Foundations of Construction: Transforming melted stones into thermal wonders                          

rockwool insulalation
Compressing a slice of #4 rock wool insulation. Photography by Achim Hering, Wikimedia Commons. Retrieved from

By Susanna McLeod

Special to Ontario Construction News

Hard and cold, stones are unyielding. The natural material does not suggest warmth and comfort, and it gives no hint of insulating qualities. Yet when first transformed into a woolly fibre, processed rock had remarkable thermal qualities for use in machinery and pipelines. Attracting a new audience, stone wool is experiencing a revival as insulation in homes and commercial buildings.

Ancient peoples searched for natural materials to efficiently insulate their homes, employing straw and reeds, cork, asbestos, and others. In the Hawaiian Islands, mineral wool was discovered on Kilauea when “the fibers came from volcanic deposits, where escaping steam had broken the molten lava into fluffy fibers” wrote Richard T. Bynum in Insulation Handbook (McGraw Hill, New York 2001). Known as Pele’s Hair, the volcanic glass strands provided insulation over island huts.

A patent for commercial mineral wool was awarded in 1840 to John Perry in Wales, but he gave up on the project when workers were harmed by glass fibres. Thirty years later, a patent was awarded to American John Player for a process of manufacturing slag wool.

“The liquid slag issuing from a tap in the pig-iron furnace is conducted through a runnel formed by coal ashes on iron plates,” describes Appleton’s Annual Cyclopaedia for 1891. The slag reaches “the point where it is to be blown and allowed to fall in a stream about 1 centimetre thick for a distance of 15 centimetres, where it is met by a powerful blast of steam.” The steam “separates it into long filaments, as fine as hair and as white as wool, in which form it drops into the room constructed for its reception.”

At that time when few safety precautions were in place, health issues surfaced. “The handling of mineral wool is attended with some danger to the health, as the fine threads penetrate the skin easily, producing inflammation, and the dust when inhaled irritates the respiratory organs,” Appleton’s said.

Charles Corydon, a chemical engineer in Indiana, developed a process to transform molten rocks into strands; in 1897, he established Crystal Chemical Works to produce stone wool. “It was a very popular in-fill insulation in light frame structures,” said David Bozsaky in “The historical development of thermal insulation materials,” Periodica Polytechnica Architecture, January 2010 (ResearchGate).

Factories opened across North America to produce the fibre, including a plant at Thorold, Ontario. Located on the Welland Canal, Spun Rock Wools, Limited enjoyed low electricity costs to transform the rocks into a viscous state with energy-draining furnaces.

Distant from specific natural resources, many plant operators blended rocks to achieve a suitable admixture. “Rocks that are thus blended are shales, dolomites, sandstones, clays, and calcium limestones,” according to J.M. Cummings in a 1929 report for British Columbia Department of Mines.

However, there were disadvantages. It was better “to use a single rock of uniform composition and definite melting-point, rather than to use mixtures of rocks having diverse melting-points,” Cummings advised. “…trouble is often encountered in cupola-furnace operations, owing to a gradual accumulation of the more infusible rock in the bottom of the cupola.”

Nearly a century later, manufacturing of stone wool is safe for both workers and the environment. Coke-fired or electrically operated furnaces operate at about 1,600° C to melt briquette-sized rocks. Basalt may be combined with binders, bauxite and slag, and then the melted rock is spun at high speed into hair-like threads. The process is reminiscent of cotton candy at the fair.

The rock wool is continuously layered by a large machine arm into a thick batt on a lengthy conveyor belt. Cooling, the wool is compressed and trimmed, and cut to specific industry and customer measurements. The batts are packaged and warehoused, ready to ship.

Environmental considerations are foremost as stone wool plants modernize. Manufacturing practices “incorporate production scrap back into the primary production process,” stated North American Insulation Manufacturers Association. The scrap may also be processed into other products. “Technology, engineering, and process control have played key roles in developing a cleaner, stronger, and easier-to-handle product over the last 20 years.”

Insulating customers from bone-chilling winters and sweltering summer heat, stone wool has low thermal conductivity rates of “between 0.020 and 0.040W/m-K,” according to Thermal Engineering. Low values are achieved by “a large number of gas-filled pockets which prevent large-scale convection.”  Since the insulation is approximately 75% natural rock, it resists growth of bacteria, fungus, and mold.

Differing from fluffy fiberglass batts, stone wool batts are firm and push into place. R-values between fiberglass and stone wool are similar, and both are fire resistant. However, there is a melting point difference—fibreglass resists melting up to 704° C whereas stone wool melts at 1,177° C.

Transformed into stone wool, those hard, cold rocks are thermal wonders… and can keep the neighbour’s noisy garage band music at bay, too.  END  800 words

© Susanna McLeod 2021. McLeod is is a Kingston-based freelance writer who specializes in Canadian History


Compressing a slice of #4 rock wool insulation. Photography by Achim Hering, Wikimedia Commons. Retrieved from


Please enter your comment!
Please enter your name here

I accept the Privacy Policy

This site uses Akismet to reduce spam. Learn how your comment data is processed.