Dear Evil Engineer: Could I build a solid gold castle?

Dear Evil Engineer,

After a long and lucrative career in real estate, I am ready to put my wealth to good use. I would like to build myself a castle from gold, à la El Dorado. When I say gold, I should specify that I mean solid gold, not gold leaf. I didn’t spend 50 years exploiting my workers, squashing my competition beneath falling pianos, fibbing to regulators, and siphoning off my employees’ pension to live out my retirement like a poseur in a shiny concrete chateau.

I am conveniently very rich, so money is no object. However, there may be technical impediments that I should take into account. To what extent will the tedious laws of nature force me to compromise on my dream retirement project?

Yours,

No relation to other noteworthy gold-loving villains

Dear villain,

Nothing says good taste quite like a gold house.

There are many famous gold structures – from the Dome of the Rock to the Padmanabhaswamy Temple – although they are, of course, not made from solid gold. From this we can conclude that, even when one has seemingly infinite resources at one’s disposal, building with gold was concluded long ago to be a Bad Idea.

We all know that there is not much gold in the world. It’s gold’s thing. How much is not much? A widely cited statement by Warren Buffett is that all the mined gold in the world could fit into a cube measuring 20m on each side. The US Geological Survey, which includes the more than 50,000 tonnes in underground reserves, suggests that the cube could be expanded to measure 28m on each side. Estimates range from as low as 150,000 tonnes (Gold Money) to 2.5 million tonnes (the Gold Standard Institute). Why do these estimates vary so wildly? It is very difficult to estimate how much gold was mined over thousands of years of history – How much was reused? What was its purity? – and it is moderately difficult to estimate current levels of gold extraction, with some countries reluctant to disclose figures and considerable illegal mining activity slipping under the radar.

Even at the most conservative estimates, however, there is enough gold for your retirement project. For instance, the walls of a cuboid keep measuring 50m tall, 20m on each side, with walls 2m thick would use just 3,800 cubic metres of building material, which works out at just over 70,000 tonnes of gold. So, if you can gather up a significant fraction of all the gold that has ever been mined, you would have enough to make a modestly sized castle.

The problem that you cannot buy your way out of, however, is that gold is a terrible building material. We cannot say precisely how gold would behave if used on a construction scale in the real world, although from its properties – the opposite of those that make a good building material, such as lightness, strength, and durability – we can say that it would not go well at all.

Gold is very dense. We tend not to notice this, as it is rare to handle significant quantities of gold. But it is denser than lead (19.3 tonnes per cubic metre, vs. 11.3 tonnes per cubic metre) and much, much denser than common construction materials like concrete (2.4 tonnes per cubic metre) or steel (7.9 tonnes per cubic metre). A building made from such a dense material would be prone to sinking into its foundations, warping out of shape, and generally collapsing under its own colossal weight. Gold is very soft, making it a poor choice for providing the support necessary for holding up such a heavy structure. You would certainly need to add structural elements made from lighter, stronger materials to prevent your entire castle falling down. It also has a relatively low melting point of just above 1,000°C: low enough to cause concern when you light a big crackling fire in your banqueting hall.

Of course, there is also its worth to consider. Even lead roofing was a popular target for thieves. El Dorado avoided being stripped bare like the Happy Prince on account of its non-existence, but that didn’t stop the likes of Raleigh giving it their best shot.

The good news is that there are plenty of uses for gold in buildings; just not as a replacement for concrete and steel. It is beautiful, practically immune to corrosion, and highly reflective of heat and light (gold reflects 99 per cent of infrared light). This is why thin gold coatings are applied to the visors of astronauts’ helmets and spacecraft mirrors – think of the famous gold hexagonal mirrors of the James Webb Space Telescope! Today, gold nanoparticle coatings are sometimes applied to windows to keep interiors cool in summer and reduce heat loss in winter. For instance, the gleaming Royal Bank Plaza building in Toronto used 70kg of gold to coat its thousands of windows.

So, could you construct a castle from solid gold? Possibly. Should you construct a castle from solid gold? Absolutely not. The extreme density and softness of the material would severely limit the size and shape of your castle. There are few worse building materials out there; you may as well build a castle from marshmallows.

I would suggest following the example of the grandiose men (and women, but mostly men) that came before you: build a castle from practical materials, then cover it with gold leaf. There is a good reason why solid gold castles do not exist.

Yours,

The Evil Engineer