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By H. M. Lewis

The current status of the international zinc market is summarized, and a supply /demand outlook is presented for the next five years. A number of technical problems facing the industry are outlined. Introduction ZINC has sometimes been called the unknown metal. Considering that on a world basis it ranks third among non-ferrous metals in production and consumption, zinc retains a certain modesty. It has no distinctive colour like copper, lacks the low density of aluminum or magnesium and is not always easy to identify as a product. The 'spangle' of galvanized steel is probably zinc's most distinguishing feature. The purpose of this article is to develop an over-all review of zinc. Starting with the current position of the metal, the major factors that will shape its future will be described so that an assessment of the outlook for zinc can be made.

Jan 1, 1978

By Sulanto J, Talonen T

In the mid 1960's Outokumpu considered building a lead Flash Smelter on the basis of Flash Smelting technology developed first in the late 1940s for smelting copper concentrates. Pilot scale tests were carried out and the process was found technically sound. The project was however not economically viable under Finnish conditions and it was cancelled. In 1981 pilot plant tests were restarted after many enquiries from the primary lead producers. All the know-how and development made in copper and nickel smelters was applied to the Lead Flash Smelting process and the tests were successful. Outokumpu has so far tested several types of lead concentrate with lead contents ranging from 28 to 75 per cent. The concentrate is oxidized and smelted to metallic lead and slag directly in the Flash Smelting Furnace using high oxygen enrichment of tonnage oxygen. Fossile fuel is used in the Flash Smelting Furnace in cases when the feed material does not contain enough latent heat to smelt the furnace products even though pure oxygen is used for oxidation. Thus the process results in a low consumption of extraneous energy. Use of oxygen results in a small gas flow but it is rich in sulphur dioxide. The gas treatment facilities and sulphur recovery plant will be small in size and low in investment cost. The process is flexible and can produce stable quality of products from different types of raw materials. Quality of lead bullion especially the sulphur content can be regulated over wide a range, from 0.1 per cent upwards according to the needs of the refinery.

Jan 1, 1984

By Jukka S. Sulanto

Outokumpu has applied its well -known copper Fl ash Smelling Process also for smelting su1phide lead . concentrates. The lead in the Flash Smelting Furnace slag is recovered by reducing the oxygen potential of the slag in an electric furnace by injecting reductant into the slag bath. A series of tests have been carried out in a 1000 KVA electric furnace? adjacent to a pilot Flash Smelting Furnace. The furnace has been fed continuously with molten Flash Furnace Slag. The feed rate has varied from 0.5 to 3.1 MTPH (0.55 - 3..4 STPH) depending on the feed rate and the quality of concentrate to the Flash Smelting Furnace. The inlet slag contained 15 to 40% of lead and the cleaned slag 1. 5 to 3. 0%. The lead recovery from sol id raw material was tested in the same continuous process by feeding lead calcine into the furnace. Feed rate was 0.7 to 0.95 MTPH (0.8 - 1.05 STPH) and the calcine contained 33 to 34% of Pb. Several reduction agents and procedures were tested. The process proved to be suitable for continuous recovery of lead from solid oxide raw materials.

Jan 1, 1990

By J. -P. Lempainen, K. Haavanlammi, K. Valkama

The Outotec nickel matte chloride leaching process is especially suitable for small capacity standalone nickel refineries. The matte is leached with hydrochloric acid and oxygen in multiple steps in atmospheric OKTOP® reactors. Oxygen feed prevents reductive conditions and hydrogen formation in the leaching process. A sufficiently high acid concentration keeps the iron dissolved in the solution. The iron is removed after leaching by precipitation and the pregnant leach solution continues to metals recovery. The metals recovery from the chloride solution is done by solvent extractions, where ammonia is used for pH control. Copper and nickel are stripped from the organic phases into sulphate solutions. Metals can be recovered from the purified solution by conventional electrowinning. Other alternatives are precipitation or hydrogen reduction processes. The ammonium chloride in the solvent extraction raffinate is regenerated into ammonia. The calcium chloride solution from ammonia regeneration is routed to acid regeneration where sulphuric acid is added and ultra-pure gypsum is precipitated. The hydrochloric acid solution from this step is recycled back to leaching. The process yields good overall metal recoveries and expensive

