CHAPTER XVII

THE INCANDESCENT MANTLE INDUSTRY--HISTORICAL AND GENERAL INTRODUCTION

The group of elements which we are considering can be divided, from the point of view of technical application, into two classes. The first of these contains one element only, titanium, which in its technology, as in its chemistry, stands apart from the others; it will, accordingly, be treated in a separate chapter. The second class contains the yttrium and cerium metals, with zirconium and thorium; the technical importance of these elements is due chiefly to the use of their oxides in illumination, to a small extent in Nernst lamps, and to a much greater extent in the so-called Incandescent Lighting. The manufacture of incandescent mantles[487] is a large and ever-extending industry, intimately bound up with the older process of coal-distillation, with its innumerable ramifications; indeed, it may be said that but for the ingenious invention of Dr. Auer, illumination by means of coal-gas would to-day have been almost obsolete. The discovery which resulted in the production of the familiar incandescent mantle of the present day may be regarded as the culmination of a century’s effort to increase the value of coal-gas as an illuminating agent. In the present chapter it is proposed to outline the history of these endeavours, and to give a short general account of Auer’s work and its results.

[487] The term ‘incandescent mantle’ is not, perhaps, scientifically very desirable. It is used here, not only on account of its general acceptance, but also because there seems to be no brief and convenient term which might be used in its stead.

Soon after the introduction of gas as an illuminating agent it was realised that the luminosity of the flame is dependent on the presence of solid particles, which by the heat of combustion of the gas are raised to a temperature at which they emit radiations of wave-lengths corresponding to the ‘luminous rays’ of the spectrum. A non-luminous flame of sufficiently high temperature, therefore, can be rendered luminous by the introduction of suitable solids, and numberless investigators have striven, during the past century, to discover the most suitable method of increasing the luminosity of a flame in this way. The luminosity of the ordinary ‘bats-wing’ or ‘flat’ flame, now so rapidly going out of use, is due to the presence in the outer zone of the flame of heated particles of carbon, produced by the decomposition--or partial combustion--of ‘dense’ hydrocarbons, _i.e._ of hydrocarbons having a high percentage of carbon. Ordinary coal-gas consists largely of a mixture of hydrogen and methane, both of which burn with practically non-luminous flames, with small quantities of olefines, acetylenes, etc., to which the luminosity is chiefly due. It would appear, then, that by the introduction of dense hydrocarbons, a gas of poor illuminating power might be made much more valuable as a source of light. On the other hand, it is also apparent that the same end might be achieved by the introduction into a non-luminous or feebly luminous flame of an altogether foreign substance, introduced as such, and not continuously consumed, as is the carbon in the former method. Both these directions of improvement have been followed; since, however, the results achieved by the latter method have become recently of far greater importance, the applications of the first method will be dismissed quite briefly, and the history of the second will then be treated somewhat fully.

The first important attempt to increase the illuminating power of gases burning with feebly luminous flames was that of Faraday, who in the course of an investigation into the causes of the variations in luminosity of ‘portable gas,’ discovered benzene, or bicarburet of hydrogen, as he called it, in 1826. In 1830 an engineer named Dunnovan undertook to illuminate Dublin by means of water-gas[488] which he ‘carburised’ by addition of dense hydrocarbons. During the latter half of the nineteenth century this method became of some importance. It has been applied, in particular, to enrich the ‘natural gas’ of Ohio, North America. The dense hydrocarbons necessary for this purpose are obtained by the process known as ‘cracking.’ The viscous residues from the distillation of the mineral oil of the district are allowed to drop into a brick chamber, of which the walls are raised to a bright red heat, and the dense hydrocarbons which are evolved are removed by a current of the gas to be enriched. In this way a gas of relatively high illuminating power is obtained.

[488] Water-gas is a mixture of equal volumes of carbon monoxide and hydrogen, obtained by blowing steam through a glowing coke furnace. At intervals the steam is shut off, and air is blown through to raise the temperature of the coke.

