Ed. Note: The following is a condensed excerpt from the book Minneapolis: Murder & Mayhem (History PR) by Marie Wu (M.A. ’12, M.A. ’15). Writing here under the pen name Ron de Beaulieu, Wu is also a doctoral candidate in the Department of Sociology. This section details the key role U of M researchers played in explaining how the historic Minneapolis Mill District exploded, killing 18.

Thunder cracked over the city of Minneapolis at 7:20 p.m. on May 2, 1878. Or at least that’s what the residents and workers farther from the falls would have thought for a split second. But then, for a half-mile around, Minneapolitans’ windows shattered. As far away as St. Paul, houses shook. 

Thousands of Minneapolitans and St. Paulites ran out of their homes. The St. Paulites, fearing that it was an earthquake, made for the St. Anthony Falls. The streets teemed with terrified men, women, and children. 

Earlier that evening, at 6 p.m., the day shift workers began to clock out at the Mill Complex at 700–709 South First and Second Streets, in the Mill District of Minneapolis. Among them were all of the shipping and receiving clerks, maintenance workers and packers for transport, as well as the day shift millers. The night shift consisted of skeleton crews. 

At the largest mill in the complex, the Washburn A, 14 millers would keep the grist ground overnight. They finished clocking in by about 7:12 p.m. Eight minutes after the shift change, a column of smoke, laced with flames, shot up hundreds of feet in the air over the Washburn A. 

That was the first shock. At the second shock, the roof shuddered upward and then fell. As it went, it took out the floors below it, and the walls in turn crashed down. The Diamond and the Humboldt Mills, to the west and the south, respectively, went down as the walls of the Washburn A collapsed on them. The A went up in flames, cremating the entire night shift. 

The Pettit, Zenith, and Galaxy Mills, farther away on the other side of the canal, burst into flames within minutes. The wooden elevator near the A caught on fire and then became fire, a tower of kindling. Its iron roof fractured, and a fragment flew two miles away. 

Within five minutes, Winslow M. Brackett, chief engineer of the volunteer firefighters, arrived at the Mill District. The tongues of flame threatened to devour the whole district, but Brackett and his men stood in its way. There was no salvaging the Washburn A. The firefighters focused their attention on the Washburn B mill, which had caught fire immediately after the A explosion. 

At last, on the afternoon of Sunday, May 5, the firefighters' work was done. The explosion knocked out an entire third of Minneapolis’s milling capacity. In addition to the mills themselves, the fire, shocks, and falling debris destroyed lumberyards, a railroad roundhouse, and a machine shop. Many Mill City residents lost their homes. 

The Washburn A, the colossus, the biggest flour mill in the world at that time, had been reduced to a “chaotic pile of huge limestone rocks … interwoven with slivered timbers, shafts and broken machinery from which [poured] forth steam and water,” according to a newspaper at the time. The explosion and its aftereffects leveled the Humboldt and Diamond Mills. Three other mills were skeletons. In addition to the 14 A millers, four nearby workers died. 

The mills sought to collect on their fire insurance policies. The estimated financial loss tallied more than $1 million in 1878 figures, which translates to $27 million in today’s currency. The insurers didn’t cough it up on the grounds that their policies were for fire, not explosions with chemical origins. 

Hennepin County empaneled a coroner’s jury. On May 20, it convened in the Agricultural College Building of the University of Minnesota. Two scholars from the University appeared before the jury. Louis W. Peck, a physics instructor, and chemistry professor Stephen F. Peckham performed an experiment for the jurors. They held, one after another, nine products of a flour mill over a Bunsen burner. All caught fire but fizzled in less than five seconds. 

This was the first of a series of demonstrations. In another, an uncovered lighted lamp was placed in a strong wooden box, over which a heavy, loose cover was placed. Two men stood on the cover. A small charge of any one of the dusts, except coarse bran [which, prior demonstrations had shown, was less responsive to heat and flame], blown in by a bellows, would take fire from the lamp, explode, lift the cover with the two men, and spread a sheet of flame several feet in all directions. 

Peck explained to the jurors that dust from flour milling can, in the presence of flame, explode like gunpowder. Peckham added that a 98-pound flour sack, in combination with 4,000 cubic feet of air force, could create an explosion that could blow a 2,500-ton weight 100 feet high. 

What did this mean for the Washburn A? Peckham drew the jurors’ attention to the structure of the mill. 

On the east side, six runs of millstones reground middlings. Middlings, commonly called “farina,” are a byproduct of flour production. (If you’ve ever eaten Cream of Wheat or Malt-O-Meal, you’ve had middlings.) 

The meal from these runs emptied onto a single conveyor, which deposited it in an elevator, which in turn brought it upstairs. There, at the conveyor’s terminus, a spout with a revolving fan bore off dust and the hot air from the friction of grinding and swept it along to a small room by the end of the spout for collecting flour dust, called a “dust house,” on the side of the mill. 

All of that was standard operating procedure. The problem, said Peckham, began at the stones. Either they ran low from insufficient middlings to grind, or the grain had been contaminated with a small scrap of metal or gravel. Friction between the surface of the stones and another hard surface generated sparks. Those sparks were carried along the conveyor, up the elevator, through the fan and into the spout. Here, the sparks ignited the dust particles in the dust house. No one had ever cleaned it out. The dust had been accumulating for years, and it was dense and flammable. 

That was the first explosion. It shook the building and dislodged more dust. Then came the explosion heard ’round the cities, throwing the roof up and pushing out the walls. The A mill flames jumped 25 feet over to the Diamond Mill’s windows. 

Having just felt the shock, the Diamond’s dust had already loosened when the ignition hit, and the air was thick with flammable material. The Diamond gave the Humboldt the same treatment. Three mills caught fire within a matter of seconds. (It had taken observation of how the walls of each mill overlapped one another to prove that they had fallen successively. ) 

The jurors took more than two weeks in their deliberations, delivering their verdict on May 22. They commended Peckham’s work and unanimously agreed that his interpretation of events was correct. 

All insurance companies that could afford to pay up, paid up, but the mill owners bore the brunt of the loss—to say nothing of the survivors of the 18 dead. 

At least the mill operators had learned. They now kept the mills cleared of dust. Safety standards are written in blood, and “[u]ntil the explosion occurred, the latent possibilities of flour dust in this connection were unsuspected.” 

Another good thing to come out of the tragedy was that the local volunteer firefighters realized that this was a job for professionals. 

In 1879, a year after the mill explosion, the volunteers requested that the municipal government establish a fire department, funded by the city, with paid firefighters. The Minneapolis Fire Department was born, and Winslow Brackett remained the chief engineer. 

And for years to come, the men of MFD would fear another explosion every time an alarm sounded in the Mill District.