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The working of metal is ubiquitous but the heavier and dirtier processes tend to

predominate in the older industrial areas. Some processes have changed little

over the years but modern technology, particularly computer aided, has

introduced the potential for large losses to occur. The industry includes many

entirely different processes and the Consultant needs to have a broad knowledge

of each to understand the potential fire hazards involved.


Some of the more common manufacturing processes and their related fire

hazards are briefly described as follows:

1. Foundries

Melting. Metal is smelted from the ore, often including scrap, in furnaces

heated by oil, electricity or gas. The material is loaded into the top of the

furnace (cupola) and is generally fed slowly into the main heating area

where it becomes molten. Additives are included to control the final

properties of the material or to assist the process before it is tapped off

either directly into moulds via channels or into a container (ladle) for transfer

to separate areas for casting. Melting can also be done in smaller tilting

furnaces which may be operated by hydraulics.

Due to environmental requirements filters have to be incorporated to

remove particles from the flue gases. This is especially prevalent where

reprocessing of scrap material occurs. Fires have occurred within these

filters and special protection considerations may apply, including the

installation of spark detection and suppression systems – Technical Bulletin

16 refers. Furnaces are maintained at high temperature at all times to

prevent thermal cracking of the brick linings during cooling or heating and

commonly will be running 24/7. The furnace can be water cooled to help

protect the lining. The melting of some metals can be very hazardous in

which case the process is carried out under inert atmospheres or vacuum.

Casting. When the metal is molten it is cast into moulds which are

generally made of sand or steel. Sand casting usually involves a wooden,

plastic or metal pattern around which sand is forced. The pattern is

removed leaving the shape of the item to be cast. To ensure that the sand

remains in the required shape and that the mould releases properly, various

liquid additives or resins can be added, some of which are highly


Sand cores (used to produce holes in the casting) are manufactured by

mixing special sand with flammable resins and liquids (often alcohol based).

Large stocks of wooden dies and patterns can be found in foundries and

are generally of high value.

For intricate casting, an expanded polystyrene mould is used around which

the sand is compacted. In this instance the polystyrene is left in the mould,

vaporising on contact with the molten metal.

Investment (lost wax) casting. This consists of a precision casting technique

with considerable use of wax patterns and highly flammable liquids.

Traditionally, a hazardous process requiring special fire safety


Die casting. Frequently used for low melting point metals and alloys such

as aluminium and zinc and carried out by pressure or gravity. Pressure die

casting is the more hazardous in that molten metal is injected under

pressure into a metal mould, where it cools and is then ejected (similar to

plastic injection moulding). The pressure is applied using hydraulic oil. The

process, particularly for fancy goods manufacture, is frequently

accompanied by lacquering using highly flammable solutions and

extensive packing operations.

Potential Hazards

  • Highly flammable liquids and gases

  • Hot metal

  • Over heating

  • Hydraulic oil

  • Pattern making and storage

  • Transportable heating commonly employed

  • Extraction systems

2. Forging

This essentially consists of the shaping of metals by hammering or

squeezing, whether as part of the initial refining and working process or for

the manufacture of semi-finished components is carried out in a forge.

Metal is heated in a furnace and transferred to an anvil located in a hammer

press where it is struck by a large weight (tup). The striking action can be

either by gravity (drop forging) or under power. In either case the weight is

lifted and or lowered by electric, pneumatic, steam or hydraulic power.

Presses are usually of massive construction.

Where manufacturing semi-finished components various types of presses

and reheating furnaces will be encountered.

Forging can be done cold, for example by extrusion forging where metal is

forced through a die under extremely high pressures.

Potential Hazards

  • Vulnerable fuel lines requiring emergency shut-off procedures

  • Hydraulic oils

  • Hot metal

  • Transportable heating commonly employed

3. Rolling Mills

Converting large ingots of metal to bars or beams of various cross section is

carried out in rolling mills. Rolling is usually done hot; the ingot is heated to

above red heat in a furnace and is transferred to the mill where is passes

between water cooled rollers to progressively reduce the cross section.

The rollers can be shaped to produce the different profiles.

Machines in rolling mills are of massive construction and can be

hydraulically or electrically powered. Control of the mill is from a centrally

computerised operations room and the position of this and its fire

protection is of importance.

Non-ferrous metals are often cold rolled, sometimes with the use of light

grade cooling and lubrication fluids.

Potential Hazards

  • Hydraulic oils

  • Hot metal

  • Flammable oils

  • Transportable heating

4. Pressing and Stamping

Metal can be shaped whilst at room temperature providing sufficient force

is used. Presses vary from manual fly presses to power presses applying

thousands of tonnes pressure used, for example, in the motor industry. In

drop stamping, the upper part of the die is dropped by gravity in a similar

fashion of drop forging.

In hot stamping, small ingots are heated in furnaces and placed in the die

for shaping.

