The term iron ore is limited to oxides of iron, either as such or in combination with water or carbonic acid. Other compounds of iron, as for example the sulphide, are not adapted for iron making. The term is further limited to deposits of sufficient purity and richness to render smelting profitable. These factors differ in different localities. Both the ferric and magnetic oxides occur in nature, sometimes nearly pure; they are called respectively hematite (red or anhydrous hematite) and magnetite. Ferric oxide also occurs in combination with water in various definite proportions; these compounds are called hydrous or brown hematites. Ferrous oxide is a component of many minerals, but only the ferrous carbonate is important as an ore of iron; it is known as spathic ore. Mineralogi-cally iron ores may be grouped as follows:
Color of powder.
Ferric Oxide-Red Hematite.
Cherry red to reddish brown...
Ferric Oxide Hydrated - Brown Hematite.
Yellowish brown to rusty yellow
Massive or fibrous.
Massive or earthy.
Brownish to ochre yellow........
Ferrous Carbonate - Spathic Ore.
Magnetic Oxide - Magnetite.
Iron ores rarely occur in masses of mineralogi-cal purity. The hydrous hematites are so closely related in their formation, occurrence, and physical appearance, that their distinction is sometimes impossible without chemical analysis, but generally the groups are readily distinguishable. The color of the powder, or streak, is very characteristic. Turgite here forms an exception to the rule; but it is easily recognized on heating, since it decrepitates and gives off water. Nearly all the iron ores contain earthy substances. These are commonly silica, alumina, lime, magnesia, etc.; silica usually predominating. These substances are removed in the cinder on smelting. Manganese accompanies iron in nearly all its ores, but for the most part in small quantities; the spathic ores contain the largest proportion. Under favorable conditions the manganese is reduced in the furnace and unites with the iron; usually, however, the greater part goes into the cinder. Sulphur is found in many ores, either in the form of sulphuric acid or as iron pyrites. According to the conditions of the smelting, the sulphur may enter either the iron or the cinder.
Phosphorus, in the form of phosphoric acid, is present in most iron ores, either combined with the oxide of iron, or mechanically disseminated as apatite (calcic phosphate). It is the most dreaded of all the impurities of iron ores, since no method has been discovered of eliminating it in the blast furnace; nearly the total amount of phosphorus in the ore goes into the iron. Titanium, as titanic acid, is present in many ores, especially magnetites. It renders the ores very difficult to smelt in the blast furnace ; it is generally mainly removed in the cinder, but occasionally some of it unites with the pig iron. Chromium in small amount is not an unusual ingredient of iron ores; on smelting it probably passes mainly into the pig iron. Zinc is very commonly present in minute amounts; it is completely volatilized, and forms incrustations of white oxide around the furnaces. A brief description of the leading varieties of iron ore deposits, and their distribution, particularly in the United States, is all that will be attempted in this article.
The Term Hematite (Gr. blood) is properly applied to the sesquioxide only, since this has a red powder ; but Theophrastus speaks of or yellow hematite - probably an ochreous limonite. The ferric or sesquioxide occurs in several varieties. The specular ore has a crystalline structure, often forming beautiful splendent rhombohedral crystals. The famous mines on the island of Elba, worked before the beginning of the Christian era, furnish this variety in great purity. Sometimes the structure is foliated or micaceous, giving the ore a greasy appearance and feel; it is then called micaceous hematite. The more common varieties are the compact, columnar, and fibrous. It occurs also in concretions or botryoidal masses. Its color is brownish red to iron black (red hematite). Occasionally it is earthy in character (red ochre). An argillaceous variety is known as clay ironstone, or argillaceous hematite. It is also often oolitic in character. All the varieties have the red streak in common. Hematite is found with the iron partially replaced by titanium, giving rise to various mineral species, such as mena-cannite and ilmenite. They have the general formula (Ti, Fe, Mn, Mg)203, and contain from 3.5 to 59 per cent. of titanic acid. The hematite ores are as a rule of great purity, and from them is made a large proportion of the finer varieties of iron and steel.
