Can mirror-finish, outdoor stonework be 100 years old?
November 8, 2017 2:08 PM   Subscribe

Can mirror-finish, outdoor stonework be 100 years old?

A friend and I were examining a stone surface on the outside of a building in the northeastern United States. I would expect that it was made of granite because the color and pattern were similar to that of granite curbstones.

It was polished to a perfectly-flat, mirror-like surface and so when viewed from the correct angle, you could see an undistorted reflected image. Some parts of it were vertical and some facets were at a 45° angle, exposed to the sky without anything sheltering it.

The building itself dates to around the first World War and my friend proposed that the stonework we were examining would be of the same age. I disagreed and said I thought that a perfect mirror finish of that sort would be a sign that it's a much more recent modification; and that anyhow, even if mass-produced decorative stone finished that way had been available a hundred years ago, at the very least the angled pieces would no longer be glossy and reflective after a hundred years of weathering and exposure.

So, I guess I'm looking for sources and informed opinions on 1) whether the processes used for cutting flat stone surfaces en masse and polishing them a century ago in the United States would produce a glossy, perfectly flat surface like this, and if not 2) even if a construction like this was made with gem-cutting tools or some other unusually expensive process, would granite or any other type of dimension stone maintain such a finish after a hundred years of outdoor weathering?
posted by XMLicious to Science & Nature (8 answers total) 1 user marked this as a favorite
Yes, Vitrolite can. It's a highly polished, pigmented hard glass.
posted by scruss at 2:39 PM on November 8, 2017 [4 favorites]

Hi I'm a heritage researcher with a bit of knowledge about fabric.

'Mass produced' is a bit of a confusing way to put it; stonemasonry and stone polishing techniques to make large highly polished flat surfaces are very old. Marble floors are a common example. It's done with progressively finer abrasive material; it was a typical prison industry in the 19thC and was being partially mechanised at the turn of the century. To answer the question of can mirror-image stone surfaces be more than a hundred years old: absolutely yes.

Weathering of stone and other outdoor material can be difficult to judge. Textured surfaces like brick, slate, sandstone, cement, wear and show patina much more than smooth, highly polished ones like granite and glass; particularly where there's been regular cleaning and the surface hasn't been subject to abrasion (being walked on, or handled, typically) or chemical processes like acid rain (which is extremely bad for some stones like limestone, not so much for granite). Old cemeteries are full of such vertical surfaces. Can a polished surface last in good condition for a century? With a bit of care, yes.

To get a better answer about the age of the specific stone you'd really have to make a more detailed survey---is some of the stone more worn than other parts, indicating replacement? Is it mortared with modern cements or older lime cement?---and go to documentary sources about the specific building, and look for e.g. old plans, photographs, maintenance schedules.
posted by Fiasco da Gama at 3:40 PM on November 8, 2017 [9 favorites]

Granodiorite in the Sierra Nevada polished by ice 10,000 years ago is still pretty smooth, so I think 100 years really isn't that long in the life of a rock. I've also seen hard metamorphic rock (Eclogite) that was possibly polished by mammoths using it as a scratching post more than 10,000 years ago that was still shiny.
posted by Long Way To Go at 7:49 PM on November 8, 2017 [1 favorite]

My understanding, which perhaps isn't correct, is that it's difficult to produce an optically-perfect mirror via hand polishing; so that during most of human history, polished metal mirrors (purpose-made ones, for people to look at themselves in) have usually shown an image that's slightly distorted.

So I was probably unclear in the OP, in that these stone surfaces appeared to me to be about as flat as a mirror made with modern float glass, which Wikipedia claims was developed in the 1950s, and hence did not distort the reflected image to any degree I could perceive.
posted by XMLicious at 8:02 PM on November 8, 2017

Are you familiar with a pantograph (an arrangement of rods to transfer/enlarge an image)? They would have used a similar sort of rig (with a grindstone or polishing cloth instead of a pen) to achieve a perfectly flat surface. The rig would be held level with a wooden (or in modern times steel) frame clamped to the surface to be polished. This kind of set-up has been used for literally thousands of years.
posted by sexyrobot at 10:53 PM on November 8, 2017 [1 favorite]

I'm not totally clear why people are responding as though I've suggested it was impossible to highly polish anything a hundred years ago or at some other point in the past; in the OP I specifically propose that at the very least, the same tools and techniques used for cutting gemstones could obtain the type of surface I'm talking about, and I'm not saying that the practice of cutting gemstones isn't thousands of years old.

But there was some sort of resource constraint which resulted in at least some manufactured and worked surfaces not being polished to an optically-perfect flatness, at some point in the past.

None of the monuments made of granite from the end of the nineteenth century or the beginning of the twentieth century which I've visited personally—for example the Bunker Hill monument in Boston, the Soldiers and Sailors Monument on Boston Common, or the Eternal Light Flagstaff in Madison Square Park in NYC—even have glossy polished finishes on their stone components, much less an optically-perfect mirror-like finish on flat surfaces as does the single replacement pane of float glass out of sixteen in the photo I linked to above.

But in contrast the black granite Space Mirror Memorial at the Kennedy Space Center, constructed in 1991, produces a fairly good image in its reflection, at least better than the non-replacement window panes in that photo.

So my reluctance to accept the "Oh yeah, no problem, it was possible thousands of years ago!" assurances is partly due to the fact that, if true, it should be fairly easy for someone to corroborate by mentioning as an example an outdoor public monument finished this way with a known construction date and verifiable history, from a century or more ago. If someone has an example like that to accompany details such as the pantograph-type mechanisms and Fiasco da Gama's research expertise, even an example from outside North America where I've seen most granite stonework I've taken a close look at, it might set my mind at ease?
posted by XMLicious at 10:44 AM on November 9, 2017

Hmmm now this is getting interesting! The kind of polishing I had in mind was of the kind that you can see on, for example, Queen Victoria's pedestal (1888) or Lenin's mausoleum (1930). They're reflective and, particularly in Lenin's case, exposed to weather/smog equivalent or worse to that of NE America (though they've had had a life of exceptional care and maintenance). This kind of polished stone is fairly common on outdoor headstones and plaques (like on this War Memorial near where I grew up), but that doesn't seem to be what you're describing, which is mirror-like rather than just reflectively glossy? I'd suggest some other potential scenarios for this:

- the façade is a gloss ceramic or other non-stone material (like scruss suggested) made to look like a mineral surface, or
- the stone surface is original, but has been been since polished.

In cases like these, if plans aren't easily accessible to establish fabric, I go and pick the brains of local historical societies at local libraries---they often have amazing documentary and photographic collections. It's probably the only way to establish it for sure.
posted by Fiasco da Gama at 4:03 PM on November 9, 2017

Thank you for your persistence, Fiasco da Gama, that's exactly the thing! In this Wikipedia photo of the Lenin Mausoleum you can see the reflection of what appears to be the GUM department store, based on this panorama. And per this diagram, the reflective red surfaces are going to be either granite or porphyry.

I think I must have at some point incorrectly inferred that the reflective perfectly-flat or near-perfectly-flat finish on large pieces of hard stone was a result of newer technology because in the cemeteries near to where I grew up only the more recent gravestones looked like that. But, it was probably just the result of one or more local gravestone-making companies or suppliers not buying the appropriate polishing machine until that point.

Thank you to everyone else who answered too, each of these bits of information was also quite interesting.
posted by XMLicious at 5:34 PM on November 9, 2017

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