Potential Hazards

  • Hydraulic oils

  • Vulnerable fuel lines requiring emergency shut-off procedures

5. Sheet Metalworking

Fabrication. The production of items from sheet metal involves essentially

simple processes such as cutting, folding and welding. Guillotines of

varying sophistication are used from simple hand operated ones to fully

computerised equipment that cut and fold to pre-set sizes. Thicker sheets

of metal can be cut using oxy-acetylene or oxy-propane. Where large

quantities of a particular shape are required a profile cutter is used which

has a number of gas jets mounted on to a frame. Control can be

mechanical following a template or may involve sophisticated computer

numerically controlled (CNC) equipment.

Increasing use has been made of laser cutting which, whilst being no more

hazardous than conventional means, can involve very expensive equipment

and control plant.

Fabrication will often be followed by spray painting/stove enamelling or

electro –static powder coating of components or finished products.

Metal spinning. Metal can be formed into hollowware by spinning. A

mandrel is mounted on a modified lathe; a disc of metal is held between

the mandrel and the chuck and is rotated at high speed. The disc is drawn

over the mandrel by the use of levers. Materials up to a quarter of an inch

thick can be used to producing anything from kettles to cement mixer

barrels. There is frequently considerable use of combustible packaging.

Welding and brazing. Metals can be welded by use of gas flames or

electric arc. In gas welding the fuel is usually acetylene but others such as

hydrogen or various proprietary mixtures can be used. In electric arc

welding an inert shielding gas such as argon or nitrogen is frequently used.

Brazing and soldering is done at lower temperatures but frequently

involves the use of flammable fluxes. In some systems the fuel gas is

passed through a liquid flux entraining it for delivery to the burner. This

precludes the need for flux to be applied to the joint prior to brazing or


Potential Hazards

  • Highly flammable liquids and gases

  • Combustible packaging

  • Welding and cutting processes

  • Spray painting/powder coating.

6. Machining

The machining of metal using a cutting head or tool is carried out on a wide

variety of equipment from a simple lathe to a multi-headed CNC machining

centre. Consultants when reporting should clearly differentiate between

"low-tech" and "hi-tech" plant.

Various cutting fluids can be used to lubricate and cool the tool. "Suds" oil is

predominantly water and presents little hazard, but others may be neat

mineral oils. In some processes there can be considerable splashing or

leakage on to floors necessitating the use of metal drip trays.

Aluminium machining can involve the use of light grade coolants such as

kerosene with the associated fire risk. Floors can get soaked providing a

considerable risk. Ancillary machining of plastics can be encountered.

Some metal swarf such as titanium and magnesium turnings, can become

ignitable if machined finely enough so coupled with inadequately

sharpened or worn cutting tools and insufficient coolant, this can lead to

fires developing within the machining area and subsequently into swarf

collection compartments.

Precision engineering will frequently involve the use of spark erosion

machining for which special fire safety considerations apply.

Potential Hazards

  • Reactive and/or ignitable metals

  • Machining of plastics

  • Friction caused by blunt or worn cutting tools

  • Oil based coolants

  • Spark erosion machining

  • Degreasing

  • Heat treatment

  • Unattended processes

7. Heat Treatment

This process is carried out to relieve stresses and strains imparted during

the working of the metal or to impart certain properties such as hardness to

the metal. It involves heating and then cooling under controlled conditions,

frequently by quenching in oil.

It is sometimes carried out "in-house" in a separate heat treatment

department but more often sub-contracted to specialist heat treatment

companies. Heat treatment can involve some potentially extremely

hazardous operations and a separate section of the Manual is in course of


8. Polishing and Grinding

Metal goods can be finished by polishing. A greasy cutting paste is applied

to a rotating cloth mop and the item held against it manually. The process

can be automated in which case the machinery is enclosed within a booth.

A considerable amount of greasy fluff is created which should be removed

by an extraction system.

The process is frequently sub-contracted to specialist polishers who often

operate on low budgets in poor buildings and with unfavourable fire safety


Metal components are often finished by a process of precision grinding

which can be carried out on a variety of different manually or semiautomatic machines to include surface grinders (typically for flat workpieces,) cylindrical grinders (where the cylindrical workpiece rotates and is held at each end,) centreless grinders (using a slave or control wheel to rotate a cylindrical workpiece while a second grinding wheel machines

the surface,) and internal grinders (where the workpiece is hollow and the

internal surface is ground using a rotating grinding wheel.)

As with any grinding process, the grinding wheels are abrasive and will

generate sparks when coming into contact with the metal component,

therefore machining coolants are required.

Potential Hazards

  • Poor housekeeping

  • Combustible extraction ducting

  • Accumulation of fluff and fly

  • Sparks

  • Dust explosion


The following features, amongst others, require consideration by Consultants as

part of an effective risk assessment and control programme:

1. Construction

To prevent corrosion from the elements or from fumes given off in the

process, many old foundries, drop forges and the like were built of bitumen

coated metal, usually Robertson’s Protected Metal (RPM) providing the

potential for rapid fire spread. These materials are unlikely to be around in

any quantity today, although Consultants should be aware.

