A safety lamp is any of several types of lamp that provides illumination in coal mines and is designed to operate in air that may contain coal dust or gases both of which are potentially flammable or explosive. Until the development of effective electric lamps in the early 1900s miners used flame lamps to provide illumination. Open flame lamps could ignite flammable gases which collected in mines, causing explosions and so safety lamps were developed to enclose the flame and prevent it from igniting the surrounding atmosphere. Flame safety lamps have been replaced in mining with sealed explosion-proof electric lights.
Miners have traditionally referred to the various gases encountered during mining as damps, from the Middle Low German word dampf (meaning "vapour"). Damps are variable mixtures and are historic terms.
Before the invention of safety lamps, miners used candles with open flames. This gave rise to frequent explosions. For example, at one colliery (Killingworth) in the north east of England, 10 miners were killed in 1806, 12 in 1809. In 1812, 90 men and boys were suffocated or burnt to death in the Felling Pit near Gateshead and 22 in the following year.
Wood 1853 describes the testing of a mine for firedamp. A candle is prepared by being trimmed and excess fat removed. It is held at arm's length at floor level in one hand, the other hand shielding out all except the tip of the flame. As the candle is raised the tip is observed and if unchanged the atmosphere is safe. If however the tip turns bluish-gray increasing in height to a thin extended point becoming a deeper blue, then firedamp is present. Upon detecting firedamp the candle is lowered and arrangements made for the ventilating of the area or the deliberate firing of the firedamp after the end of a shift. A man edged forward with a candle on the end of a stick. He kept his head down to allow the explosion to pass over him, but as soon as the explosion had occurred stood upright as much as possible to avoid the afterdamp. Officially known as a fireman he was also referred to as a penitent or monk from the hooded garb he wore as protection. The protective clothing was made of wool or leather and well dampened. As can be imagined this procedure did not always preserve the life of the man so employed.
When they came into regular use, barometers were used to tell if atmospheric pressure was low which could lead to more firedamp seeping out of the coal seams into the mine galleries. Even after the introduction of safety lamps this was still essential information, see Trimdon Grange for details of an accident where pressure was involved.
The lack of good lighting was a major cause of the eye affliction nystagmus. Miners working in thin seams or when undercutting the coal had to lie on their side in cramped conditions. The pick was swung horizontally to a point beyond the top of their head. In order to see where they were aiming (and accurate blows were needed), the eyes needed to be straining in what would normally be the upwards and slightly to one side direction. This straining led first to temporary nystagmus and then to a permanent disability. Mild nystagmus would self-correct if a miner ceased to perform this work but if left untreated would force a man to give up mining. The lower levels of light associated with safety lamps caused an increase in the incidence of nystagmus.
Both on the continent of Europe and in the UK dried fish skins were used. From them a faint bioluminescence (often called phosphorescence) occurs. Another safe source of illumination in mines was bottles containing fireflies.
Flint and steel mills introduced by Carlisle Spedding (1696-1755) before 1733 had been tried with limited success. An example of a Spedding steel mill may be seen in the museum at Whitehaven where Spedding was manager of the collieries of Sir James Lowther, 4th Baronet. A steel disk was rotated at high speed by a crank mechanism. Pressing a flint against the disk produced a shower of sparks and dim illumination. These mills were troublesome to use and were often worked by a boy, whose only task was to provide light for a group of miners. It was assumed that the sparks had insufficient energy to ignite firedamp until a series of explosions at Wallsend colliery in 1784; a further explosion in June 1785 which the operator of the mill (John Selkirk) survived showed that ignition was possible.
The first safety lamp made by William Reid Clanny used a pair of bellows to pump air through water to a candle burning in a metal case with a glass window. Exhaust gases passed out through water. The lamp gave out only a weak light though it was intrinsically safe provided it was kept upright. It was heavy and ungainly and required a man to pump it continuously. It was not a practical success and Clanny subsequently changed the basis of operation of later lamps in the light of the Davy and Stephenson lamps.
Safety lamps have to address the following issues:
Fire requires three elements to burn: fuel, oxidant and heat; the triangle of fire. Remove one element of this triangle and the burning will stop. A safety lamp has to ensure that the triangle of fire is maintained inside the lamp, but cannot pass to the outside.