Jan 1, 2014

By Y. K. Yen

The most serious problem for on-line coarse particle size measurement from a conveyor belt by image analysis is the overlap phenomenon that occurs between adjacent particles. A methodology is developed and described in this paper to estimate the sieve size distribution from the overlapped projected particle images as might be obtained from a conveyor belt. Two aspects of the overlap problem have been taken into consideration. First, several image processing algorithms were studied to find the most suitable technique to separate the projected overlapped particle images for narrow sized particles. These image segmentation algorithms include two-dimensional thresholding, edge detection and Watershed discrimination techniques. The other aspect of the overlap problem is related to the determination of the general kernel function for the partial particle images obtained from the projection of the overlapped particle bed. In this regard, a computer simulation technique was applied to mimic the overlapped particle bed for narrow sized particles, then the general kernel functions of the partial particle images were determined.

Jan 1, 1994

By Kirsi M. Riihilahti

The oxidation characteristics of solid copper matte particles under simulated Kennecott-Outokumpu Flash Converting conditions are presented. The effects of feed matte grade, matte particle size, oxygen-to-matte mass ratio, oxygen content of the process gas and temperature on the particle oxidation rate, quality of conversion, particle size change and particle morphology were determined. The converting quality was highest (~68) with the 58% copper matte at 1100°C. At similar conditions, the 72% copper matte was more likely to produce copper oxides and leave unoxidized particles rather than directly produce sulfur dioxide at 1100°C, resulting in an average converting quality of 55. The quality of conversion was poorest for the 72% matte at 920°C (~45); the sulfur removal was extremely low even with a relatively high degree of oxidation. Fragmentation of the particles was significant in all cases with the 72% matte. A higher O2/matte .ratio and higher temperature, in general, resulted in increased fragmentation while oxygen enrichment had no significant effect. With the 58% matte, particle size was found to increase for the finest fractions rather than to decrease due to particle expansion.

Jan 1, 1998

By E. Krause

The oxidation of Fe(II) with S(IV) and oxygen can be used advantageously in leaching systems requiring both Fe(III) and sulfuric acid; e.g., heap leaching of sulfide-containing ores or concentrates. The oxidation of Fe(II) with SO2/air is several orders of magnitude faster than with air alone; rates of up to ?20 g/L'h were observed at 25°C. The reaction proceeds via an intermediate orange-red mixed-valence Fe(II)/Fe(III) sulfite complex, which is present at an essentially constant concentration throughout most of the oxidation process. The oxidation rate of Fe(II) is approximated by the formula -d[Fe(II)]/dt = k [S(IV)] [02]0'3. The concentration of the mixed-valence complex, and thus the iron oxidation rate, decreases with increasing acidity. Certain impurities, such as SCN- and thiourea, negatively affect the rate. The oxidation reaction 2Fe2 + SO2 + 02 4 2Fe3+ + S042- is accompanied by the side reaction H20 + SO2 + 1/202 4 H2SO4. The side reaction becomes particularly important at low temperatures and whenever the oxygen mass transfer is insufficient with respect to the rate of addition of S(IV). Thus, effective oxygen mass transfer is essential for achieving high oxidation rates with minimal acid formation. The activation energy was estimated as 141 kJ/mol over the temperature range 10-35°C.

Jan 1, 2006

By B. H. Good

This paper describes the research that has been conducted -in an effort to understand the nature and causes of sulphide fires -in the Sullivan Mine. Results of the investigation have led to several theories of the origin: of the fires, the nature of the chemistry and a method for assessing: the fire potential. Recommendations and the implications for the mining operation are also discussed.

