Aluminium is the most abundant metal in the earth’s crust. It is also, for a brief period in the nineteenth century, one of the most expensive substances on earth.

The distance between those two facts is one of the stranger stories in industrial history, and it ends with the material in almost every modern window.

More Valuable Than Gold

Aluminium is never found in metallic form in nature. It is locked into compounds, and separating it out defeated chemists for decades.

The small quantities that could be produced were treated as precious. Napoleon III is said to have reserved aluminium cutlery for his most honoured guests, seating everyone else with gold. When the Washington Monument was capped in 1884, the builders chose an aluminium apex  at the time, a metal comparable in price to silver.

The Discovery That Collapsed the Price

In 1886, Charles Martin Hall in the United States and Paul Héroult in France independently arrived at the same electrolytic process for extracting aluminium at scale.

The effect was immediate and dramatic. A metal that had been a curiosity for emperors became an industrial commodity within a generation. The Hall-Héroult process remains the basis of aluminium production today.

From Aircraft to Architecture

Aluminium’s first great application was aviation, where its strength-to-weight ratio was transformative.

Construction followed, and the reasons transferred directly:

•     It is strong for its weight, allowing slim frames to carry heavy glass without bulk.
•     It can be extruded into complex profiles, so drainage channels, gasket races and hardware seats form part of the section itself.
•     It does not rust, forming a protective oxide layer instead of corroding through.
•     It holds its dimensions, neither swelling with humidity nor shrinking with heat.
•     It takes finishes exceptionally well, through powder coating and anodising.
•     It is almost endlessly recyclable, losing none of its properties in the process, which is why a large share of the aluminium in use today has been through a previous life.

The Problem That Nearly Stopped It

Aluminium had one serious weakness as a window material. Being a metal, it conducts heat readily, carrying outdoor temperature straight through the frame.

The solution was the thermal break  a non-conductive polyamide strip built into the profile, separating the inner and outer halves so heat cannot cross. Its adoption from the 1970s onward removed the last real argument against aluminium, and modern thermally broken systems now compete directly with any material on thermal performance.

It is a small component to have settled such a large question, but it is the reason aluminium held its position rather than losing it to newer materials.

Final Thought

Aluminium did not become the default window material because it was fashionable. It became the default because it solved more problems at once than anything else available.

There is a pleasing symmetry in the fact that a metal once too precious for anything but ceremony is now the quiet, unremarkable frame around the view from your living room.