Safety Lamp

A safety lamp is any of several types of lamp that provides illumination in places such as coal mines where the air may carry coal dust or a build-up of flammable gases, which may explode if ignited, possibly by an electric spark. 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; safety lamps were developed to enclose the flame to prevent it from igniting the explosive gases. Flame safety lamps have been replaced for lighting in mining with sealed explosion-proof electric lights, but continue to be used to detect gases.

Background

Damps or gases

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.

* '' Firedamp'' Naturally occurring flammable mixtures, principally methane.
* '' Blackdamp'' or ''Chokedamp'' Nitrogen and carbon dioxide with no oxygen. Formed by complete combustion of firedamp or occurring naturally. Coal in contact with air will oxidize slowly and, if unused workings are not ventilated, pockets of blackdamp may develop. Also referred to as ''azotic air'' in some 19th-century papers.
* '' Whitedamp'' Formed by the incomplete combustion of coal, or firedamp. The mixture may contain significant amounts of carbon monoxide, which is toxic and potentially explosive.
* ''Stinkdamp'' Naturally occurring hydrogen sulphide and other gases. The hydrogen sulphide is highly toxic, but easily detected by smell. The other gases with it may be firedamp or blackdamp.
* ''Afterdamp'' The gas from an explosion of firedamp or coal dust. Contains varying proportions of blackdamp and whitedamp and is therefore suffocating, toxic, or explosive, or any combination of these. Afterdamp may also contain stinkdamp. Afterdamp may be a bigger killer following an explosion than the explosion itself.

Open-flame illumination

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 and 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.

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. To fire the gas, a man edged forward with a lit 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 as upright 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 well-dampened wool or leather. This was a job with risk of injury, or to life.

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. This continued to be essential information even after the introduction of safety lamps; at Trimdon Grange there was an accident involving pressure.

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 emitted by safety lamps caused an increase in the incidence of nystagmus.

First attempts at safe lamps

In Europe and Britain dried fish skins which emitted a faint bioluminescence (often called phosphorescence) were used.

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 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 was intrinsically safe provided it was kept upright, but gave out only a weak light. 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.

Oil lamps

Principles of operation

Safety lamps have to address the following issues:
* Provide adequate light
* Do not trigger explosions
* Warn of a dangerous atmosphere

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.

Since any breathable atmosphere contains oxygen, and a safety lamp's ''raison d'être'' is to operate in an atmosphere also containing fuel (firedamp or coal dust), the element which must be blocked is heat. The key to manufacturing a successful safety lamp is to control the transfer of heat while still allowing air (the necessary oxidant) to enter and leave the lamp.

There are three main paths by which heat must be prevented from leaving the lamp:
* convection of the exhaust gases,
* conduction through the lamp body, and
* burning of the incoming firedamp passing back through the inlet.

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 metal mesh or 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 wire 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 metal gauze surrounds the flame and extends for a distance above forming a cage; flames do not pass through a fine enough mesh. 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; if firedamp is present it will get through the mesh and burn within the lamp itself, but without igniting gas outside. As the lamp burns brighter in dangerous atmospheres it acts 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 are made of materials such as brass or tinned steel, which do not make a spark if they strike rock.

History and development

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. 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 the inability of flames to pass through mesh; 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. The Newcastle committee also awarded Stephenson a £1,000 prize collected by subscription. 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.

Improper use of safety lamps defeated the purpose of the safety lamp and caused risk. When the flame went out in a lamp there was a temptation for the collier to open and relight it. Some opened the lamps to light tobacco pipes underground. Both of these practices were strictly forbidden. The miner was expected to return to the shaft, a round trip of up to a few miles, to relight an extinguished lamp. For men on piece work paid for what they produced, a relighting could cost them perhaps 10% of their day's pay, encouraging them to take the risk. Miners would also damage the mesh to make the lamp brighter. To prevent dangerous relighting (or opening the lamp to light a pipe), 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 was 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 lit 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 Davy's, obscured by the gauze); early lamps gave less light than candles. This was not resolved 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.

The gauze in early lamps (the Davy, Geordie and Clanny) was exposed to air currents. It was quickly discovered that an air current could cause the flame, in effect, to pass through the gauze: the flame playing directly on the mesh heats it faster than the heat can be conducted away, increasing its temperature until sufficient to ignite the gas outside the lamp.

The following data is compiled from :

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 "bonnet" (typically of tinned iron) in the shape of a truncated cone covering the gauze above the glass cylinder. The important point was that no direct current of air could impinge on the gauze itself. The shields 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.

Davy used a fine wire gauze with a mesh of 784 holes per square inch (28 mesh). The required fineness of the mesh was scrutinised by the Miners' Lamp Committee in 1924, 109 years after Davy's work, and a recommendation to use a coarser mesh of 400 holes/sq in (20 mesh) of 27 SWG wire was made. Lamps tested were as safe, and illumination increased, depending on lamp type, by between 16% and 32%.

Timeline of development

Examples of lamps

Davy lamp

In the Davy lamp

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