Fire Safety

Decide before you reach for anything

An extinguisher is for a small fire you caught early.

Fight it only if all four are true

• The fire is small and contained — a bin, a rag, a puddle. Not spreading.
• You know what is burning, and you have the right extinguisher for it.
• You have a clear exit behind you and the fire is not between you and it.
• Someone else has already raised the alarm.

Metal fires, and what it takes to start one

Metal burns only when it is divided finely enough, and then only when several conditions line up at once. Ordinary aluminum chips and swarf will not do this, and neither will the quantities of fine material a lab space like this one generates. Solid stock does not burn at all. The cases below are the ones that are genuinely different.

The form matters more than the metal

• Magnesium is the real Class D case. Thin sections, chips and turnings ignite readily and burn with intense heat and a brilliant white light, and dull tools alone can produce enough heat during machining to start it.
• Titanium turnings can ignite, and titanium fines and grinding dust are worse — fine enough that a static discharge is a credible ignition source.
• Aluminum fines — the dust off sanding, grinding, buffing and polishing — are a genuine hazard, but only through the specific mechanisms below.
• Aluminum chips and swarf are hard to ignite. Aluminum carries heat away from a hot spot fast, it wears a tough oxide skin that holds together while the metal underneath melts, and a chip has little surface area for its mass. Put a torch to a pile of chips and they melt.
• Solid stock does not burn. A block of aluminum is not fuel.

Size is the usual line — shape is the exception

The line in the standard is about half a millimeter, measured across a particle’s smallest dimension — which different editions write as 500 microns or as 420 microns (fine enough to pass a 40-mesh sieve), not two separate limits. Below it, metal dust can carry an explosion. Above it, generally it cannot — and generally is the word the standard uses, not never.

Flakes are the exception, and buffing and polishing make flakes. A flake is wide but thin, so it is too big for the sieve and still burns like dust. Treat anything off a buffing or polishing wheel as fines however big it looks. Machining chips are genuinely coarse — that is the difference between a chip tray and a dust collector.

The ways fine metal actually ignites

• A suspended dust cloud meeting an ignition source. Fines sitting in a tray are not the hazard. Fines thrown into the air, at the right concentration, reaching a flame, a torch, a spark or a static discharge, are — burning travels from one particle to the next with extreme speed. It is why you never blow fine dust around, and why you do not raise a cloud of it cleaning up.
• Fines heavily mixed with rust or grinding scale. Aluminum plus iron oxide — rust, or the scale off steel grinding — is a thermite reaction: the aluminum strips the oxygen off the iron, so the mixture burns without needing air from outside. That is what makes it a metal fire rather than an ordinary one, and why water and an ABC extinguisher do nothing to it. It takes a real mixture to go — something near three parts iron oxide to one part aluminum by weight, and an ignition source far hotter than a spark. That is a rust-caked pile, not a trace. See Metal Chips and Swarf for how far apart that threshold and the recycling one actually sit.
• Wet fines. Aluminum fines in contact with water slowly give off hydrogen. That is a reason not to seal damp fines into a closed container, and part of the reason water is the wrong thing to put on a metal fire.

If fine metal does ignite

Why dry sand is on the list, and why it is probably not on yours

Sand does work. The Aluminum Association’s Guidelines for Handling Aluminum Fines (F-1) lists fine dry sand, “preferably finer than 20 mesh,” next to approved dry powders as effective for isolating an aluminum fines fire. It also says what has to be true first.

• Dry. F-1 says dry sand. Damp sand is water, and water “will react with the aluminum fines to form flammable hydrogen gas.”
• Already in the room. “A plentiful supply should be easily available in the operating area and stored in covered bins or drums.” Sand fetched after the fire starts is not that.
• With the right tool waiting beside it. “Long handled shovels should be available at each bin or drum, and should be constructed of non-sparking metal.”
• Laid, not thrown. The shovels “lay the sand or powder around the edge or perimeter of the fire.” You are walling it in. Anything thrown at the pile raises the dust cloud that is the actual explosion.
• Decided before it started, by someone senior. “Qualified supervisory personnel should decide in advance whether it is better to attempt to fight the fire or to withdraw to a safe location and allow it to burn itself out.”

Those are preconditions, not tips. If you cannot point at the covered sand bin and the non-sparking shovel beside it right now, you do not have this option — and the call to fight it was never yours to make on the spot. Alarm, evacuate, report.

What that means day to day

• Do not let fine dust accumulate. Empty trays, vacuums and collectors rather than letting them fill — see Machine Hazards and Housekeeping for housekeeping.
• Chips and fines are disposed of differently, and the difference matters — see Metal Chips and Swarf.
• Let hot chips and debris cool before they go anywhere near a bin of paper or rag.
• Ask before you machine an unfamiliar metal. If you do not know what it is, that is the answer: ask.