dozen crucibles is sometimes used. In recent times the oil fired tilting furnace to hold one large crucible of No. 300 or 400 size has found considerable favor.

At the Nipissing Mill, the bullion from all sources is melted down in an oil-fired reverberatory. After the slag has been removed the metal is refined by playing a blast of air over the surface of the molten bath of metal by which means silver of 998 to 999 fineness is obtained.

The Nipissing type of furnace has recently found favor in Mexico and in many large plants has entirely displaced crucible melting, with a considerable saving on account of crucibles, fuel and labor. The precipitate cakes from the presses without any preliminary drying are placed on trays and covered with about 5% of flux, usually composed of borax and bottle glass and charged into the furnace at intervals until the desired amount of silver has been accumulated. The Nipissing refining procedure is however, omitted, and as soon as the charge is fused, it is tapped out into shipping-bar moulds, the slag following at the end of the pour. About 70 shipping bars, each 35 kilos in weight, are produced at each fusion. When melting about 8000 kilos of fine bullion a month, the furnace bottom will last for 6 to 8 months before it has to be replaced.

Fluxing the Charge.-The flux ordinarily used will vary with the composition of the precipitate and in order to determine the best mixture a number of small fusions may be made with varying proportions and the results noted. Borax is the most generally useful reagent but if used in large quantity, it makes the fusion rather expensive. Its place may be taken to some extent by carbonate of soda and sand, utilizing the tendency of bases to form fusible double silicates. For this purpose only sufficient soda should be used to form the second base in the double silicate. When the solutions are not properly filtered before precipitation there may be a considerable amount of silica in the precipitate and it may not be necessary to add any sand, but such a state of things usually implies bad working conditions. A little fluor-spar is sometimes used to flux difficultly

fusible substances. For soda most text books recommend the bi-carbonate though for what reason it is difficult to see, and the writer prefers the normal carbonate in the form of commercial soda ash. Sodium bi-carbonate evolves a large volume of carbonic acid gas at a temperature below that of sintering of the charge, and if the precipitate is too dry, a considerable loss by dusting may occur; moreover, soda ash is less bulky and also cheaper per unit of fluxing power.

Julian and Smart1 give the following as a typical flux mixture.

If sodium carbonate had been used instead of the bi-carbonate only about 2% of the above amounts of soda would have been needed.

By adding an oxidizing agent to the flux it is often possible to raise the fineness of the bullion by oxidizing some of the base metal and throwing it into the slag. Nitre is sometimes used for this purpose but it has a low efficiency owing to the rapidity of its reaction and E. H. Johnson and W. A. Caldecott3 have sub

1 Cyaniding Gold and Silver Ores, page 168 (Second Edition).

2 The Cyanide Process, page 188 (Fifth Edition).

3 Proceedings Chem. Metall. and Min. Soc, of S. A., Vol. III, pages 46-63 (July, 1902).

stituted manganese dioxide, and suggest the following mixtures:

They state that manganese dioxide has a tendency to carry silver into the slag, so if much silver is present, it should be used cautiously or not at all.

When an oxidizing flux is used the operation should be carried out in crucibles with removable or permanent fire-clay linings, as graphite would tend to counteract the effect of the oxidizing agent.

To one accustomed to the high-grade precipitates produced in the silver mills of Mexico the proportions of flux previously mentioned will seem excessive, and in such cases the quantity may be considerably reduced. A typical flux mixture for these rich silver precipitates is that used by Walter Neal1 at the Dos Estrellas mine in Mexico.

The iron is used to remove the silver from the matte which is so often produced in melting these precipitates even when no acid treatment is given. The same writer, in cleaning-up, makes two short-zinc products. He first screens his material with a 20mesh sieve and afterward with a 60-mesh. The zinc remaining on 20 he returns to the boxes and that passing 20 but remaining on 60 he melts separately with a special flux:

The resulting metal, which contains 20% of zinc, is added to the high-grade bullion when the latter is remelted for running into bars.

A flux the writer has often found useful for clean silver precipitates consists of

Preparation of The Charge.-Opinions vary as to the degree of mixture necessary. Some metallurgists break up the lumps of precipitate and pass the whole through a fine sieve, varying from 14 in. to 20 mesh, before or during the addition of the fluxes, making a thorough mixture before charging into the crucibles, while others, avoiding breaking the lumps as far as possible, spread the precipitate in a layer on the fluxing table, sprinkle the fluxes evenly on top and shovel at once into the pot. The latter method has the advantage of reducing the dust loss to a minimum, but on the other hand is probably more suitable for a pure high-grade precipitate than for a base and dirty one, and the choice of procedure must be determined by the circumstances of each mill.

Briquetting has much to recommend it in the avoidance of accidental spilling of product outside the edges of the pot and minimizing of dust losses in charging, especially where it is the custom to add fresh batches of the material on top of a charge already partly fused, but it is expensive and also conducive to losses due to the additional handling necessary.

Charging. The practice in some mills is to charge a crucible only at the beginning of each fusion and to pour it as soon as ebullition ceases. It is a wise precaution against dust loss but is less efficient in fuel, time and labor, and its adoption should be determined by the value of the precipitate per unit of weight. To avoid spilling when charging, the fluxed material is sometimes put into stout paper bags holding from one to two pounds, which can be placed in the crucible with a pair of tongs.

Pouring. The most usual procedure is to pour the fused charge, slag and all, into a conical mould, and when cool to knock off the bottom and put aside for remelting into a shipping bar. At the Butters Divisadero Company's mill, however, G. H. Clevenger some years ago introduced the practice of carefully tilting the crucible to decant the bulk of the slag into a conical mould, stopping when about an inch only of slag remained on top of the metal: a little dry sand was then thrown in to cool and thicken the slag, which was raked off with a small scraper, the pot being kept tilted so as to bring the level of the molten metal almost to the lip. The clean bullion was poured direct into the shipping-bar mould and a remelting avoided, resulting in a marked saving in fuel and labor. The first batch of slag removed was low grade and free from metallics, but the thickened slag had to be remelted and the thickening agent, being clean quartz, instead of bone ash, was easily fluxed with a little soda. The resulting bullion averaged from 900 to 950 fine in gold and silver.

Sampling. There are various methods in use for sampling the bullion bars for assay. Probably the most common is that of chipping off two diagonally opposite corners of the bar and averaging the resulting assays. Sometimes, however, the difference between them is so great as to throw doubts on the accuracy of both and in that case it will often be possible to obtain a better sample by stirring the molten metal and dipping out a little in a small well-heated ladle just before pouring. This sample may be granulated by pouring it while molten onto a sloping board immersed in a bucket of water, or better still, by pouring into a conical assay mould and when cold drilling a hole right through the centre of the button and assaying the borings.

The by-products of the melt consist of slag, sometimes of matte, and when coke or charcoal is used for firing, of ashes.

When the slag contains visible metallics it is usually ground up and concentrated either in a sluice box, a cradle, or on a Wilfley table and the tailings shipped to a smelter or thrown into the mill bins. The latter course has in some cases been known