In the Geordie lamp the inlet and exhausts are kept separate. Restrictions in the inlet ensure that only just enough air for combustion passes through the lamp. A tall chimney contains the spent gases above the flame. If the percentage of firedamp starts to rise, less oxygen is available in the air and combustion is diminished or extinguished. Early Geordie lamps had a simple pierced copper cap over the chimney to further restrict the flow and to ensure that the vital spent gas did not escape too quickly. Later designs used gauze for the same purpose and also as a barrier in itself. The inlet is through a number of fine tubes (early) or through a gallery (later). In the case of the gallery system air passes through a number of small holes into the gallery and through gauze to the lamp. The tubes both restrict the flow and ensure that any back flow is cooled. The flame front travels more slowly in narrow tubes (a key Stephenson observation) and allows the tubes to effectively stop such a flow.
In the Davy system a gauze surrounds the flame and extends for a distance above forming a cage. All except the very earliest Davy lamps have a double layer at the top of the cage. Rising hot gases are cooled by the gauze, the metal conducting the heat away and being itself cooled by the incoming air. There is no restriction on the air entering the lamp and so if firedamp is entrained it will burn within the lamp itself. Indeed, the lamp burns brighter in dangerous atmospheres thus acting as a warning to miners of rising firedamp levels. The Clanny configuration uses a short glass section around the flame with a gauze cylinder above it. Air is drawn in and descends just inside the glass, passing up through the flame in the centre of the lamp.
The outer casings of lamps have been made of brass or tinned steel. If a lamp bangs against a hard piece of rock a spark could be generated if iron or untinned steel were employed.
Within months of Clanny's demonstration of his first lamp, two improved designs had been announced: one by George Stephenson, which later became the Geordie lamp, and the Davy lamp, invented by Sir Humphry Davy. Subsequently, Clanny incorporated aspects of both lamps and produced the ancestor of all modern oil safety lamps.
George Stephenson came from a mining family and by 1804 had secured the post of brakesman at Killingworth colliery. He was present at both the 1806 and 1809 explosions in the pit. By 1810, he was engineman and responsible for machinery both above and below ground. The pit was a gassy pit and Stephenson took the lead in work to extinguish a fire in 1814. For some years prior to 1815 he had been experimenting on the blowers or fissures from which gas erupted. He reasoned that a lamp in a chimney could create a sufficient updraft that firedamp would not penetrate down the chimney. Further observations of the speed of flame fronts in fissures and passageways led him to design a lamp with fine tubes admitting the air.
Sir Humphry Davy was asked to consider the problems of a safety lamp following the Felling explosion. Previous experimenters had used coal gas (chiefly carbon monoxide) incorrectly, believing it to be the same as firedamp. Davy however performed his experiments with samples of firedamp collected from pits. As an experimental chemist, he was familiar with flames not passing through gauze; his experiments enabled him to determine the correct size and fineness for a miner's lamp.
Davy was awarded the Rumford Medal and £1,000 by the Royal Society in 1816 and a £2,000 prize by the country's colliery owners, who also awarded 100 guineas (£105) to Stephenson. However, the Newcastle committee also awarded Stephenson a £1,000 prize collected by subscription. Dr. Clanny was awarded a medal by the Royal Society of Arts in 1816.
Both the Davy and Stephenson lamps were fragile. The gauze in the Davy lamp rusted in the damp air of a coal pit and became unsafe, while the glass in the Stephenson lamp was easily broken, and allowed the flame to ignite firedamp in the mine. Later Stephenson designs also incorporated a gauze screen as a protection against glass breakage. Developments, including the Gray, Mueseler and Marsaut lamps, tried to overcome these problems by using multiple gauze cylinders, but glass remained a problem until toughened glass became available.
Were the flame to go out in a lamp, then there was a temptation for the collier to relight it. Some opened the lamps to light tobacco pipes underground. Both of these practices were strictly forbidden; they clearly negated the whole point of the safety lamp. The miner was expected to return to the shaft to perform relighting, a round trip of up to a few miles. For men on piece work, this meant a loss of income (perhaps 10% of their day's pay) and so was unpopular. From the mid-century onwards, and particularly after the 1872 act, lamps had to have a lock mechanism which prevented the miner opening the lamp. Two schemes existed; either a special tool was required which kept at the pit head or else opening the lamp extinguished the flame. The latter mechanism can be seen in the Mueseler, Landau and Yates lamps below. Such a lamp was known as a protector lamp, a term picked up and used as a company name. Only on the return to the bank could the lamp man open the lamp for refilling and service. Various different locking mechanisms were developed; miners tended to be ingenious in finding ways of circumventing them. A number of additional lamps were supposed to accompany each gang of men, but restricting the number was an obvious economy for the pit owners.