Jan 1, 1977

By J. C. Allen, E. V. Herring

CHILE EXPLORATION CO's. oxide plant at Chuquicamata, the largest of its kind, started in 1915 with initial operations at the rate of 10,000 tons of ore per day. Anaconda Copper Mining Co. acquired the controlling interest in 1923 and in the next few years modernized the plant and mine, extending operations to a nominal capacity of 490 million lb of electrolytic copper yearly by 1942. Plant production averaging 495.7 million lb of copper per year was maintained for the eight-year period extending from 1941 to 1948 inclusive. During the World War II production reached its highest level when in 1944 the output was 531.8 million lb. The maximum production for any one month was 49.6 million lb, achieved in May 1947. E. A. Cappelen Smith, consulting metallurgist for M. Guggenheim's Sons, worked out the first process for the treatment of Chuquicamata copper oxide ore about 1913, and directed a staff of 20 young engineers operating a pilot plant at Perth Amboy, N. J., on three shifts for an entire year. The copper is extracted from the oxide ore by a hydro-metallurgical treatment described in detail by T. A. Campbell in AIME Transactions, Vol. 106, Copper Metallurgy, 1933. The Oxide Plant is divided into five main operations as follows: (1) Crushing, (2) Leaching, including dechloridizing and sulphur dioxide treatment, (3) Tailing disposal, (4) Electrolytic precipitation, (5) Smelting and melting. Crushing The present Primary Crushing Plant has been in operation 25 years, during which time 4,342,112 cars of 70 tons net capacity have passed through the dumpers. The ore as received from the mine is frequently blocky and up to 5 feet in section. It is reduced to 9-in. size by two 60-in. gyratory crushers, and transported to the east and west ore bins by 60-in. conveyors. In the secondary crushing operation the ore is first reduced to 2 to 3-in. size by five standard 7-ft cone crushers and two No. 10 gyratories. This product is then reduced to 3/8-in. size by fifty-two 48-in. vertical Symons disc crushers and is transported by belt conveyors to the loading bridges from which it is charged° into the leaching vats. Conveyor belt No. 14 which serves the eight vats in the east section of the leaching plant has transported over 125 million tons of ore, which is a record for any type of conveyor. This 60-in. belt travels at a speed of 600 fpm and carries loads up to 5500 tons per hr. Leaching The ore is leached in 14 vats, averaging 13,500 short tons net capacity each. These vats are lined with a natural mastic which adequately resists the sulphuric acid leaching solutions. The leaching process as presently conducted comprises soaking the ore with successive volumes of leaching solution and spent electrolyte and withdrawing successive volumes of enriched solution from the vat to a total of approximately 3.2 million gal. The first 1.6 million gal of this volume withdrawn is called strong solution and contains about 35 parts of copper per 1000. The balance of 1.6 million gal subsequently withdrawn is stored in a

Jan 1, 1952

By T. Oberthür

The Great Dyke of Zimbabwe constitutes the world?s second largest reserve of platinum group elements (PGE) after the Bushveld Complex in neighbouring South Africa. Within the Great Dyke, economic concentrations of PGE are restricted to sulphide disseminations of the Main Sulphide Zone (MSZ), which are currently mined at the Ngezi, Unki, and Mimosa mines. Near-surface oxidized MSZ ores have a large potential. Their total resources are in the range of ca. 160 ? 250 Mt; however, all previous attempts to extract the PGE from this ore type proved uneconomic due to low PGE recoveries (<< 50 per cent) achieved by conventional metallurgical methods. Within the ores of pristine, sulphide-bearing MSZ, the PGE are bimodally distributed. Platinum occurs mainly in the form of discrete platinum group mineral (PGM) grains (mainly bismuthotellurides, sulphides, and arsenides), whereas approximately 80 per cent of the Pd (and some Rh) is hosted in pentlandite. Within the oxidized MSZ ores, the PGE are polymodally distributed. Whereas arsenide- and sulphide-PGMs, making up ca. 25 per cent of the original Pt content of the ore, largely remain stable (relict PGMs), the remaining PGMs are disintegrated. The base metal sulphides are destroyed, partly releasing their base metal and PGE contents, and are replaced by iron oxides or hydroxides. Unspecified amounts of the PGE are redistributed and either form secondary PGMs, are found in chemically and mineralogically ill-defined (Pt/Pd)-oxides or hydroxides, or in iron-hydroxides, Mn?Co-hydroxides, and in secondary silicates. The problematic processing of oxidized MSZ ores is attributable to their complex nature and polymodal distribution of the PGE, prohibiting a significant upgrading of the ores by conventional metallurgical methods. Therefore, only bulk leaching methods are viable for ore treatment, and novel metallurgical methods have to be developed for the processing of these ores. Our ongoing work aims at locating the PGE in their mineralogical form in order to understand the mineralogical balance of the PGE in the ores and thereby facilitate the evaluation of metallurgical options for their recovery. A short overview on options and recent advances regarding the recovery of the PGE from oxidized ores is given.

Jan 1, 2012

By Adrien Antenen, Didier Villette

CITRON has developed a new process for the recycling of hydroxide sludge, household batteries, catalysts, automobile shredding residues, grinding sludge and other metal containing organic and inorganic waste. A pilot plant with a capacity of 1,000 tons per year has started operations in April 1998. In August 1999, an industrial plant became operational with an initial treatment capacity of 23,000 tons.