Nearly all the Bessemer pig iron in England and America is made from red hematite. " It occurs in rocks of all ages. The specular variety is mostly confined to crystalline or metamorphic rocks, but is also a result of igneous action about some volcanoes, as at Vesuvius. Many of the geological formations contain the argillaceous variety or clay ironstone, which is mostly a marsh formation, or the deposit over the bottom of shallow, stagnant water; but this kind of clay ironstone (that giving a red powder) is less common than the corresponding variety of limon-ite or siderite. The beds that occur in metamorphic rocks are sometimes of very great thickness, and, like those of magnetite in the same situation, have resulted from the alteration of stratified beds of ore, originally of marsh origin, which were formed at the same time with the enclosing rocks, and underwent meta-morphism, or a change to the crystalline condition, at the same time." (Dana.) The hematite ores are widely distributed. Immense beds occur in Chili, and it is said in other South American states. The mines of Norway, Sweden, Lorraine, Switzerland, Saxony, Bohemia, and the Hartz also contain this ore.
Unusually pure varieties are found in the mountain limestone of the carboniferous system in Cumberland and North Lancashire, England; and remarkably fine fibrous hematite is mined in Wales. At Bona, Algeria, there are extensive deposits of pure hematite, which is exported to France, England, and the United States for the manufacture of Bessemer steel. In the United States there are immense deposits of specular ore in the azoic rocks of the Marquette region, south of Lake Superior. These deposits probably consist chiefly of martite, which is sesquioxide of iron crystallizing in isometric forms, and supposed to be pseudo-morphous after magnetite. According to this hypothesis, the Marquette ore beds were once all magnetite in composition, and have been changed to sesquioxide by the addition of oxygen. Some of these deposits present masses of absolutely pure ferric oxide; the majority, however, are more or less silicious, containing streaks and masses of jasper. The amount of sulphur and phosphorus is small, and the ores are consequently well adapted for the manufacture of steel.
They furnish a large proportion of the Bessemer pig iron of the United States. Missouri, which is one of the richest states in iron ores on the North American continent, contains specular ore in porphyry and in sandstone, as well as in disturbed and drifted deposits, and also strata and drifted deposits of compact and earthy red hematites, supposed to be in many cases the product of an alteration of the specular ores. The most famous deposits are those of Iron mountain and Pilot Knob. At Iron mountain, which is the largest ore deposit in Missouri, a hill of decomposed porphyry 250 ft. high is traversed by numerous ore seams, and cut in two by an enormous vein of specular ore from 40 to 60 ft. thick, besides being covered with surface ore in rounded bowlders and fragments of variable size, in a stratum usually from 1 to 5 ft. thick. At Pilot Knob the ore is not in veins, but forms a regular bed in the porphyry of blue conglomerate. Shepherd mountain, Cedar hill, and other localities show similar deposits. The Missouri specular deposits in sandstone belong to the lower Silurian formation, and seem to have been originally formed in lenticular shape. The red hematites of the carboniferous formation of Missouri extend over large areas, as beds impregnating or replacing the ferruginous sandstone.
All the Missouri specular iron is more or less magnetic, and in some cases it possesses polarity. Specular ores and massive or earthy and oolitic red hematites occur in the great azoic region of northern New York, in St. Lawrence, Clinton, and other counties. The Sterling, Parish, and other mines are famous. The Rossie hematites are now brought in considerable quantity to the Hudson river, for smelting with the magnetites of Lake Champlain. It is said that these hematites are so nearly pure as to permit the use of a considerable portion of them in the manufacture of Bessemer pig. In North and South Carolina a micaceous ferric oxide in schistose rocks, called itabiryte or specular schist, occurs. It is found also in great beds in Canada. In some parts it is a rich ore of iron, and in others passes into ordinary chlo-ritic schists. The Laurentian system in Canada contains beds of hematites, or oligist iron ore, in large irregular masses, as on Lake Ni-pissing, arranged in the planes of stratification. Similar ore occurs in small beds in the Potsdam sandstone. Specular ores occur in Maryland, Virginia, and other southern states, but do not yet constitute so important a source of iron production as the brown hematites, magnetites, and argillaceous carbonates.
Maine and New Hampshire also present red hematite deposits, the largest of which is on the Aroostook river in the former state. The finest iron ore of this variety yet discovered west of the Missouri river is the deposit of red hematite near Rawlins, Wyoming territory. It is massive and very pure, and has been mined to a considerable extent and shipped to Salt Lake, where it has been charged in the lead-smelting furnaces, as a flux for the production of an iron slag.
