CHAPTER V.
ASSAY OF ORES.
WHEN AN ORE WILL PAY--WASHING FOR GOLD AND PLATINUM--HOW TO ASSAY GOLD IN THE SIMPLEST WAY--TO TEST ANY ROCK FOR GOLD AND SILVER--TO FIND THE PURITY OF GOLD--TO DETECT AND ASSAY SILVER ORES--ASSAY OF COPPER, IRON, ZINC, TIN AND LEAD ORES--READY METHOD OF TESTING GRAPHITE.
One of the first questions asked after the discovery of a metallic ore, is--“will it pay?” We propose to state in plain words a method of determining the character and value of the principal ores, so that any intelligent man, however unscientific, may answer his own question. The chemical analysis or exact assaying of ores is too complicated, and must be left to professional assayers.
“Will it pay?” is an important query; for many ores of even precious metals, are not “paying.” Whether an ore is profitable depends not so much upon the relative value of the metal as upon the ease of separating it from the rock or “gangue” as it is called. Thus the minimum percentage of metal, below which the working of the ore ceases to be profitable is--
Of Iron, 25 per cent. Zinc, 20 “ Lead, 20 “ Antimony, 20 “ Copper, 02 “ Tin, 01½ “ Quicksilver, 01 “ Silver, 1/2000 “ Platinum, 1/10000 “ Gold, 1/100000 “
That is, an ore of iron which contains less than 25 per cent. of metal will not pay for working; for the reduction of iron in comparison with copper ore is very difficult. Gold is very easily extracted, and hence some quartz rocks which do not apparently contain a particle of gold, pay well, a bushel of rock often yielding half an ounce.
Iron occurs in large masses or beds; but the other metals are scattered in fragments through sand or soil, or exist in veins running through rocks.
WASHING FOR GOLD AND PLATINUM.--This operation, called “panning,” is the oldest and simplest method of extracting the precious metals. At the present time, it furnishes to Russia nearly all the gold produced in that empire. It is based on the principle that substances of different weights may be separated by means of water,--the heaviest going to the bottom first. To examine the bank or bed of a river, suspected to contain gold, fill a milk-pan with the sands and carry it to a tub or pool of quiet water. Dip it under, stirring the mass with one hand or a stick. Then pour off the muddy water, fill with fresh water stirring again, and again pour off the light sand, clay, etc. Scales of gold will sink fast; mica flakes will take their time. Repeat this process till all the fine particles are washed off; then allow just enough water to enter the pan as will cover the sand. By shaking the pan and gradually lowering the side by which it is held, the light sand will flow off, leaving in the corner a heap of coarse sand. Put in a small quantity of water and turn the pan around so as to create a gentle current, when the precious metal, if there be any, can be easily detected,--the gold by its bright lustre, the platinum by its lead color, and both by their malleability. Particles of gold are of uniform color and are either flat or rounded; while other yellow grains are angular. Holding the pan in the sunshine, secure any glittering glassy crystals, and test them for diamonds or rock-crystals. A magnet will remove any particles of magnetic iron-ore.
ASSAY OF GOLD ORE.--Gold may be found in quartz rock, in iron and copper pyrites, and in silver ores.
To ascertain if any gold is present in quartz, reduce the rock to powder and sift it. A certain quantity, say half a peck, is then washed as above described, till a manageable quantity of sand is left. If there is any show of gold, dry the mass and put it in a bowl or glass dish, and add an ounce of quicksilver, stirring the mixture well with a wooden rod. The quicksilver, which will unite with every particle of gold which may be there, is then poured off into a soft leather (chamois) bag. This is squeezed to remove superfluous quicksilver, and a pasty amalgam is left, which is put into an iron vessel and heated red hot. The yellow powder remaining is mixed with saltpetre and melted, when a button of pure gold will be found in the crucible. Quartz ores should yield $6 to the ton in order to pay.
To test pyrites for gold, reduce a given quantity to powder and wash as before; then roast the residue at a red heat. Upon cooling, add quicksilver and treat as just described. Pyrites should yield $1 of gold to the bushel of ore to be profitable.
Native silver often contains gold. To separate them, carefully flatten the alloy with a smooth hammer on an anvil, and then boil it in strong nitric acid in a glass flask for about ten minutes. Carefully pour off the acid into a vial, and wash the powder in the flask (which is fine gold) with water and dry. To the liquid in the vial add a solution of common salt. The white powder which falls should be removed, washed with water, and fused with powdered chalk or iron filings; a button of pure silver is the result.
Any substance supposed to be or to contain gold may be tested by dissolving it powdered in aqua regia and then pouring in a solution of copperas; if there is gold, the reddish-brown precipitate, by rubbing, assumes a bright metallic lustre.
To tell whether a globule of silver has any gold in it, put it on a white porcelain dish and moisten it with a drop of nitric acid: if it is pure silver, it will dissolve and retain its white color; if mixed with gold, it will soon turn gray or black.
To test the purity of gold, rub some of it off on a hard black flint slate, and apply to the mark a drop of aqua fortis. If the gold is pure, the yellow streak remains unchanged, but if alloyed it partly disappears; if it is only an imitation of gold, it vanishes altogether.