Jan 1, 2000

By Charles Jones

Discussion of the paper of CHARLES COLCOCK JONES, presented at the San Francisco meeting, September, 1915, and printed in Bulletin No. 105, September, 1915, pp. 1887 to 1898. D. A. LYON, Salt Lake City, Utah.-For some years I have been interested in the iron-ore deposits of the Pacific Coast, and I feel very much indebted to Mr. Jones for the information he has given us on this subject. I think that we are also very much indebted to him for having presented this paper, as he has given us reliable data in shape of facts and figures, which is just what we need in considering whether or not it would be commercially feasible to attempt to establish an iron and steel industry on the Pacific Coast. Personally, due to lack of market, I do not think it is a commercially feasible proposition at the present time. Mr. Jones states that pig iron can be produced on this coast as cheaply as it can be produced in Pittsburgh. Even. so, what can be done, with it after it is produced? To begin with, iron produced on the. Pacific Coast has to compete with iron produced in China. Last year I had occasion to investigate the subject of the market for iron and, steel on the Pacific slope, and found that it was possible for the producers of Chinese iron to sell it for much less than the price for which iron can be produced in the West, and also for less than eastern pig iron is now sold for, even when shipped through the Panama Canal. I was informed that the agents for Chinese iron had instructions to sell it for $1 or $2 less per ton than that asked for other irons on the market. Of course, I realize that the competition of Chinese iron may not be important, but even if it is not, and even though pig iron ,can be produced at the cost given by Mr. Jones in his paper, I do not see any immediate chance of its being so produced, due to lack of market. C. C. JONES, Los Angeles, Cal.-Mr. Lyon is right. It is a fact that Chinese pig is being sold in San Francisco on some large contracts at $18.50 a ton, while $19 is the usual price. We ought to produce iron. on this coast at $14. Personally I do not think Chinese iron is ever going to be an important factor. It is a bugbear out here now. You are not going to have much Chinese pig iron in .this country. Our little friend Japan is going to absorb everything it can. The price of Chinese pig iron at $18 is about as low as it will ever get. If we can show you that we can produce this iron here at the same price as at Pittsburgh, I. think you will have a good opportunity to compete with anything China or India can ever do. Costs there are very low now. They are going to come up by leaps and bounds in the next few years. The labor cost over there may be very low now, but I. have seen some men from there quite recently

Jan 12, 1915

By Thomas Kilpatrick

The liability created by building this railway is the great financial problem before the people of British Columbia at the present time. In dealing with the subject, I propose to divide it into three periods: The Past - The Present - and The Future. The Past The primary objects in building the railway were to provide an independent and direct route for the Grand Trunk Pacific railway into the city of Vancouver, and to open the central and north-eastern portion of the province.. This vast territory, almost one-half the entire area of British Columbia, has great wealth in fruit, agricultural, and grazing lands; timber, coal, water-power and minerals of all kinds, including some of the richest placer deposits in the world. Rivers and lakes teem with fish. Big game, including moose, elk, caribou, deer, grizzly and black bear, mountain goat and sheep, is plentiful and there is an abundance of small game and fur-bearing animals. In addition, its scenic attractions are unsurpassed in Canada. To develop these resources it was considered necessary to build a railway from south to north as a main artery of transportation and, if necessary, branch lines to carry heavy .shipments of coal or other commodities. Lighter freight may be handled by motor trucks from distant points to the railway.

Jan 1, 1925

By Neil J. Sullivan, Goode. George E.

"The Adams mine, operated by Jones & Laughlin Mining Company, Ltd. and located near Kirkland Lake, Ontario, produces one million tons of iron ore pellets per year. This paper briefly describes the mining and concentrating operation and deals mainly with the operation of the pellet plant. The orebody consists of fine-grained magnetite and some hematite, with alternating layers of chert, and is mined by conventional open-pit methods. After crushing and grinding, the ore is concentrated by magnetic separation and flotation. Agglomeration is accomplished by the grate-kiln pelletizing process. The plant layout and general operation will be discussed in detail."