2. Management

Key considerations include the following:

Good organisation with separate storage areas for combustible packing,

wood patterns, flammable liquids and any other combustibles. In larger

risks separation should be by compartment walls of at least 120min fire


Good housekeeping standards are necessary to prevent accumulations of

waste, spillages of oil etc, especially in the pits that surround larger presses

and stamps. The use of sawdust for soaking up oil needs to be discouraged

in preference to the use of proprietary oil absorbent granules.

Planned preventative maintenance systems are desirable in all cases but

are essential where chances of metal break outs exist or where machines

are run 24 hours per day. Specialist heat treatment facilities need high

standards of management and maintenance, as do foundries and forges

where in both cases high temperatures are used with their potential


If water comes into contact with molten metal severe explosions can result.

Water services need to be sited so that leaks or bursts will not present a

hazard. Where scrap is fed directly into furnaces that already contain

molten metal it should be checked for water contamination, especially

when it has been stored in the open.

  • Where light grade oils such as kerosene are used for cooling and

    lubrication special care is necessary to control spillage and to

    provide the necessary fire protection. Cold rolling mills are normally

    protected by fixed gaseous systems.

  • Transportable heating appliances fuelled by paraffin or LPG are

    commonly encountered in foundries and similar risks for localised

    heating. Subject to these being correctly operated and maintained,

    Underwriters will often consider these to be tolerable.

3. Overheating

Within the foundry industry the metal melting pots are usually water cooled

and it is essential to maintain a reserve supply of water in the event of

pump or power failure. Should the cooling supply be interrupted the metal

can break out with dire consequences. Overheating may be caused by a

number of factors including:

  • Failure of cooling circuits.

  • Faulty heating arrangements or thermostats.

  • Operator failure - incorrect training or incompetent personnel.

4. Flammable Liquids

wide range of flammable and highly flammable liquids are employed in

many metalworking and engineering facilities, and the need to carry out an

effective risk assessment in accordance with the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) is paramount to ensuring that these are stored and used in a safe manner. Technical Bulletin 21 refers Pattern making is usually done by specialist companies, but foundries will often have a small repair shop. If plastic patterns are employed, this may include the use of use of resins, catalysts, and solvents.

5. Reactive Metals

All metals, including iron are explosive in a very finely divided state. Certain

metals, mainly magnesium, titanium and aluminium are particularly

hazardous because their swarf or other small particles are combustible and

once ignited are difficult to extinguish.

The machining of these metals must be conducted under carefully

controlled conditions and it needs to be established that the operators are

fully aware of the dangers and the importance of correct cutting speeds

and tool maintenance. Swarf must be cleared regularly and stored

separately in a safe area. Special dry powder extinguishers must be


Grinding and polishing of these metals is particularly hazardous and may

lead to explosions in the extraction system. Wet systems are normally used,

especially for titanium, but care is needed as certain metals may react with

water to form hydrogen. Specialist advice may be necessary in these cases.

6. Hydraulic Fluids

Mineral hydraulic fluids can be used under considerable pressures usually

in conjunction with flexible hoses where moving parts are involved. Bursts

or leakages are likely to give rise to fine mists or sprays of oil which can be

easily ignited. The problem is especially relevant where hot metal is being

worked or where hot services are nearby and an effective planned

maintenance programme is very important in these situations.

Large hydraulic presses and forges often have the pumps and reservoirs

located in cellars below the machines. These would normally be critical

areas, difficult to fight a fire in and where provision of fixed extinguishing

systems would usually be considered essential.

Further guidance regarding hydraulic fluids can be found in Technical Bulletin 14.

7. Degreasing

Removal of oily films produced in machining operations is often carried out

in vapour degreasing tanks. They are usually heated by electricity or gas

and need to be kept clear of combustible materials.

Although the solvents used are normally non-flammable, fires can occur

due to a build-up of oily sludge at the bottom on the tank. This results in a

rapid boiling of the solvents and the evolution of acidic fumes which can

cause severe corrosion problems. Regular cleaning as part of a planned

maintenance programme is essential.

8. Welding and Cutting Equipment

Safe practice should include:

  • The provision of non-combustible or proprietary fire retardant/flame

    resistant welding screens as required.

  • Gas cylinders stored externally in a safe manner.

  • Gas cylinders in use mounted secure on a cylinder trolley or secured

    upright to a workbench or the building fabric.

  • Flashback arrestors fitted as required and all flexible welding hoses

    which can become perished or damaged, examined and replaced as

    needed. In respect of oxy-acetylene equipment, Technical Bulletin

    53 refers.

9. Spark Erosion Machining (Electro Discharge Machining)

Technical Bulletin 10 refers.

10. Unattended Operations

With the development of automation many machines are capable of

running for long periods without human supervision. Combined with the

large capital sums invested in these machines, such as spark erosion

machines, it is becoming common for these to be run unattended

overnight. Technical Bulletin 15 refers.

These notes are intended to provide general guidance on the key trade specific,

survey considerations across a wide range of metalworking and general

engineering processes. For more information and advice concerning a specific

case, Consultants should contact the Technical Helpline.

Metalworking & General Engineering

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