The light given out by these lamps was poor (particularly the Davy where it passed through the gauze); indeed, in early lamps worse than candles. The problem not solved until the introduction of electric lighting around 1900 and the introduction of battery-powered helmet lamps in 1930. The poor light provided yet another reason for miners to try to circumvent the locks.
Early lamps (the Davy, Geordie and Clanny) had the gauze exposed to air currents. It was quickly discovered that an air current could cause the flame to pass through the gauze. The flame playing directly on the gauze heats it faster than the heat can be conducted away, eventually igniting the gas outside the lamp.
The following table is compiled from Hunt 1879, article: Safety lamps:
|Lamp||Air velocity to cause flame to pass the gauze (ft/s)||Time to explosion (s)||Number of lamps to equal 1 standard candle||Hours to burn 2 oz of oil|
Following accidents such as Wallsend (1818), Trimdon Grange (1882) and the Bedford Colliery Disaster (1886), lamps had to be shielded against such currents. In the case of the Davy, a "Tin-can Davy" was developed which had a metal cylinder with perforations at the bottom and a glass window for the light from the gauze. Clanny derived lamps had a metal shield (typically tinned iron) in the shape of a truncated cone, called a bonnet covering the gauze above the glass cylinder. The important principle is that no direct current of air can impinge on the gauze. The shield had the disadvantage of not allowing the collier or the deputy to check that the gauze was in place and clean. Lamps were therefore made so that they could be inspected and then the bonnet placed on and locked.
In the Davy lamp a standard oil lamp is surrounded by fine wire gauze, the top being closed by a double layer of gauze.
If firedamp is drawn into the flame it will burn more brightly and if the proportions are correct may even detonate. The flame on reaching the gauze fails to pass through and so the mine atmosphere is not ignited. However, if the flame is allowed to play on the gauze for a significant period, then it will heat up, sometimes as far as red heat. At this point it is effective, but in a dangerous state. Any further increase in temperature to white heat will ignite the external atmosphere. A sudden draught will case a localised hot spot and the flame will pass through. At a draught of between 4 and 6 feet per second the lamp becomes unsafe. At Wallsend in 1818 lamps were burning red hot (indicating significant firedamp). A boy (Thomas Elliott) was employed to carry hot lamps to the fresh air and bring cool lamps back. For some reason he stumbled; the gauze was damaged and the damaged lamp triggered the explosion. At Trimdon Grange (1882) a roof fall caused a sudden blast of air and the flame passed through the gauze with fatal results (69 killed).
Poor copies and ill-advised "improvements" were known, but changing dimensions either reduced the illumination or the safety. The poor light compared to either the Geordie or Clanny eventually led to the Davy being regarded as not a lamp but a scientific instrument for detecting the presence of firedamp. Some pits continued to use candles for illumination, relying on the Davy to warn men when to extinguish them.
In the earlier Geordie lamps an oil lamp is surrounded by glass. The top of the glass has a perforated copper cap with a gauze screen above that. The glass is surrounded by a perforated metal tube to protect it.
Later versions had an annular chamber around the base of the lamp into which air entered through small (") holes and from which air passed through gauze into the lamp. The glass was surrounded by gauze so that in the event of a glass breakage the Geordie became a Davy.
In a strong enough current of air enough air could be forced in through the tubes (later holes and gallery) to enlarge the flame and the lamp could get red-hot. The lamp becomes unsafe in a current of from 8 to 12 feet per second, about twice that of the Davy.
A development of the Geordie lamp was the Purdy. A galley with gauze provided the inlet, above the glass was a chimney with perforated copper cap and gauze outer. A brass tube protected the upper works, shielded them and kept them locked in position. A sprung pin locked the whole together. The pin could only be released by applying a vacuum to a captive hollow screw; not something that a nicotine starved miner could do at the coal face.
Clanny abandoned his pumps and candles and developed a safety lamp which combined features of both the Davy and Geordie. The oil lamp was surrounded by a glass chimney with no ventilation from below. Above the chimney is a gauze cylinder with a double top. Air enters from the side and spent gases exit from the top. In the presence of firedamp the flame intensifies. The flame must be kept fairly high in normal use, a small flame permits the enclosed space to fill with firedamp/air mixture and the subsequent detonation may pass through the gauze. A larger flame will keep the upper part full of burnt gas. The Clanny gives more light than the Davy and can be carried more easily in a draught. Lupton notes however it is superior in no other respect, particularly as a test instrument.