The brown hematites belong to the most recent iron formations, and occur in great abundance. They constitute a series, in which the physical characters vary as the proportion of water diminishes, passing from earthy varieties having a yellowish streak to compact masses, with brown streak inclining to red. Turgite, which has the lowest amount of water, and is therefore nearest to red hematite, has already a red streak. The usual condition of hydration is that of limonite, which may be regarded as the typical brown hematite. The other varieties enumerated are exceptional. This ore occurs in a great variety of conditions, as earthy or ochreous masses, or in concretionary, stalactitic, or hard, compact, mammillary, and botryoidal aggregations. It has often a distinct fossiliferous character, and is associated with vegetable and animal remains. All the varieties yield water when heated, and all except turgite give a yellowish or brownish streak. Brown hematites differ widely as regards purity. Usually they contain considerable silica and phosphoric and sometimes sulphuric acid, and are consequently rarely employed exclusively for the finer varieties of iron and steel.
They however supply a large proportion of. the iron that is used for castings. "Limonite occurs in secondary or more recent deposits, in beds associated at times with barite, siderite, calcite, aragonite, and quartz, and often with ores of manganese; also as a modern marsh deposit. It is in all cases a result of the alteration of other ores, through exposure to moisture, air, and carbonic or organic acids, and is derived largely from the change of pyrite, siderite, magnetite, and various mineral species (such as mica, hornblende, and augite), which contain iron in the protoxide state. It consequently occupies, as a bog ore, marshy places over most countries of the globe, into which it has been borne by streamlets from the hills around; and in the more compact form it occurs in stalactites as well as in tuberose and other concretionary forms, frequently making beds in the rocks which contain the minerals that have been altered into it. In moist places, where a sluggish streamlet flows into a marsh or pool, a rust-yellow or brownish yellow deposit often covers the bottom, and an iridescent film the surface of the water; the deposit is a growing bed of bog ore. The iron is transported in solution as a protoxide carbonate in carbonated waters, a sulphate, or as a salt of an organic acid.
The limonite beds of the Green mountain region were shown by Percival to be altered beds of pyritiferous micaceous and argillaceous schist; and the same is held by Lesley as true also of the other beds of the Atlantic border, from New England and New York, through Pennsylvania (Mt. Alto region and others), to Tennessee and Alabama." (Dana.) Brown hematite is the most universally diffused of all the ores of iron, and furnishes a large proportion of the iron of the world. In the United States it is distributed very widely and abundantly. Large deposits occur in New England, particularly in the neighborhood of Salisbury and Kent, Conn., and in Columbia and Dutchess counties, N. Y. The ores of these deposits are highly prized. Similar deposits of limonite are traced in a zone extending from the Hudson river to Alabama, along the line of the north flank of the South mountain, Blue Ridge, and Smoky mountain range, and also in the lower Silurian limestone valleys of Pennsylvania and Virginia, Nittany valley, Kishicoquilis, etc.; and again, under similar geological conditions, in East Tennessee, where the deposits near Embreeville are estimated by Lesley, Maynard, and others to contain many millions of tons of excellent ore.
Western North Carolina and northwestern Georgia contain portions of the same zone, which ends in the magnificent deposits of Alabama. The siderite clay ironstones of the carboniferous and other rocks frequently furnish by oxidation deposits of brown hematite. This is the case particularly near the outcrop, but sometimes also throughout large deposits. The lignites of New Mexico, Colorado, and Montana are accompanied by ores of this character. The same is the case in the Appalachian region, for instance at Brady's Bend, Pennsylvania, in "West Virginia, and elsewhere, and among the carboniferous iron ore deposits of Ohio, Indiana, Kentucky, etc. Large deposits of limonite occur in dolomite, associated with zinc ores, in Arkansas. Texas is also abundantly supplied with this ore. Brown hematite (bog ore and ochre) is found in large quantities in Canada, particularly in the St. Lawrence valley, where it overlies superficial deposits of clay and sand. The distribution of brown iron ores in other countries is so nearly universal that the localities need not be named.