A ready method of finding the amount of gold in a quartz rock with considerable accuracy, is by taking the specific gravity of the rock (well cleaned) as given on page 13. If the gravity is not over 2.7, it contains little or no gold. If it is 3, it very likely is gold-bearing, although pyrites may be present. But if it is over 5, it is undoubtedly auriferous, and if 12, it is very rich in gold.
It is generally considered that the sand of any river is worth working for the gold it contains, provided it will yield twenty-four grains to the hundred weight.
ASSAY OF SILVER ORE.--Pure silver is easily recognized. But lead and copper ores often contain a large percentage of the precious metal.
To detect silver in lead ore, dissolve the powdered ore in strong nitric acid; pour off the liquid and insert a piece of pure copper. If silver is present, it will go to the bottom. Or, add to the liquid a solution of common salt, and it will instantly become cloudy or white. If lead ore yields three ounces of silver to a ton, it may be worked for the silver as well as the lead. In Colorado, the average value of silver-bearing galena is $100 per ton.
To test the copper ores for silver, dissolve them in nitric acid; then add a few drops of muriatic acid, and if silver is present, a white curdy precipitate will fall to the bottom. Native copper, when polished, often shows white spots of silver.
To estimate the proportion of silver in lead ore, reduce a known quantity of the clear ore to powder, mix with a little dry soda and a few nails, and heat in a round-bottomed iron pot or crucible. The lead which is obtained should then be put in a cup having ashes at the bottom, and strongly heated in an open furnace. A globule of silver will be left, if any is present, and being weighed, the percentage can be found.
Rich silver ores may be reduced by mixing them with ten parts of common salt, and exposing the mass for hours in an open furnace, stirring it frequently. When cold reduce to powder and mix with an equal quantity of quicksilver and enough water to make a paste, and agitate the mixture for two days, when the amalgam will fall to the bottom. The amalgam is then squeezed in a leather bag and washed.
Silver glance will yield its metal by heating it before a blow-pipe.
ASSAY OF COPPER ORE.--When the ore is native copper and rock, as at Lake Superior, it should be pounded and the earthy matter washed away. Then mix with a little potash or soda and bring to a high heat in a crucible.
Other copper ores may be tested by dissolving them powdered in dilute aqua regia. The presence of silver will be shown by a white powder on the bottom. Then add considerable ammonia. If there is any copper a blue liquor will be produced. Strain this through tissue paper, and evaporate to dryness. Dissolve the residue in muriatic acid, and by putting in a piece of iron or zinc, the copper will fall down. Or, add to this solution pure potash; dry and weigh the powder thrown down; every 5 parts of it contains 4 parts of copper.
Gray copper and red copper ores may be assayed by heating with charcoal, (both powdered,) in a furnace. Malachite and azurite should be smelted with borax; Copper pyrites and silicate of copper with soda or powdered marble.
A ton of copper ore which contains ten per cent. of metal, pays $25 at the furnace. The ore of copper when roasted, turns black; and when thrown into nitric acid makes a sky-blue solution. A clean knife-blade put into this solution will be coated with copper.
ASSAY OF IRON ORE.--Take a known quantity of the ore in fine powder and mix thoroughly with dry borax (or with one part of fluor spar, one of charcoal and four of salt,) and expose it for an hour in a covered crucible lined with charcoal to a white heat in a wind-furnace for an hour. A button of iron will be found at the bottom, which determines the percentage.
ASSAY OF ZINC ORE.--If the weighed ore is roasted with powdered charcoal, white flowers of zinc will be formed on a piece of cold iron held over it. After thorough roasting, the residue should be weighed; the loss is the oxide of zinc, and every 100 parts of this contain 81 of metal.
All the ores of zinc will dissolve in either nitric or hot sulphuric acid.
ASSAY OF TIN ORE.--Tin-stone will yield up its metal if mixed with charcoal, borax and soda, and heated on the hearth of a furnace or before a blow-pipe.
The presence of tin may be tested by dissolving the metal thus roasted out, in aqua regia and adding a decoction of Brazil-wood: if the metal was tin, the liquid will be colored a beautiful crimson.
ASSAY OF LEAD ORE.--Both galena and cerussite are rich ores, and when abundant pay well. They are easily reduced by heat, the former being usually mixed with charcoal and iron filings. If a western backwoodsman wants shot or bullets, he kindles a fire in a hollow tree or an old stump, puts some galena on the charred wood, and melts it down. After cooling, he finds the metal at the bottom. The smelting of a ton of lead costs about $6. The average price per ton of galena is $30. When galena is dissolved in warm nitric acid, a clean plate of zinc placed in it will be coated with brilliant blades of lead; if the galena contains silver, a plate of copper will be served in the same way. A solution of chromate of potash poured into a solution of lead ore in nitric acid will throw down a yellow powder.
TO TEST THE PURITY OF GRAPHITE.--Its value depends upon the amount of its carbon. Pulverize and then dry at a heat of about 350 degrees, twenty grains of it; then place it in a tube of hard glass four or five inches long, half an inch wide and closed at one end. Add twenty times as much well dried oxide of lead and well mix. Weigh the tube and contents, and afterwards heat before a blow-pipe till the contents are completely fused and no longer evolve gases. Ten minutes will suffice for this. Allow the tube to cool and weigh it. The loss in weight is carbonic acid. For every twenty-eight parts of loss there must have been twelve of carbon.