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This is something I've been thinking about for a while.

Say a block of marble is used to sculpt a statue. Most of the stone is chipped away and is effectively useless. Instead of it being tossed, can it perhaps be melted back down into bricks?

I ask this, because it would probably require a lot of energy and heat. I'm also not sure if the melting and cooling process will change the material, such as making it more brittle.

Edit: To clarify, I don't mean marble specifically. I want to know what it would generally be required to melt down the stone, if the cooling process will affect it and if it would generally be practical to do this

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    $\begingroup$ Are you asking if one can melt stone and cool it again, are you asking about marble specifically, are you asking if this makes sense economically, are you asking if this is good for the environment, are you asking how some kinds of rocks are created geologically? I can think of a dozen more interpretations for your question, maybe you should be more specific $\endgroup$
    – Raditz_35
    Jun 30, 2017 at 8:48
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    $\begingroup$ Marble, being Calcium Carbonate, is the one example that would NOT work.. $\endgroup$
    – Andrew Dodds
    Jun 30, 2017 at 9:36
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    $\begingroup$ Rocks chips are useful too. And there is no economic reason to recycle rock -- after all, Earth is a huge piece of rock... On the other hand, rock recycling is exactly what the rock cycle does; it just takes a very long time. $\endgroup$
    – AlexP
    Jun 30, 2017 at 9:50
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    $\begingroup$ @AlexP Glass is made of a material that is abundant in the Earth's crust; yet we recycle it. $\endgroup$
    – Kaz
    Jun 30, 2017 at 15:16
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    $\begingroup$ @Kaz: "Made from" != "is". Table salt is made from chlorine (a poisonous gas) and sodium (a metal which reacts violently with water). To make glass from sand we spend huge amounts of energy; it makes perfect sense to avoid doing that over and over when we can reuse the glass. $\endgroup$
    – AlexP
    Jun 30, 2017 at 16:00

7 Answers 7

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It depends on your rock.

Rocks like Granite, with large crystal sizes, are the result of VERY slow cooling and crystallization. So although in theory you could remelt and recrystallize this kind of rock, you'd probably need hundreds or thousands of years to do it.

Basalt, a fine grained igneous rock, would be fine. It would still need quite a long cool down time.

Obsidian and volcanic glass would be very easy - by definition, this cools quickly in an eruption. No problem recycling, apart from the heat needed.

Now the problems..

Sandstone (and other sedimentary rocks) - you could not melt these and re-form them, obviously. You could grind them down to sand grains, THEN try to press them back together with the appropriate cement (silica or carbonate, depending on the original rock). This would take pressure and quite a bit of time.

Slate Now, not only would you have to grind this down, but also slightly recrystallize this under pressure at a few hundred degrees, with more pressure in the direction normal to the cleavage. For a long time.

Marble You can't melt marble at surface pressures, it decomposes into calcium oxide and CO2. If you had a very high pressure crucible and a means of heating it, you could melt marble and re-crystallize it.

Blueschist This is getting a bit hard. You need a pressure equivalent to about 20km of rock, and a temperature about 400 degrees C.

Eclogite A type of very high grade metamorphic rock. 45km depth and c. 700 degrees C. For years, to get the crystal size.

So... unless you specifically want volcanic glasses to work with, it would probably be a lot easier to just buy some more. Rocks take a long time to form, and usually under conditions of heat and pressure that are not cheap to reproduce.

