farther away. | Coating at times is composed of white sulphite and sulphate of lead in addition to the oxide. |
Lead Iodide. PbI2 | Chrome-yellow. | Volatile. |
Bismuth Oxide. BI2OI3. | Yellow near the mineral and white farther away. | TO be told from the lead – oxide coating by iodine test. |
Bismuth Iodide, BiI3. | Bright red with yellow ring near assay. |
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Composition of coating.
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