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Nov 29th 2021 / General Stone Care

Hot Stuff: High Heat Can Damage Natural Stone

Many types of natural stone form in hot environments. Granite comes from molten magma that’s 1,000 to 2,000 degrees Fahrenheit, and marble is the beautiful result of limestone compressed and heated to somewhere between 600 and 1,500 degrees.

man holding melted metal

Clearly, that must mean these stones can withstand heat, right? A hot casserole pan is no match for a subterranean magma chamber, after all. But unfortunately, this is one of those cases where one’s intuition turns out to be false. Although granite starts to melt around 1,100 degrees, it can be damaged at temperatures much lower than that. Some stones can start to show mild effects of heat damage when exposed to temperatures around 400 degrees for an hour or more.

There are four ways heat can damage even the toughest granites: The minerals can undergo physical changes, the stone can become more porous, the sealer can react to the heat, and the slab can crack. We’ll set aside the last one for another time and focus on the first three.

Destroying Stone in The Name Of Science

Geologists love rocks, but that doesn’t stop us from subjecting them to all manner of terrible conditions to see what happens.

Geologist testing granite

As a result, there’s an entire body of research devoted to the ways that heat can damage granite, marble, and other stones. Much of this work is in reference to buildings and monuments damaged by fire, but the experiments are totally relevant to less-disastrous events too.

A key takeaway from the research is this: “Granite submitted to high temperatures may lead to the loss of aesthetic values even before structural damage is caused.” (Vazquez et al., 2016)

In other words, just because the stone didn’t get hot enough to melt or crack doesn’t mean it won’t start to change appearance.

One particularly useful study took 13 types of sawn, unpolished, unsealed commercial granites, put them in a furnace at increasingly hot temperatures, then measured the effects. An important difference between experiments on rocks and a real-world kitchen “incident” is that stones were exposed to high heat for an hour or more in the experiments. In the kitchen, it’s likely that the heat exposure would be only a matter of minutes.

But on the flip side, the experiments gradually raised the stone’s temperature, whereas, in the kitchen, the stone could be heated rapidly, which can be more damaging. Nevertheless, the experiments offer useful insight into what can happen to various stones when heated.

Heat Can Make Stone Redder, Yellower, Lighter, or Darker

Many types of stone change color when exposed to high heat. In experiments, several of the stone samples started to show color changes after spending 3 hours at temperatures as low as 400 degrees, but the effects were subtle. At temperatures hotter than that, changes in color became increasingly apparent.

The most common change was a shift toward a redder overall color. The fancy name for this is ‘rubefaction,’ which is just a highbrow way of saying something turns red. The reddening is caused by the oxidation of iron within the minerals. In some cases, green mica (chlorite) turned yellow, and some dark, mica-rich granites also turned yellow.

In experiments on four types of labradorite, a black granite, three of the sample stones started to lighten when exposed to heat. In one sample, the heat made the stone darker. In either case, the effects from exposure to 400 degrees were barely noticeable. But by 500 degrees or more, changes were more pronounced.

Fire damage on limestone

Fire damage on limestone