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Analysing Blazes

To most of us, a burnt-out building is just a charred mess, but for those who can read them, there are clues as to where the fire began and what might have caused it.

By Janine Griffin

Fires cost both money and human lives. Fire claimed 30 lives in 1990, and a third of those killed were under 10 years old.

One of the Fire Service's functions is to study and reduce the major causes of fires, says John Sinclair, the Regional Fire Safety Officer for Canterbury. The Fire Service investigates every fire to determine a point of origin and cause.

Structure fires, those that occur in buildings, represented nearly a quarter of all fires in New Zealand in 1990. In a building, and in any fire, the spread of a fire will differ according to the conditions at the time the fire started.

If doors or windows are open, the fire will burn faster than if they are closed, and the size of the room in which the fire starts will influence the fire's rate of growth. The materials in a room, such as furniture, appliances or stored stock, will also affect the spread of the fire.

The physical evidence left behind once the fire has been put out is the fire investigator's working material. The investigator also uses information from the witnesses to the fire, the firefighters, the building's owner or occupants and many others.

The fire investigator isn't necessarily a fire officer. Fires are often investigated for insurance purposes. In some cases, the specific cause may not be required, as it may be sufficient to say what didn't cause the fire.

Sometimes the cause is obvious, and sometimes it is impossible to determine with any degree of certainty. A cause could not be determined for 500 of the 4,500 structure fires in New Zealand in 1990.

"All fires leave traces in the debris," says Sinclair. "The art of fire-tracing is finding these and identifying them."

Many fires are started accidentally, and sometimes the investigation may be hindered by people, say Sinclair and Tony Doust, a Senior Station Officer in Christchurch. People aren't necessarily malicious, they say, but might be trying not to look foolish. Parents may try to protect children who were playing with matches or giving smoking a try, or the cook who left the stove on while talking on the phone might try to avoid blame.

Fires come in all shapes and sizes, and different fires leave different types of physical evidence. The investigator must first narrow down the area where the fire originated. Then the source can be determined.

The investigator needs to go in with a clear mind and consider all possibilities. Fire indicators will give the point of origin, says Doust, and from there it's a process of elimination.

An investigator develops a feel for a fire, states Doust. If that feeling doesn't develop, he says, the suspected cause may not be correct.

Finding the Origin

An examination of the outside of a building and information given by people who first saw the fire will provide some information about it. Eyewitness accounts may tell the investigator the general area where the fire started, the intensity of the fire or the colour of the smoke.

Charring on the outside of the building is a strong indication that the fire started outside and spread inwards. The investigator can then concentrate on looking for evidence outside the building.

Charring and smoke deposits on door edges and around windows, vents and eaves give further information about the fire. If the inside edges of a door show no charring and the side of the door exposed to the fire does, then it is likely the door was closed during the fire, says Doust.

Once inside, the investigator starts with the least severely damaged areas and moves toward the most damaged area.

Fire burns longest in the area of origin, and the area showing the greatest damage will be where the fire started. Charring of timber, smoke deposits on windows and walls, and burning of plaster will all be more severe closer to the fire's origin.

This narrows down the area of origin. Once in the most damaged area, the fire's origin can be pinpointed. But finding this area can be difficult, especially if the building is severely damaged or has collapsed.

Fire doesn't burn randomly but tends to follow a pattern. Heat burns upwards more readily than downwards. The area beneath the point of origin will thus be less damaged than that above it.

Fire usually will not spread laterally from the point of origin but will tend to burn upwards and outwards. As hot gases rise from the origin they spread out, and as these gases burn they leave a V-pattern. The point of the V will be the point of origin. This can be seen by holding a match to the centre of a piece of paper. The fire fans out from the bottom of the paper, reaching the top before the sides in the classic V-pattern.

So, if the fire began near a wall or other vertical surface, such as the side of a stove, there will be a V-pattern near the point of origin. But, if the fire started in the centre of the room, or away from a wall, there may be no V-patterns. The ceiling over the origin may be damaged more than other parts, or may be lighter than other parts if it was subject to intense heat.

Heat flow is primarily along the ceiling, and lights and light bulbs may yield some clues. When exposed to heat, light bulbs swell and may blow out in the direction of the heat source, leaving an important clue to the direction of the fire. But if temperatures are very high, the glass will flow and this type of clue will not be found.

Window glass also yields important clues. Windows closer to the fire will have a lighter smoke buildup than those farther away. As the fire grows, window glass will expand and shatter. Large pieces with heavy smoke show that the fire developed slowly while smaller pieces with rounded edges indicate a rapidly developing, high temperature fire. This might help to narrow down the possible causes of the fire. Thin glass melts more readily than thick glass and this must be taken into account.

Determining the Cause

Once the area where the fire started is pinpointed, only the cause remains to be found. The contents of the room and the burn patterns may fit a specific cause. More often than not, though, more than one possibility will present itself.

The cause of a fire can be intentional or unintentional, and can be anything from a lit cigarette falling down the back of a sofa to sparks from a chimney. The possibilities are almost endless. Heaters, stoves, electrical faults, flammable liquids, cigarettes, electric blankets, soldering, candles, natural gas, and hazardous substances stored improperly can all result in a fire.

The Fire Service uses information from a wide range of people in determining the cause of a fire. For example, says Doust, printers will be able to tell the investigator what sort of chemicals and equipment would normally be found in a print shop. This sort of information can lead the investigator to a cause not normally considered, or it can indicate if the fire may have been suspicious.

A fire in a kitchen, where the damage is most severe in the area of the stove, will suggest to the investigator that someone left a hotplate on. The knobs may be found in the "on" position or, if burned away, the position of the flat edge of the pin will tell whether the hotplate was on or off. If the stove is severely damaged, the investigator will have the contacts examined to see if they are open or closed. If the contacts are found to be closed, the hotplate was on and was very likely the cause of the fire.

Often, though, determining the cause is not possible. A lit cigarette that causes a fire may leave behind a cylinder of ash which will be obliterated by the water used to put out the fire. Other signs may lead the investigator to conclude that the fire was probably started by a cigarette, and the building's occupant might remember leaving a lit cigarette lying around. But the physical evidence that would confirm this conclusion is often destroyed.


Fire Service statistics show that arsons are on the increase in New Zealand. Known or suspected arsons have increased over 60% in the last five years.

Arsons present a special case to the fire investigator. In an arson investigation, the Fire Service will report to the police the suspected cause of the fire. The police then work on the evidence provided by the Fire Service to build a case against their suspect.

Various signs can point towards an arson. Obvious ones are signs of forced entry such as windows broken before the fire started, broken doors, fire separation doors prevented from closing, sprinklers or fire alarms turned off and fire trails. Accelerants in places where they wouldn't normally be found or unusual behaviour of burning material also suggest arson. Accelerant spread around a room will leave a splash-like burn pattern which strongly suggests arson.

The investigator has to be careful not to jump to the conclusion of arson too quickly. It may be obvious that a door has been forced, but further investigation might reveal that the firefighters used that door to enter the building to fight the fire.

There are no hard and fast rules for determining the cause of a fire. The evidence left comes in many pieces and must be put together, much like a jigsaw puzzle, often with pieces missing.

Janine Griffin is a freelance journalist specialising in science issues.