The glass on a Clanny was secured by a large diameter brass ring which could be hard to tighten securely. If a splinter occurred at the end of a crack, or indeed any other unevenness, then the seal might be compromised. Such an incident occurred at Nicholson Pit in 1856 on a lamp being used by an overman to test for firedamp. The mines inspector recommended that only Stephenson lamps were used for illumination and Davys for testing. In particular "overmen ... whose lamps are mostly used to detect the presence gas [sic], should avoid such [Clanny] lamps".
The lamp is a modified Clanny designed by the Belgian Mathieu-Louis Mueseler. The flame is surrounded by a glass tube surmounted by a gauze capped cylinder. Air enters from the side above the glass and flows down to the flame before rising to exit at the top of the lamp. So far this is just a Clanny, but in the Mueseler a metal chimney supported on an internal gauze shelf conducts the combustion products to the top of the lamp. Some Mueseler lamps were fitted with a mechanism which locked the base of the lamp. Turning down the wick eventually released the base, but by then the flame was extinguished and therefore safe.
The lamp was patented in 1840 and in 1864 the Belgian government made this type of lamp compulsory.
In the presence of firedamp the explosive mixture is drawn through two gauzes (cylinder and shelf), burnt and then within the chimney are only burnt gases, not explosive mixture. Like a Clanny, and the Davy before it, it acts as an indicator of firedamp, burning more brightly in its presence. Later models had graduated shields by which the deputy could determine the concentration of firedamp from the heightening of the flame. Whilst the Clanny will continue to burn if laid on its side, potentially cracking the glass; the Mueseler will extinguish itself due to the stoppage of convection currents. The lamp is safe in currents up to 15 feet per second.
The Marsaut lamp is a Clanny with multiple gauzes. Two or three gauzes are fitted inside each other which improves the safety in a draught. Multiple gauzes will however interfere with the flow of air. The Marsaut was one of the first lamps to be fitted with a shield, in the illustration (right) the bonnet can be seen surrounding the gauzes. A shielded Marsaut lamp can resist a current of 30 feet per second.
The Bainbridge is a development of the Stephenson. A tapered glass cylinder surrounds the flame, and above that the body is a brass tube. The top of the tube is closed by a horizontal gauze attached to the body of the lamp by small bars to conduct heat away. Air enters through a series of small holes drilled in the lower brass ring supporting the glass.
The lamp is in part a development of the Geordie. Air enters into a ring near the base which is protected by gauze or perforated plate. The air passes down the side of the lamp passing through a series of gauze covered holes and enters the base through another yet another series of gauze covered holes. Any attempt to unscrew the base causes the lever (shown at f in the illustration) to extinguish the flame. The gauze covered holes and passageways restrict the flow to that required for combustion, so if any part of the oxygen is replaced by firedamp, then the flame is extinguished for want of oxidant.
The upper portion of the lamp uses a chimney like Mueseler and Morgan lamps. The rising gases pass up the chimney and through a gauze. At the top of the chimney a dished reflector diverts the gases out sideways through a number of holes in the chimney. The gases then start to rise up the intermediate chimney before exiting through another gauze. Gas finally passes down between the outermost chimney and the intermediate chimney, exiting a little above the glass. The outer chimney is therefore effectively a shield.
The Yates lamp is a development of the Clanny. Air enters through the lower part of the gauze top and leaves through the upper part; there is no chimney. The lower glass part of the lamp has seen some development however. It is replaced by a silvered reflector having a strong lens or bull's-eye in it to allow the light out. The result was a claimed 20 fold improvement in lighting over the Davy. Yates claimed "the temptation to expose the flame to obtain more light is removed".
The base also contained an interlocking mechanism to ensure that the wick was lowered and the lamp extinguished by any attempt to open it.
The lamp was "much more expensive than the forms of lamp now in general use, but Mr, Yates states that the saving of oil effected by its use will in one year pay the additional cost".
The lamp devised and manufactured by Evan Thomas of Aberdare is similar to a shielded Clanny, but there is a brass cylinder outside the gauze above the glass. It resists draughts well but the flame is dull.