Jan 1, 1968

By S. S. PRENZ

Discussion held at the sessions of the Canal Zone meeting, November, 1910. [SECRETARY&apos;S NOTE.-As a result of this discussion, an expression of opinion covering all points of unanimous agreement was adopted and signed by members of the Institute party as individuals. This statement, which was printed in the Bulletin of December, 1910, is reprinted below, as an introduction to ,the detailed report of the discussion. It should be clearly understood that it does not constitute an official declaration by the Institute, the rules of which forbid any such action. At the New Haven meeting of February, 1909, a free discussion of the plan of the canal took place, in which the critics of the present scheme had full opportunity to state their views. That discussion was continued at considerable length in the newspapers ; but the Institute was never committed to either side. In this latter instance, the situation is precisely similar. Certain members and guests of the Institute, having had the advantage of later developments and of personal inspection, have expressed the conclusions at which they have arrived on their own responsibility ; and these conclusions are published by the Institute, subject to further discussion, like any other part of its proceedings. The general statement referred to contained a promise that the full discussion, of which it was a result, would be subsequently printed ; and this promise is fulfilled, so far as practicable, in the subjoined report, concerning which I beg to offer the following explanations: 1. It is made up of the remarks of the members of our party, as subsequently written out and mailed to the Secretary. 2. In editing these contributions, I have condensed the material considerably by cutting out much repetition of opinions and phrases ; and in such excision, I, frankly confess, the authors whose communications were delayed, have been the sufferers. Nevertheless, there is a good deal of repetition left, as there must be, when so many persons had talked so much, and come to agreement upon so many points. 3. It has been found impossible to preserve in this report the actual order of the free and informal oral discussion represented. Consequently the several contributions have been arranged in alphabetical order under the names of their authors. References to "preceding speakers," etc., must therefore be construed as referring to other speakers, preceding or following.-R. W. R.] S. S. PRINZ AUGUST WILHELM, AT SEA, November 14, 1910. We, the undersigned, members and guests of the American Institute of Mining Engineers, after a visit to the Isthmus of Panama, and inspection of the work of the United States Isthmian Canal Commission, and after full discussion of our individual impressions, find ourselves in unanimous agreement as to the following conclusions:

Jan 1, 1911

By P. Molera

"In Catalonia, between the 10th century and 1878, some installations were developed which at first were called ""factories"" and later ""fargas"", since they manufactured iron and steel of low carbon content. Because of its excellent quality, the fame of this metal spread from Catalonia to many countries. Fargas were started in Corsica, the Low Countries, Germany, Oriental India, Madagascar, Finland and along the rivers Orinoco and Mississippi among others. The extractive process used in these fargas has passed into history as the ""Catalan Farga Process"", ""Catalan Process"", or just ""Farga Catalana"". With this technology, raw material was obtained for the fabrication of tools, nails, weapons, grilles, etc. to supply the national and export markets. Aside from meteorites and native iron, found as metallic nodules in basalts in Greenland and Palestine, the ancient civilizations, which had no means to reach the fusion temperature of iron (1535°C), obtained iron by a direct process. Similar processes were used until quite recently (1930s) by certain tribes in Central Africa; holes in the ground were filled with iron ore and coal, and combustion encouraged by blowing air by mouth or with bellows made of animal skins. Thus without melting the iron it is possible to reduce the mineral and obtain a porous and impure metallic mass, which is purified on forging by mechanical squeezing. With a process very likely similar, the famous column at Dehli weighing 6.4 m.t. was built about 300 B.C."

Jan 1, 1999

By Bernard MacDonald

Introduction. OF the treatment of the slime-pulp of gold- and silver-ores by cyanidation, agitation is an essential part. When prepared for treatment, this pulp, consisting of ore reduced to such fineness that approximately 80 per cent. of it will pass through a 200-mesh screen, is mixed with a certain proportion of water, carrying in solution the quantity of cyanide (KCN) and other chemicals required. The water-constituent of the pulp thus prepared usually ranges from 1 to 2 parts by weight to 1 of the dry ore. Thus constituted&apos;, the pulp is charged into treatment-tanks, the shape and capacity of which vary, according to the quantity of pulp to be treated daily and the method of agitation to be employed. Tanks have no other function in a cyanide-plant than that of being economical and convenient containers or receptacles for holding the pulp, solution, or water used in the operations. It is by agitation that the solids in the pulp charged into the tanks are kept in suspension in, and mixed with, the solution in the proper proportions required for the treatment. If the proper mixture of solution to solids be determined to be 2 to 1 by weight (which is, approximately, 5 to 1 by volume), this proportion should be maintained in every part of the charge ; -that is, each solid particle of the pulp, whether it be of 180- or 400-mesh size, should, be surrounded by five times its own volume of solution throughout the whole period of treatment. The reason is, that the amount of chemicals ascertained to be necessary for dissolving the gold and silver contained in the solids, is held in uniform solution in the water-constituent of the pulp, and, therefore, the determined proportions of solution and solids must be maintained at all times during treat-