It may be remarked that the extensive deposits of Styria and Carinthia in Austria, and of Nassau on the Rhine, are celebrated for their purity and freedom from phosphorus-an element which, by reason of the usual organic origin of such deposits, is most likely to be found in them. The universality of this ore naturally follows from the fact that it is the ultimate result of the chemical metamorphosis of all other kinds of iron ore; so that wherever any ore of iron is exposed to oxidizing agencies and moisture, some form of limonite or hydrated ferric oxide of iron is certain to occur. The term limonite is derived from the Greek moist grassy land, and refers to that variety which is known as bog ore or marsh ore. Famous ochreous deposits occur at Brandon, Vt. At Pointe du Lac and St. Ann, Montmorenci, Canada, are remarkable localities of the ochre, and at the latter place it is seen in the process of formation. The deposit varies from 4 to 17 ft. in thickness, and covers an area of four acres.-
This ore is never found as pure ferrous carbonate, part of the iron being invariably replaced by manganese, lime, or magnesia. The percentage of iron given in the table above is therefore theoretical, and is never perfectly attained. The ore is found crystallized, massive, and concretionary; in the latter form it is called sphaeroside-rite. It is for many purposes the most valuable ore of iron, owing to its general freedom from injurious ingredients, its easy reduci-bility, and the presence of manganese (from 1 to 10 per cent. of oxide, exceptionally as high as 25 per cent.), which always enhances its value. It is not very extensively distributed in nature, but a few localities contain it in large deposits. It is almost the only material used in the preparation of spiegeleisen. (See Iron.) Ferrous carbonate also forms the basis of the carboniferous blackband ores, and of most of the clay ironstones, which are very extensively distributed. The ferrous carbonate is in these ores intimately associated with argillaceous, silicious, and often carbonaceous matter. It frequently contains also sulphur as iron pyrites, and phosphorus as calcic phosphate. These ores are therefore much less pure than the spathic ore properly so called, and yield irons of a much inferior character.
The carbonated ores, when heated, lose their carbonic acid, and their ferrous oxide is converted into magnetic oxide. They are always calcined before smelting. The carboniferous blackbands contain usually from 15 to 20 per cent. of carbon, and may be roasted without the addition of fuel. On roasting they lose half their weight. Spathic ore becomes brown or brownish black on exposure, owing to a peroxidation of the iron and its passing into limonite; and by a subsequent loss of water it may pass into red hematite. The occurrence of spathic ore is limited principally to the crystalline slates and the older sedimentary rocks, the most extensive and characteristic deposits being in the Devonian formation. The most noted localities are Siegen, Rhenish Prussia; Kamsdorf and Stahlberg in Thurin-gia; Osnabruck in Westphalia; the Erzberg near Eisenerz in Styria, in the Devonian; and the Erzberg near Huttenberg in Carinthia. England has also deposits in the Brendon hills in Somersetshire, and at Exmoor, South Moulton, and Walscott in Devonshire; also at Weerdale, Durham. A remarkable series of deposits of impure or earthy carbonate is found in the different members of the lias formation in the Cleveland hills, North riding of Yorkshire, England. The main deposit is in the middle lias, showing a workable seam from 8 to 13 ft. thick; it is believed to extend throughout the whole of Cleveland proper.
In this region of England the manufacture of iron has reached a higher stage of development than in any other part of the world. The principal deposit of siderite in the United States is at Roxbury, Conn., in a vein of quartz, traversing gneiss.
The clay ironstones are met with in both the carboniferous and tertiary (brown coal) formations. In England, Scotland, Westphalia, and other regions, the blackband ore (carbonaceous carbonate) forms the basis of an extensive industry. This ore, as found in Westphalia, contains an extraordinary amount of phosphoric acid, in some layers as much as 30 to 60 per cent., and in others 20 to 25 per cent. The blackband ores are of subordinate importance in the United States, though they have been found in the coal regions of western Pennsylvania. Earthy carbonates occur extensively in Pennsylvania, West Virginia, Ohio, etc.
Magnetic iron ore occurs generally in large masses, but with distinctly crystalline structure. It also occurs in the form of sand, concentrated by fluviatile or tidal action from the debris of rocks containing it. It is readily recognized by its black color and streak, and by its being attracted by the magnet. It derives its name from the Thessalian district of Magnesia, bordering on Macedonia, or, according to Pliny, from Magnes, who first discovered it. There is a magnesian variety in which part of the ferrous oxide is replaced by magnesia, and a titaniferous variety in which a part of the iron is replaced by titanium. This variety bears the same relation to magnetite as iserine to hematite. The amount of titanic acid varies through wide limits. Magnetic ore is often found in a state of almost absolute purity; more generally it is associated with apatite (calcic phosphate), iron pyrites and other sulphides, quartz, and earthy ingredients. It supplies a large amount of the finest iron and steel of commerce. The iron industry of Sweden is based almostly entirely on magnetic ores. "Magnetite is mostly confined to crystalline rocks, and is most abundant in metamorphic rocks, though found also in grains in eruptive rocks.
