Mad About Science

Mineral Wool

By Brenden Bobby
Reader Columnist

Humans are very resourceful creatures. We manage to pull fibers from animals to create textiles for clothing, such as wool from sheep. These textiles are great for clothing if you can stand feeling itchy all the time.

Animals aren’t the only source of natural fibers. Did you know we can get fibers from rock, too?

Asbestos is a naturally occurring mineral fiber that has been used for a whole host of things, from brake pads to roofing and insulation. It’s also unfortunately extremely carcinogenic, as tiny microfibers break off the main structure and end up embedded in your lungs, which causes chronic swelling that eventually leads to a cancer known as mesothelioma. 

Asbestos isn’t malicious or evil, it’s just a rock we shouldn’t be using to insulate our homes.

Instead, many manufacturers have been shifting to a different kind of mineral wool made from basalt — volcanic rock formed by lava spilling or erupting to the surface all around the world. It’s likely that you’ve encountered basalt at some point in your life, and it’s just as likely that you are now wondering how a rock can become fiber.

This actually occurs naturally at a number of places around the globe. In Hawaiian legend, it is said that these fibers are strands of hair from the goddess Pele. These strands form when tiny droplets of molten basalt are carried and stretched out by the wind, catching on obstructions like metal antennae or natural structures like cliffs and caves. In vast numbers they begin to appear like matted bird nests but be wary when trying to handle this volcanic wool. The strands are brittle and sharper than newly crafted syringes.

Despite the dangers of handling mineral wool, it’s an incredibly important material for a vast number of applications. Insulation is one of the most important functions of mineral wool, but it has many properties that make it a fantastic building material. Basalt wool is naturally fire retardant, requiring temperatures of more than 1,500 degrees Celsius in order to melt. It won’t catch fire like straw or pine shavings. It’s also extremely hydrophobic, which means it isn’t likely to harbor mold in your walls. It also has great properties for soundproofing structures. Lastly, it’s virtually infinitely recyclable.

All of these great features, why don’t we use rockwool everywhere?

It’s worth noting, that even though it’s considered ecologically friendly due to its high recyclability and nonexistent toxicity compared to asbestos, creating rock wool artificially requires a very large energy input at virtually every stage of the process. It’s far from carbon neutral, but that isn’t necessarily a cause to totally dismiss it, either.

The first step is securing basalt. Large basalt deposits are almost always buried by a large amount of soil that has to be excavated first. This soil can be harvested and resold as topsoil for gardeners, or it can be placed back down where the basalt once stood in order to re-seed a natural space.

Once the basalt has been exposed, engineers will drill a grid of holes into the structure with high powered drills. These drilling rigs can chew through up to five feet of rock per minute. As an avid North Idaho gardener, I can tell you that is a lot faster than what I could ever do with a shovel.

Dynamite is lowered into the grid and the basalt formation is blasted into loose chunks of rocks in seconds. You may think this is where the bulk of the ecological heartache comes from, but you’d be wrong. This part can be pretty easily reversed by backfilling the area after it has been harvested and allowing it to return to nature. Instead, the bulk of energy requirements go into the transportation and processing of the basalt.

The process to create basalt fibers is effectively the same process that goes into creating basalt to begin with. Humans create what is effectively an artificial volcano in the form of a foundry, feeding small chunks of basalt into a huge furnace that pumps molten streams of rock through steel pipes lined with refractory bricks that are extremely heat resistant. These machines maintain the same level of heat for 24 hours a day, seven days a week for one simple reason: It would require a tremendous amount of energy to heat the system from a cooled state, and less energy is lost by simply running it continuously.

This molten rock is dropped onto high-speed wheels. Upon striking the wheels, droplets are flung away from the wheel and caught by a blast of pressurized hot air that stretches and elongates the droplets into strands thinner than a human hair. This blast of air works to rapidly cool the fibers and also transports them onto a huge conveyor that lays the fibers into mats, like the world’s ugliest shag carpet.

This ugly carpet is very thin — just a few centimeters thick — so the machines will place down multiple layers to achieve a desired thickness. Once layered, the sheet is fed through two conveyors with mechanical arms that press all sides of the wool inward to create a densely packed structure. Depending on the product being sold, it can range in thickness from eight to 12 inches.

After compression, the wool is cut by huge high-speed circular saws. Some run parallel to the sheet to cut it into long ribbons, while two saws swing around on perpendicular arms to cut the ribbons into rectangular panels. Once they’ve been cut, they’re sent off to be packaged and distributed to builders.

Stay curious, 7B.