Apr 1, 1912

By MERLE HOWARD GUISE

WHEN I was a boy I walked into Fairbanks in 1905. I was but a soft chechako, and arrived with blisters covering my feet, as a result of "mushing" the 400-mile trail on foot. Because of them, the display windows of the trading posts were first to catch my eye-dog and horse collars, harness and sled parts! And because of my enthusiasm for things adventurous, I looked long upon the prospectors&apos; gold pans, shovels, rifles, camp equipment, and what not. Today those windows are filled with motor car accessories, tires and repair parts; yes, even thermos bottles and vacuum cleaners! There were many chance meetings with little parties of prospectors in my youth, and it always set my blood to tingling. Sometimes it was a 70-year-old sun-baked son of the Arizona desert, wrangling his stubborn, faithful jenny away from some outfitting store, at others a shovel-ended poling b0at in northern Canada, \vith the partners and the dogs towing upstream along the bank, or an old sourdough, with tug-rope around neck and hand on the spruce sapling gee-pole, mushing along behind his two or three old, plodding malamutes, dragging a little Yukon sled piled high with rusty sheet-iron stove, pick and shovel, cross-cut saw and axe, rifle, bales of dog salmon, and other paraphernalia. These meetings envisioned the panning of old creek beds and draws in the search for as yet undiscovered pay-channels; they meant the toughest kind of work sinking a hole to bedrock in some far-away valley of promise; they suggested wood fires to thaw the frozen muck and gravel and crude windlass and hand-made tub to hoist the water and the mud; but loudest of all they spoke of Adventure.

Jan 1, 1929

By RICKARD HANSEN

Vehicle fires in underground mines pose a certain risk for which measures to prevent or mitigate the fires are highly important. Fire protection measures can be divided into active and passive measures, where active measures require an external power to work in the event of a fire, and passive measures could include an existing partition preventing fire spread. As passive fire protection does not rely on activation, it is expected to provide high reliability and a robust solution, and should be considered during the design process. This paper investigates passive fire protection measures that could be implemented in the design of mining vehicles. It was found that passive fire protection should include the use of several measures simultaneously to ensure fire protection throughout the different phases of a fire. Threshold distances for fuel components were calculated, and it was found that combining hydraulic hoses and electrical cables resulted in larger threshold distances. A pool fire along the underside of a vehicle will pose a certain risk, and passive measures should be focused on ignition prevention, making high demands on the insulation/shielding along the underside. Any occurring tire fire may pose a challenge, and the priority should be on preventing ignition. Potential measures include shielding the wheelhouse and applying fire-resistant sealings.

By Ulick Evans

I SHOULD like to commence by saying how much I appreciate the honor which the American Institute of Mining and Metallurgical Engineers has done me in inviting me to visit your country, and to deliver this lecture before you. In choosing a subject, and in deciding what to include and what to omit, I have endeavored to discuss as far as possible work carried out recently in Europe with which you may not be unduly familiar. It would be poor return for your hospitality, and a mistaken sort of compliment indeed, if I were to load the lecture with accounts of researches &apos;carried out in your own country-researches with which you are probably better acquainted than I. Not that I have in any way excluded work done on, this side of the Atlantic from my survey; one would indeed find it difficult to write on this, or on any other branch of science, without mentioning American work. But I have endeavored as far as possible to concentrate on aspects of the case which will, I hope, be comparatively fresh to you, and I feel fairly certain that this will be in accordance with your own wishes. If a strip of iron is heated strongly at one end, the other end remaining cold, there is produced over the heated portion a layer of oxide scale, the thickness of which diminishes steadily with the distance from the hot end. Somewhere in the middle of the strip, the thickness of the oxidized layer will become comparable .to the wave length of light, and over this region the thin films give rise to a series of beautiful interference tints, commonly known as "temper colors." The colors are caused by the fact that two trains of light waves are reflected respectively from the outer and inner surfaces of the oxide film, .and the possibility arises that the "crests" of one series of waves may coincide with the "troughs" of the other, thus diminishing the intensity of the light of some particular wave length. The state of affairs in this part of the strip is shown diagrammatically in Fig. 1, in which the film thickness is enormously exaggerated, of course, in comparison with the length of the strip; the

Jan 1, 1929