Oily rag safety

Overview

Drying oils harden by reacting with oxygen, and that reaction gives off heat. In a thin film on a board the heat escapes. In a wadded rag or a pile of towels it cannot, so the temperature climbs until the cloth ignites. Northamptonshire Fire and Rescue describes the condition exactly: the fire needs enough air for the reaction but too little air movement for the heat to escape, which is why a folded rag in a box is worse than one hung in the open.

The US Fire Administration's report on the One Meridian Plaza fire (Philadelphia, 23 February 1991; three firefighters died) found that the fire started in a pile of linseed oil-soaked rags left by a contractor refinishing paneling, and called the hazard "long recognized".

When to use

After every session with any of: boiled or raw linseed oil, tung oil, Danish oil, oil-based stain or glaze, oil-based varnish or polyurethane, or any oil-alkyd product. The USDA Wood Handbook (ch. 16) names "oil-based stain, any other oil or oil-alkyd, or urethane finish" as particularly susceptible. Flexner, in the sources I read, says that only drying oils (linseed above all) self-heat and that varnish does not; the Forest Products Laboratory is broader. This guide follows the broader, safer line. Treat tung oil as at risk too: the sources I opened are not clear on how fast it heats, and nobody says it is safe.

Water-based finishes contain no drying oil and none of the sources I opened lists them as a spontaneous-combustion hazard; the manufacturer's SDS is the place to check a specific product.

Materials and tools

• A metal can with a tight-fitting lid (a paint can works), filled with water, for the water method
• Or a clothesline, the rim of a metal bucket, or a bare nonflammable floor away from the building, for the drying method
• Nitrile gloves; Northamptonshire Fire and Rescue says oil on clothes is the same hazard

Steps

1. Do not ball or pile the rags, even for a minute. Put them where you can deal with them straight away.
2. Pick one method.
   • Spread flat. Hang each rag singly over the edge of a metal bucket or a line, or lay them out flat on a nonflammable surface, outdoors or well away from the building, until they are dry and stiff (Flexner; Northamptonshire Fire and Rescue). Then bin them.
   • Drown and seal. Immerse them in water and seal them in a water-filled airtight metal container immediately after use (Wood Handbook). Tried & True's FAQ gives a similar procedure with a bucket or jar of water, then a sealed bag. Northamptonshire Fire and Rescue adds a sealed plastic bag for the bin.
3. Do not treat water as the last step. Flexner warns that when rags come out of the water, the oil has not dried and they can still heat up, so spread them out then.
4. Keep it out of the building. Northamptonshire Fire and Rescue says not to store oily cloths indoors and notes that heat sources such as radiators make it worse.
5. In a workshop, keep a dedicated covered metal can for oily waste and empty it daily. OSHA 1910.106 requires employers to keep combustible waste in covered metal receptacles and dispose of it daily; it does not bind a home shop but is a sound rule.

Common problems

• "I used only a little oil." I did not find any source that gives a safe amount, so assume there is none.
• A rag left overnight in a pile. Spread it out now, outdoors, and keep watching it. If it is warm or smoking, soak it in water immediately and call the fire service if it is alight.
• Sanding dust and shavings from oil finishes. The sources I opened address rags and cloths, not dust. Sweep it up and dispose of it with the same care.
• A laundry machine. None of the sources I opened endorses washing or machine-drying oily rags. Do not.

Safety and environment

Small-quantity oil and solvent waste is covered in Finishing safety and disposal. Solvent-soaked rags (mineral spirits, lacquer thinner) are a flammable-vapour hazard rather than a self-heating one; keep them in the same metal can and away from ignition sources, and see Finishing shop ventilation and PPE.

Related

• Boiled and raw linseed oil, Tung oil, Danish oil, Teak oil, Oil-based pigment stain: the finishes this applies to
• Oil-based polyurethane, Wipe-on polyurethane, Oil-based enamel paint: oil-varnish products on the same list
• Wipe-on application: the rag technique that produces the most oily rags
• Polymerized linseed oil (stand oil): a pre-reacted oil; check the maker's SDS before assuming its rags are safe
• Finishing safety and disposal, Finishing shop ventilation and PPE