In a summer of wildfires severe enough to trigger Spain’s first-ever national emergency for a forest fire, a historic stone structure called the Nevera de Castro stands in a landscape burned bare in the Castellón region. Its walls are blackened by smoke and heat. A local news outlet noted that “a technical inspection will be necessary to know its real state.”

That one sentence names a problem that plays out after every major fire.

When a wildfire reaches historic masonry buildings – stone, brick and block structures that have stood for centuries – the first photographs tell a simple story: Everything is black and looks ruined. But masonry conducts heat so poorly that the damage is often only centimeters deep.

Timber roofs, wood floors and interior framing in historic buildings are extremely vulnerable to fire and can let fire jump building to building, but the load-bearing masonry shell often survives. Preserving even a part of the original structure can dramatically reduce reconstruction costs and retain the historic fabric that gives these places their identity.

But to save these buildings, people have to act quickly.

A masonry wall that has lost its timber roof or floors is now unbraced. Wind alone might bring it down. The stone itself can also start to deteriorate. When limestone is heated to about 1,380 degrees Fahrenheit (750 degrees Celsius), the calcium carbonate, which makes up most of the rock, decomposes, leaving solid calcium oxide and carbon dioxide gas. If rain reaches that surface before it is covered, the calcium oxide can react with the water and expand by as much as 20%, enough to crack the stone from within.

Some types of masonry can start to disintegrate days after a fire if no one has weatherproofed it after the fire.

Why thick walls don’t fail as you might expect

Masonry conducts heat........

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