In the year previous to that in which Faraday first carburised water-gas, Berzelius had observed that thoria and zirconia, when heated in a non-luminous flame, emit an intense white light. Similar behaviour had long before been observed in the cases of magnesia, alumina, lime, zinc oxide, etc. The first practical application of this property of the oxides was that of Drummond, who in 1826 heated a pencil of lime in the oxy-hydrogen flame and obtained the intense white light which has since become so familiar as the Drummond or ‘lime-light.’ A further development in this direction was due to du Motay and Maréchal, who in 1867 illuminated the Place de Tuileries and the Hôtel de Ville in Paris by means of pencils of compressed zirconia--magnesia was also used--heated by means of oil vapour and oxygen.

The use of non-luminous flames to secure illumination, by raising the temperature of solids suspended in them to the point of incandescence was proposed in 1839 by Cruickshank, who used a mantle of platinum wire, covered with lime and rare earths, which he heated by means of water-gas. In 1846 Gillard employed mantles of platinum wire, raised to incandescence in the flame of burning hydrogen, which he obtained by passing steam over heated iron wire; later he used water-gas (1848), his lamps with this modification being employed in Paris and in Philadelphia. Narbonne was later illuminated (1856-1865) by a similar device, but permanent success could hardly be obtained in view of the cost of the platinum mantles, which lasted only a few months. The same mantle was proposed in 1882 by Lewis, the ordinary Bunsen flame being suggested as the source of heat. In the same year Popp exhibited at the Crystal Palace lamps in which a platinum mantle was raised to incandescence by means of a flame of coal-gas and heated air. These attempts, however, served only to show that no permanent advance could be made in this direction.

A new development was made in 1880 by Clamond. He prepared a paste by grinding up calcined and powdered magnesia with a concentrated solution of magnesium acetate; by forcing this through a press he obtained a ribbon which was then wound crosswise on a wooden shaper, dried carefully, and ignited. In his later experiments twenty per cent. of zirconia was added to the magnesia. The mantle was supported in a platinum cage and heated in the flame of a mixture of coal-gas and heated air. This mantle gave an intense light, but was too fragile for extended use. In the following year, Lundgren patented a process by which lime, magnesia, and zirconia, made into a paste by the addition of gum, were forced through a press, and the resulting thread wound on a graphite-covered shaper. The mantle so obtained was stable, and gave an intense white light, but after having been heated for some time the oxides crumbled to powder. A modification of this process was introduced by Knöfler in 1894, in an attempt to use a cellulose solution containing rare earth salts; this was forced through jets, and the cellulose precipitated as a continuous thread from which the mantle was made. A further modification of Knöfler’s process by Plaisetty in 1901 was technically successful; but these developments must be taken up in a later chapter (_vide_ p. 307).

In 1883 a process was patented by Fahnehjelm in Stockholm, by which for the first time a cheap and stable mantle of considerable efficiency was produced, and which, but for the advent of the Auer mantle, would undoubtedly have been commercially successful. Fahnehjelm’s mantle consisted of an arrangement of needles or lamellæ of magnesia, lime, zirconia, etc., suspended over a burner. The plates and needles were usually arranged in the form of a comb of suitable shape, and were found to give an intense light, and to be long-lived. In later forms the combs were made of rods of magnesia dipped into solutions of chromium salts. The great disadvantage of this invention lay in the fact that the combs required to be heated in the flame of water-gas, in order to secure a good incandescence; had it been possible to attain a sufficiently high temperature by the use of coal-gas, it is doubtful whether the Auer mantle would have ever been evolved.

The more important attempts to secure arrangements by which the radiations of heated solids could be used for illumination have now been outlined and the ground cleared for the consideration of the work of Baron von Welsbach. There remain yet to be mentioned, however, two attempts which are of especial interest in view of that work. The first is that of Frankenstein, who in 1849 made use of a ‘Light-multiplier’ obtained by impregnating gauze with a paste of chalk and magnesia ground with water. The second is that of Edison, who proposed (1878) to utilise the observations of Bahr and Bunsen (1864) and of Delafontaine (1874), of the remarkable incandescence exhibited by the yttria and erbia earths, and the terbia earths, respectively, when heated; he suggested the employment of a mantle of platinum wire covered with zirconia and the oxides of the rare earth metals, a proposal similar to that put forward nearly forty years earlier by Cruickshank.