The Morgan is a cross between the Mueseler and the Marsaut. It is a shielded lamp with a series of disks at the top to allow spent fumes out and a series of holes lower down the shield to allow air in. There is an inner and outer shield so that air cannot blow directly on the gauze but must first find its way through a slim chamber. There are multiple gauzes, like the Mersaut, and there is an internal chimney like the Mueseler. There is no "shelf" supporting the chimney, instead it hangs from an inverted cone of gauze.
The Morgan will resist air up to 53 feet per second and is "sufficiently safe for every practical purpose".
The Clifford also has a double shield, but with a plain flat top. The chimney is quite narrow with gauze covering the top. The bottom of the chimney has a glass bell covering the flame. The chimney is supported on a gauze shelf. Air enters through the lower part of the outer shield, through the passage and into the lamp through the inner shield. It is drawn down through the gauze then passes the flame and ascends the chimney. At the top it leaves through gauze and the top of the double shield. The inner chimney is made of copper coated with a fusible metal. If the lamp gets too hot the metal melts and closes up the air holes, extinguishing the lamp.
The lamp has been tested and according to Lupton "successfully resisted every effort to explode it up to a velocity of more than 100 feet per second".
It was not until tungsten filaments replaced carbon that a portable electric light became a reality. An early pioneer was Joseph Swan who exhibited his first lamp in Newcastle upon Tyne in 1881  and improved ones in subsequent years. The Royal Commission on Accidents in Mines set up in 1881 carried out extensive tests of all types of lamps and the final report in 1886 noted that there had been good progress made in producing electric lamps giving a light superior to that of oil lamps and expected economic and efficient lamps to become available soon. This turned out not to be the case and progress was slow in attaining reliability and economy. The Sussmann lamp  was introduced into Britain in 1893 and following trials at Murton Colliery in Durham it became a widely used electric lamp with 3000 or so reported by the company in use in 1900  However, by 1910 there were only 2055 electric lamps of all types in use - about 0.25% of all safety lamps. In 1911, an anonymous colliery owner, through the British government, offered a prize of £1000 (equivalent to £102,589 in 2019) for the best lamp to specified requirements. There were 195 entries. It was won by a German engineer with the CEAG lamp, which was hand-held and delivered twice the illumination of oil lamps, with a battery life of 16 hours. Awards were made to 8 other lamps that met the judges' criteria. Clearly this stimulated development and over the next few years there was a marked increase in the use of electric lamps, especially the CEAG, Gray-Sussmann and Oldham, so by 1922 there were 294593 in use in Britain.
In 1913, Thomas Edison won the Ratheman medal for inventing a lightweight storage battery that could be carried on the back, powering a parabolic reflector that could be mounted on the miner's helmet. After extensive testing, 70,000 robust designs were in use in the US by 1916,.
Early electric lamps in Britain were hand held as miners were used to this and helmet lamps became common much later than in countries like the USA where helmet (cap) lamps had been the norm.
Nowadays, safety lamps are mainly electric, and traditionally mounted on miners' helmets (such as the wheat lamp) or the Oldham headlamp, sealed to prevent gas penetrating the casing and being ignited by electrical sparks.
Although its use as a light source was superseded by electric lighting, the flame safety lamp has continued to be used in mines to detect methane and blackdamp, although many modern mines now also use sophisticated electronic gas detectors for this purpose.
As a new light source, LED has many advantages for safety lamps, including longer illumination times and reduced energy requirements. Combined with new battery technologies, such as the lithium battery, it gives much better performance in safety lamp applications. It is replacing conventional safety lamps.
The Office of Mine Safety and Health (OMSHR), a part of the National Institute for Occupational Safety and Health (NIOSH) (itself part of Centers for Disease Control and Prevention) in the United States has been investigating the benefits of LED headlamps. A problem in mining is that the average age is increasing: 43.3 years in 2013 (in the USA) and as a person ages vision degenerates. LED technology is physically robust compared to a filament light bulb, and has a longer life: 50,000 hours compared to 1,000 - 3,000. Extended life reduces light maintenance and failures; according to OMSHR an average of 28 accidents per year occur in US mines involving lighting. NIOSH has sponsored the development of cap lamp systems which they claim improve the "ability of older subjects to detect moving hazards by 15% and trip hazards by 23.7%, and discomfort glare was reduced by 45%". Conventional lights are strongly focussed in a beam, NIOSH LED lamps are designed to produce a wider more diffuse beam which is claimed to improve the perception of objects by 79.5%.