The Hidden Danger in Your Shop: Spontaneous Combustion

For nearly three decades, I worked in Loss Prevention, responsible for safeguarding people and property. While working for one company, I was responsible for multiple sites in Northern California and Northern Nevada. I had investigated my share of incidents, but one call in particular has stayed with me.

It came on a quiet Saturday while I was spending time with family. An alarm company reported multiple fire alarms at a sales office in Sacramento and confirmed the fire department was already en route. I left immediately and drove straight to the site. When I arrived, the scene was chaotic. The building was roughly 10,000 square feet, staffed by about 20 employees, and the fire sprinklers were fully engaged.

The fire itself was contained quickly, but the damage was anything but minor. More than $100,000 in water damage had spread throughout the office. In the back of the building, between the men’s and women’s restrooms, was a small janitorial storage closet. That was the point of origin.

For insurance and risk management purposes, I ordered a forensic fire investigation. When the final report came back, the cause was both surprising and unsettling. The fire had not started from faulty wiring or an open flame. It started because oily rags stored in that janitorial closet spontaneously combusted.

That experience permanently changed how I view everyday shop materials and finishing practices. Spontaneous combustion is real, it is dangerous, and it often comes from items we barely give a second thought.

Most woodworkers think of sharp blades, spinning cutters, and airborne dust as the primary hazards in the shop. One of the most dangerous risks, however, is far quieter and far less obvious. Spontaneous combustion is a real and well documented cause of shop fires, and it most often involves materials that woodworkers use every day.

Understanding how and why spontaneous combustion occurs is the first step toward preventing a devastating loss of your shop, your tools, or your home.

What Is Spontaneous Combustion

Spontaneous combustion occurs when a material generates heat internally faster than that heat can escape. As the temperature rises, the material can eventually reach its ignition point without any spark or flame.

In woodworking, the most common culprits are oily rags, paper towels, steel wool, and sawdust that have been soaked with drying oils. These include linseed oil, tung oil, Danish oil, teak oil, and many oil based wiping finishes.

Drying oils cure through a chemical reaction called oxidation. As oxygen reacts with the oil, heat is released. When that heat is trapped, such as in a crumpled rag or a pile of sawdust, temperatures can rise rapidly.

Why Oily Rags Are So Dangerous

A rag soaked with oil has an enormous surface area. When it is wadded up and tossed on a bench or into a trash can, air can still reach the oil, allowing oxidation to continue. At the same time, the folds of the rag trap heat. This creates a perfect environment for temperatures to climb until ignition occurs.

Many shop fires start hours after the work is done. The finish is applied, the lights are turned off, and the shop is quiet. Meanwhile, a forgotten rag continues to heat until it bursts into flame.

Other Materials at Risk

While oily rags are the most common cause, they are not the only risk.

Sawdust and wood shavings mixed with oil based finishes can ignite in the same way, especially if swept into a pile. Steel wool saturated with oil is particularly dangerous because the fine strands hold heat extremely well. Even paper towels used for wiping on finish should be treated with the same caution as cotton rags.

Safe Handling Practices Every Woodworker Should Follow

Preventing spontaneous combustion is straightforward, but it requires discipline and consistency.

After using any rag, paper towel, or steel wool with a drying oil, never leave it in a pile. Never throw it directly into a trash can.

The safest method is to lay rags flat outdoors on a non combustible surface, such as concrete, and allow them to fully dry and harden. Once the oil has cured completely, the risk is gone and the rags can be disposed of normally.

Another effective approach is to submerge used rags in a metal container filled with water and a small amount of dish soap. A tight fitting metal lid prevents oxygen from reaching the rags and eliminates the oxidation process. Commercial oily waste cans are designed specifically for this purpose and are an excellent investment for any shop.

I live in Cheyenne, the wind doesn’t come and go, it clocks in early, works overtime, and never takes a day off. So leaving oily rags outside isn’t a good idea. I installed an inexpensive retractable clothesline in the shop that I hang rags on to cure before discarding them. This is another safe method for preparing oily rags for disposal.

Finish Storage and General Shop Awareness

Store oil based finishes in their original containers with the lids sealed tightly. Keep them away from heat sources and out of direct sunlight.

Just as important is developing a habit of scanning your shop before you leave. Look for used rags, paper towels, or finishing materials and deal with them immediately. Five extra minutes at the end of the day can prevent a catastrophic fire.

A Risk That Deserves Respect

Spontaneous combustion is not a rare or theoretical danger. It happens every year in professional shops and home garages alike, often with devastating results. The tragedy is that it is almost always preventable.

Woodworking is about patience, care, and respect for the material. Extending that same mindset to shop safety, especially when working with finishes, protects not only your work but everything around it.

Treat oily rags as a serious fire hazard every single time. Your shop, and your peace of mind, depend on it.