About the year 1880 Dr. Carl Auer began the study of the rare earth elements. The chemical aspect of his work has already been dealt with (_vide_ p. 168); but the results obtained by the technical application of his observation that threads of cotton, impregnated with a solution of salts of the elements, leave after ignition a coherent ash of oxide, which glows brightly when heated, have been of far greater importance than the purely scientific aspect, valuable though that is. A series of experiments soon showed that a fabric of suitable shape, impregnated with a solution of nitrates or acetates of the rare earth elements, after being dried and drawn together at one end by means of a platinum wire, can be ignited in a Bunsen flame in such a way as to leave a coherent skeleton of the earth oxides, which can be formed and hardened by suitable manipulation with a high temperature burner; the mantle so prepared, when suspended from a lateral support in a Bunsen flame, gives a light of considerable intensity, the colour varying with the oxides employed from green to orange tints.

The earlier mantles, which were placed on the market about 1883, consisted chiefly of oxides of lanthanum and zirconium, with smaller quantities of the other oxides, selected according to the shade of light desired. These mantles were protected by patents taken out in France in 1884, and in Germany in 1885 and the following years. The process[489] was briefly the following: A vegetable fibre, of cylindrical form, woven from threads of about 0·22 mm. diameter, is washed with dilute hydrochloric acid, then with distilled water, and impregnated with a 30 per cent. solution of the selected salts. The fabric is then wrung out and dried, and cut into suitable lengths, allowance being made for subsequent shrinkage. One end of each cylinder is then drawn together by means of a platinum wire, and the mantle hung from a side support over a burner and incinerated. The head is then treated with a solution of aluminium and magnesium nitrates (beryllium nitrate and the corresponding phosphates are also specified) to strengthen it, and the mantle dried, and ‘formed’ by means of a very hot flame. This first patent protected several definite mixtures of salts, chosen so that the mantle should emit light of a definite known tint. The chief oxides employed were lanthana, yttria, magnesia, and zirconia. A German patent granted in 1886[490] protects the use of thorium salts, and a long list of salts of the elements with numerous acids; an important advance mentioned in this specification is the process of collodinisation of the finished mantle, by dipping in a solution of rubber in benzene or of collodion (cellulose nitrate) in ether and alcohol, which renders the product strong enough for transport. From 1885 to 1891 numerous improvements were effected; asbestos threads were substituted for platinum wire, central rods of magnesia replaced the lateral platinum support, and various mixtures of oxides were tried. None of the innumerable mixtures employed, however, was successful in establishing the struggling industry on a firm basis in face of the vigorous competition of the electric lamp, and it was not till 1891 that the introduction of the final ‘Auer Mixture,’ which is in use at the present day, gave the welcome assurance of a certain success to von Welsbach and his assistants. The discovery of this mixture was a result of the examination of a quantity of impure thoria; it was found that mantles made from the nitrate gave a light which steadily decreased in intensity as the impurities were removed. It needed only the observation that the impurities consisted chiefly of cerium compounds to turn the long and arduous investigation in the direction of final success, and our present mantles, which consist approximately of 99 per cent. thoria and 1 per cent. ceria, were placed on the market in 1891, the composition being announced by patent in 1893.[491]

[489] _Vide_ _D. R. P._ 39162. Granted September 23, 1885.

[490] _D. R. P._ 41945.

[491] _Vide_, _e.g._ Moeller, _E._ 124, 1893.

The effect of increasing or decreasing the ratio of the two oxides, and the theories which have been advanced to account for the results, must be referred to in a later chapter (_vide_ p. 294). It may be mentioned here, however, that practically no other known mixture gives such satisfactory results, though mantles have been manufactured of alumina with small quantities of chromic oxide, and ‘inverted’ mantles made of these oxides with zirconia have recently been advocated by Professor Lewes,[492] an authority on gas lighting. Mixtures of alumina and uranium oxide have also been patented, but no mantles appear to have been manufactured according to the specifications. In this connection, also, may be mentioned the various attempts to evade the Auer patents by taking advantage of the ‘discovery’ of ‘new’ elements. One enterprising firm, after having an account of a ‘new’ element, Lucium, inserted in a well-known scientific periodical, put salts on the market, and proceeded to manufacture mantles from what were proved by analysis to be cerium compounds. Similar ‘new’ elements were Russium, Kosmium, and Neo-kosmium, names which covered various mixtures of thorium and cerium compounds with other salts.

