The following estimation is given as an exercise in distillation. The method has been used to advantage, being convenient and requiring little apparatus. For refined methods of distillation and absorption of the mercury by gold plates consult “Beringer,” “Furman,” etc.
Method. — In cinnabar the mercury is combined with sulphur. To separate these, the ore is mixed either with iron filings or quick-lime and heated in a retort. The S unites with the Fe forming FeS, or with the CaO forming CaS. whilst the Hg distils over and is collected in water.
Apparatus.—A small furnace. An iron retort 200 c.c. to 500 c.c. in capacity, and with a tube about 60 cm. long. A beaker of water and a supply of water for cooling the tube.
Reagents.—Iron filings or quick-lime.
Details of the Assay.—Sample the cinnabar ore and pass the final sample through an 80 sieve. Take 5 to 20 gms. of this ore according to its richness, and carefully mix with an equal weight of iron filings or quick¬lime. Transfer the thoroughly mixed charge to the iron retort. Make a paste with whiting and water and carefully lute the edges of the retort and lid. Firmly fasten down the lid. Place the retort in the fire (a convenient fire is obtained by opening the stoking door of a portable muffle). Round the tube loosely wrap a piece of old blanket or other woollen cloth, and to the lower end of the tube loosely tie a small piece of rag. Under this end place a beaker with water, and so arranged that the end of the tube is about ¼ inch above the surface of the water (special care must be taken to prevent the tube dipping into the water, which may run back into the red-hot retort and cause an explosion). Gradually heat the retort to a dull red and allow a small stream of cold water to run down the cloth on the tube. Continue the distillation for 30 minutes, raising the temperature slightly at the last.
Remove the retort and the beaker in which the Hg has collected. Carefully remove the water from the Hg. Weigh on a balanced watch glass. When cool, open the retort and carefully examine the neck and tube for any small globules of Hg, which must be collected and weighed.
The necessary calculations are self-evident.
Accuracy.—Many data are not available, but the author has obtained results on ores between 10% and 30% Hg accurate (as checked by wet methods) to about one unit. This, again, is another of those methods which, though not of a high degree of accuracy, are yet valuable on account of their convenience, and further because the results so obtained often correspond very closely with those afterwards obtained in metallurgical operations.