In the azoic system, the beds are of immense extent, and occur under the same conditions as those of hematite. It is an ingredient of most of the massive variety of corundum called emery. By deoxidation through organic matter it is changed to protoxide, which may become a carbonate; by oxidation it becomes hematite." (Dana.) The principal European occurrences of magnetic ore are at Arendal in Norway; Dalarne, Westmanland, Wermland, Dannemora, Uto, and Smaland in Sweden; in Finland, and in the Ural. In the United States there are are vast beds in the azoic of the Adirondack region, Warren, Essex, and Clinton counties, in northern New York; also in northern New Jersey, in Morris, Sussex, Warren, and Passaic counties, where it is found in beds conformable with the azoic gneiss, and also intimately disseminated in the gneissic strata. In eastern Pennsylvania there are several localities, the most noted being Cornwall in Lebanon co. In Canada it is found at Hull, Grenville, Madoc, etc. In North Carolina, at Greensboro, there is an extensive titaniferous belt of magnetites.
This is an ore analogous in composition to magnetite, but part of the iron is replaced by manganese and zinc. Its formula is (Fe, Mn, Zn)0, (Fe, Mn)203. It crystallizes in the isometric system; specific gravity about 5; hardness 5.5 to 6.5; streak dark reddish brown. It contains about 46 per cent. of iron, 17 of manganese, and 13.5 of zinc. It occurs at Franklin furnace and Stirling Hill, N. J. It is first treated to extract the zinc, and the residues are then smelted for spiegeleisen. - The reducibility of iron ores depends more on their molecular structure than their chemical composition. While the natural magnetites are classed with the more refractory ores, owing to their dense structure, the magnetic oxide resulting from the roasting of spathic ore is reduced with ease. The same contrast is noticed between the anhydrous and hydrous hematites. - The distribution of the iron ores of the United States, with relation to the resources of the country in mineral fuel, has been well stated in the "Report on Iron and Steel" of Mr. Abram S. Hewitt, United States commissioner to the Paris exposition of 1867, as follows: "The position of the coal measures of the United States suggests the idea of a gigantic bowl filled with treasure, the outer rim of which skirts along the Atlantic to the gulf of Mexico, and thence returning by the plains which lie at the eastern base of the Rocky mountains, passes by the great lakes to the place of beginning, on the borders of Pennsylvania and New York. The rim of this basin is filled with ex-haustless stores of iron ore of every variety, and of the best quality.
In seeking the natural channels of water communication, whether on the north, east, south, or west, the coal must cut this metalliferous rim, and in turn the iron ores may be carried back to the coal, to be used in conjunction with the carboniferous ores, which are quite as abundant in the United States as they are in England, but hitherto have been left unwrought, in consequence of the cheaper rate of procuring the richer ores from the rim of the basin. Along the Atlantic slope, in the highland range from the borders of the Hudson river to the state of Georgia, a distance of 1,000 miles, is found the great magnetic range, traversing seven entire states in its length and course. Parallel with this, in the great limestone valley, which lies along the margin of the coal field, are the brown hematites, in such quantities at some points, especially in Virginia, Tennessee, and Alabama, as fairly to stagger the imagination. And finally, in the coal basin is a stratum of fossiliferous ore, beginning in a comparatively thin seam in the state of New York, and terminating in the state of Alabama, in a bed of 15 feet in thickness, over which the horseman may ride for more than 100 miles.
Beneath this bed, but still above water level, are to be found the coal seams, exposed upon mountain sides, whose flanks are covered with magnificent timber, available either for mining purposes or the manufacture of charcoal iron. Passing westward, in Arkansas and Missouri is reached that wonderful range of red oxide of iron, which, in mountains rising hundreds of feet above the surface, or in beds beneath the soil, culminates at Lake Superior in deposits of ore which excite the wonder of all beholders; and returning thence to the Atlantic slope, in the Adi-rondacks of New York, is a vast undeveloped region, watered by rivers whose beds are of iron, and traversed by mountains whose foundations are laid upon the same material; while in and among the coal beds themselves are found scattered deposits of hematite and fos-siliferous ores, which, by their close proximity to the coal, have inaugurated the iron industry of our day. From these vast treasures the world may draw its supply for centuries to come, and with these the inquirer may rest contented, without further question; for all the coal of the rest of the world might be deposited within this iron rim, and its square miles would not occupy one quarter of the coal area of the United States." - The table on the preceding page gives analyses of various ores from different localities, indicated by numbers as follows: A. Hematites. 1. Whitehaven, Cumberland, England. 2. Iron mountain, Missouri; specular ore from vein. (The phosphoric acid is the average of four determinations in as many samples, varying from 0.252 to 0.081 per cent. 3. Pilot Knob, Missouri; specular ore from main ore bed.