[492] _Vide_ _D. R. P._ 218333 of January 1910.

After the introduction in 1891 of the final Auer mixture, progress became rapid. The original mantles, made from cotton, had many disadvantages; thus after being in use for some time they were found to shrink considerably, with marked decrease in strength and light-giving power. Once the success of the new form of lighting was assured, numberless investigations were undertaken to lengthen the life and increase the efficiency of the mantles. The most important of these were connected with the endeavour to replace cotton by some fabric which on ignition would leave the oxide skeleton in a harder, more coherent and more elastic condition. The first great advance in this connection was the introduction of Ramie fibre by Buhlmann in 1898. Ramie, China-grass, or grass-cloth, as it is sometimes termed, is a fabric made from the fibres of the tschuma plant of the Yang-tse-kiang valley and other parts of Asia; mantles made from it last longer and maintain their efficiency much better than the earlier cotton mantles, which they have very largely displaced. The use of artificial silk was patented by De Mare in 1894, but his process was unworkable; it was an effort to adapt to the purposes of incandescent lighting the nitro-cellulose process introduced by Chardonnet in 1890 for the manufacture of artificial silk. In 1897 De Lery and in 1900 Plaisetty made further efforts in this direction, and finally in 1902-1903 the latter worked out a process by which mantles were made directly from the spun fabric. These mantles are superior in every way to the earlier ramie or cotton kinds, and are rapidly coming into general use, especially for lamps using high-pressure gas. Numberless patents for the manufacture and improvement of this kind of mantle have been taken out during the last ten years; the most important of these will be dealt with in a later chapter.

Attempts have been made to secure greater strength and toughness in mantles in other directions also. The use of metallic wires in the fibre has been suggested; numerous patents deal with mantles ‘strengthened’ by doubling the thread at intervals, and by special methods of weaving the fibre. One method, which follows on the lines of Glamond and Lundgren, proposes[493] the use of mantles made from various oxides mixed with silica, the whole being worked into a paste by use of a gum or soap, from which threads are prepared by pressure; mantles made from these threads are said to be very strong and porous. Another patent[494] protects the manufacture of ‘incandescence bodies’ made from plates or combs prepared from a thread obtained in a rather similar way. A third of these innumerable suggestions recommends a preliminary impregnation of the fabric with an aluminium or magnesium salt,[495] from which the oxide is precipitated on the fabric by a suitable means, impregnation with the ordinary ‘lighting fluid’ being effected after drying. Quite an early patent[496] proposes the impregnation of the prepared mantle, either after or just before burning off, with an alcoholic solution of an organic silicon compound, so that when the mantle is in use a skeleton of silica is formed to ‘strengthen’ the oxide ash. No useful purpose can be served by extending the list of these proposals; enough has been said to indicate the various directions in which so many vain attempts at improvement have been made.

[493] Laigle, _D. R. P._ 216871 of December, 1909; see also _D. R. P._ 216877 and 219640.

[494] Michaud and Delasson, _D. R. P._ 210640, June, 1909; see also _D. R. P._ 227257.

[495] Zdanowich, _E._ 27755, 1908.

[496] Jasper, _E._ 30145, 1897.

From the mechanical and physical side the recent developments have been very marked. The introduction of the ‘inverted’ lamp was a tremendous step forward, and paved the way to the second great improvement, the use of ‘high-pressure’ gas, with which such successful results are being obtained. The form of lamp now coming into use for street lighting gives 1500 candle-power per mantle, and usually carries three mantles; each lamp thus develops 4500 candle-power. The purely mechanical devices which are now used to secure ‘automatic’ lighting are rapidly bringing this form of lamp into favour for street illumination. A full account of these developments would be entirely beyond the scope of the present work. In the following chapters, therefore, no complete treatment of the incandescent lighting industry can be given; but whilst the chemical aspect is treated at some length, many points of more purely technical character, which are connected with this, have also been included.