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    $\begingroup$ Great answer. You should point out the general difference between igneous rocks (for which melting would work by definition, though as you mention the cooling times vary) and other types of rock. $\endgroup$
    – kingledion
    Jun 30, 2017 at 11:14
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    $\begingroup$ Could you add an estimate for how long "a long time" is? Currently I don't know if that is months and therefore not viable commercially or we are talking centuries where we will most likely not live to see the result. $\endgroup$
    – nwp
    Jun 30, 2017 at 11:53
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    $\begingroup$ @nwp: Given that we're not exactly running out of rocks any time soon, even one hour would not be commercially viable. Marble would be the chief exception, and that's not really a rock anyway. $\endgroup$
    – MSalters
    Jun 30, 2017 at 14:14
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    $\begingroup$ For a lot of stones, these processes might produce some that are similar in composition and physical qualities, but not in the looks. Especially for marble, the bands of impurity make it so nice looking, it would need quite an extra step of effort to put these in. $\endgroup$
    – PlasmaHH
    Jun 30, 2017 at 14:25
  • $\begingroup$ @nwp - It depends, mainly on the crystal size, hence the vagueness. The biggest crystals can take years thousands of years to form, based on how long it takes granites to cool down. $\endgroup$
    – Andrew Dodds
    Jun 30, 2017 at 15:17
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An opportunity here to link up my all time favorite episode of How It's Made: Stone Wool Insulation. It is exactly melted and recycled rock, being done commercially. YouTube thumbnail

The idea was inspired by "Pele's hair" which is a real thing found in Hawaii: molten basalt whipped into thin hairlike strands. In the video they show the making of artificial lava from crushed basalt (and slag), which is then whipped into wool and made into mats. Great stuff.

@Yagos notwithstanding, it does not look like wildly advanced tech to me: melt stone, whip into wool, trim and press, use as fireproof insulation. This is not some research project either: folks are making this stuff and want you to buy it.

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  • $\begingroup$ That's really cool. I had no idea we were even doing this sort of thing (possible, yes, actually doing it, no). $\endgroup$
    – Draco18s no longer trusts SE
    Jun 30, 2017 at 15:44
  • $\begingroup$ "folks are making this stuff and want you to buy it." I have seen this insulation advertised. I want to buy it too. Just need to find the time to install it. $\endgroup$
    – Mike Vonn
    Jun 30, 2017 at 17:43
  • $\begingroup$ How is this significantly different from standart fiberglass insulation? $\endgroup$
    – jamesqf
    Jul 1, 2017 at 17:30
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    $\begingroup$ @jamesqf I assume it would be useful in the event of a fire. Keeping the heat contained to a single room (if the ceiling is insulated) would give more time for emergency services to respond and tackle a blaze. $\endgroup$
    – SGR
    Jul 3, 2017 at 10:55
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    $\begingroup$ @jamesqf - it's a slightly more effective insulator (typically 0.038 W/mK versus 0.040 W/mK) but slightly more expensive. Of course neither of them compares with foamed plastic products (e.g. polyisocyanurate, which is usually about 0.022 W/mK if I recall correctly), but they're much cheaper than they are. It also has non-insulation applications, because it is capable of absorbing large quantities of water and holding it for a long time. Some large commercial greenhouses grow plants in it, for example. $\endgroup$
    – Jules
    Jul 4, 2017 at 9:16
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"Stone" isn't really a thing, in the way think of it. It's not a simple substance like plastic, glass, or metals that can be melted and molded.

The properties that make a stone come from a complex interplay of many factors. Stones made from exactly the same elements and molecules can behave in radically different ways depending on how quickly or slowly they cool.

Marble, specifically, is composed of calcium carbonate crystals. Normally it has started out as limestone, produced by biological processes, which is subjected to heat and pressure that causes the calcium carbonate to crystallise in an interlocking pattern. If you were to take that marble and melt it at atmospheric pressure, you'd actually destroy the calcium carbonate, and be left with calcium oxide and carbon dioxide.

To melt it down while maintaining its chemical composition, you'd have to subject it to enormous pressure while heating it to around 180°C at the same time. This wouldn't melt the marble, but it would allow you to reshape it. You would then have to control its cooling very precisely, to allow it the time to form the interlocking crystal structure that makes it marble. This would likely take a long time, though I can't find exactly how long; generally, the more slowly something cools, the larger crystals it would form.

The same answer applies to most other rocks as well. In most cases, if you simply melt down a rock and let it cool, you're going to get some form of glass. After all, we make glass by melting sand, and sand is simply pulverised rock.

The way minerals cool is actually more important to what they become than what they started as. It's all about the size of the crystals you can form in a given sample.

Take a look at this:

enter image description here

This is a piece of nickle/iron alloy. Just iron, the same as your car is made of - but you'll never be able to make a piece of iron that shows a pattern like this. It's called a Widmanstätten pattern, and it will only form when the iron cools slowly. Very slowly. Like, over millions of years-slowly. It takes that long for large iron crystals to form.