B. Hydrous Hematites. 4. Lake ore, Sweden. (Phosphoric acid varies from 0.051 to 1.213 per cent) 5. Katahdin furnace, Piscataquis co., Me., resulting from the decomposition of iron pyrites. 6. Silicious ore, York co., Pa. 7. Pennsylvania furnace ore-bank, Centre co., Pa. C. Carbonates. I. Spathic ore. 8. Musner Stahlberg, Prussia. 9. Calcined spathic ore from Altenberg, Styria, used for Bessemer process at Neuberg. 10. Brendon hill, Somersetshire, England. II. Earthy carbonates. 11. Gubbin ironstone, Dudley, S. Staffordshire, England. 12. Sphaerosiderite from Ahaus, Prussia. 13. Eston, Cleveland, England; main seam. 14. Carbonate ore from Fayette co., Pa. III. Blackband. 15. Shelton, N. Staffordshire, England. 16. West-phalian blackband, low grade. 17. Best West-phalian, roasted. D. Magnetites. 18. Danne-mora ore, Sweden. 19. Granrot ore, Sweden. 20. Lake Champlain, "new bed" ore, unusually free from apatite. 21. A sample from New Hope mine, Morris co., N. J. (The ores in northern New Jersey are very variable in regard to silica and phosphoric acid. The former varies from 2 to 40 per cent., the latter from 0 to 3 per cent.
Low percentages of both silica and phosphoric acid, and freedom from sulphur, are usual.) 22. Titaniferous, from Greensboro, N. C. The following table shows the amount of sulphur and phosphorus contained in most of the Swedish and in some of the Prussian ores:
Chromic oxide with trace of vanadium,
LOCALITIES AND ORES.
Sulphur, per cent.
Phosphorus, per ct.
Rastelp, Sweden, magnetite......
Pershyttan, " " ........
Lerberg, " " ...........
Manias, " " ..........
Hillang, " " ..........
Prague, Byberg, and Nyberg, Sweden, magnetite .....................................
Farola, Sweden, magnetite ..........
Nartorp, " " .......
Stenebo, " " .........
Dannemora," " ...........
Near Wiesbaden, Prussia, red hematite....
" Coblentz, " " " ......
" " " brown " .........
" Wiesbaden, " " " ......
Limburg, " spathic ore ......
Oberlahn, " " " ........
Werbenbach, " " " ........
Westphalia, " blackband ......
Rhine Province, " brown hematite.
• • • •
Westphalia, '" " "
Iron ores are generally used in the blast furnace in the condition in which they are mined, but sometimes they are submitted to a preliminary treatment. The carbonate ores are invariably' roasted before smelting. This drives off the carbonic acid; the ferrous is converted into magnetic oxide ; and the ore is rendered not only richer but much more porous, and thereby more readily reduced. Ores containing much sulphur are also roasted with access of air, and the greater part of the sulphur is driven off as sulphurous acid. Heavy compact ores are occasionally roasted to render them friable. Roasting may be effected in open heaps or within brick walls by piling the ore and fuel (wood or brush) in layers, and allowing it to burn out. This method is far less thorough and efficient than roasting in shaft furnaces. In the latter case the fuel (small coal) and ore may be charged alternately, or gas (from the blast furnace or suitable generators) may be used. The operation is continuous. Brown hematites often occur mixed largely with clay and other earthy matters; they are then submitted to a dressing or concentrating process by washing, in which the lighter clay is washed off and the heavier ore remains. - Forge and mill cinders, produced in puddling and heating iron, are very rich in iron, containing from 40 to 75 per cent.
Although, strictly speaking, they are not ores of iron, yet they are used for reduction in the blast furnace. Their use in large quantity is attended with disadvantage, owing to the facility with which they melt and escape reduction. Puddling cinder, moreover, contains the greater part of the impurities of the iron from which it is made, and yields therefore inferior iron. Roasting renders the cinder more infusible, and also effects a partial purification.