Marble, specifically, is similar - it needs to cool over thousands of years for its crystals to form their interlocking structure. Granite is composed of rocks that have been melted and then allowed to cool over thousands or millions of years. Sedimentary rocks that are melted can cool into granite. Rapid cooling of the same liquid rock would leave you with dense basalt or obsidian.

So, while it's not theoretically impossible to use a combination of heat and pressure to reshape a sculpture or a carving back into 'virgin' stone, and while we could possibly do it today if we had a strong need, it's nearly always going to be much more practical to simply dig a new block out of the ground.

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We produce bricks for construction by melting rock in blast furnaces

http://www.altocy.com/pdf/petro/TDS_FIBRANgeo_B-001_eng.pdf

However, most stone will melt at around 1500 degrees Celsius (2750 Fahrenheit), the previous company says they do it at 1520º C. So it is quite difficult and requires advanced technology.

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    $\begingroup$ Iron melts at 1,538°C. Since cast iron has been used in cookware for at least two thousand years, the practices of melting and cooling reasonably large quantities of materials at this temperature don't qualify as "high tech" - it might qualify for late iron age. $\endgroup$
    – pojo-guy
    Jun 30, 2017 at 13:28
  • $\begingroup$ Cast iron melts at 1,200ºC. Blast furnaces didn't appear in Europe until the 13th century. $\endgroup$
    – Alberto Yagos
    Jun 30, 2017 at 18:49
  • $\begingroup$ Thanks for the correction. cast iron does have a lower melting point than pure iron. 13th century for a blast furnace is late medieval to early renaissance technology, so it still doesn't qualify as advanced technology. $\endgroup$
    – pojo-guy
    Jul 1, 2017 at 0:37
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    $\begingroup$ @pojo-guy: "Advanced technology" does not necessarily mean what you think; it's easy to Google up examples where it's used in reference to things like "metalworking", "pottery", "astronomy", "shipbuilding", "horseback riding", and "the wheel". (I'm actually not sure quite what it does mean; I don't think it's a completely vacuous phrase, but it's still probably too vague to be terribly useful in this answer.) $\endgroup$
    – ruakh
    Jul 2, 2017 at 18:15
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Small fragments of marble or quartz are made into slabs of "engineered stone" for sale as floor tiles and kitchen bench tops.

This product is 90-95% rock. The rest is mostly plastic to glue it together, and there may be some dye to make it different colours. It's formed at a temperature that's hot enough to melt plastic but not hot enough to melt rock. It's hard like stone and when it's polished it looks very nice.

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    $\begingroup$ It does look nice, but bad things can happen if certain solvents come in contact with it... ;-) $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Jul 2, 2017 at 3:16
  • $\begingroup$ Another form of engineered stone is, of course, concrete, which uses a mineral-based binder rather than a plastic. If the surface is ground down to expose the stone aggregate pieces and then polished, the effect is in many ways similar. $\endgroup$
    – Jules
    Jul 4, 2017 at 9:24
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Speaking of marble, yes -- historically people fed old architectural marble (e.g. old Roman marble) into lime kilns: to make mortar and concrete ("lime" being the key ingredient in cement, mortar, concrete)

Spaces in Late Antiquity: Cultural, Theological and Archaeological Perspectives

Feeding Marble to the Kilns

Why did the population begin feeding the sculptural and architectural elements made of marble, which, as elsewhere, once decorated the public monuments and élite mansions in Galilee, to nearby limekilns? The main cause given by scholars for this kind of marble reuse is that it happened for economic reasons. As mentioned earlier, marble is superior to limestone when it comes to the production of lime. While that is the case, for most of antiquity marble was considered too rare and valuable a commodity to be used for this purpose and was, instead, used primarily for purposes of decoration and lavish display. When, by Late Antiquity, limekilns began to be built within the city limits, scholars reasoned that this was because, by that time, marble was amply available there in the form of architectural decoration and sculpture. Aside from marble's superior quality, reusing this stone from former urban structures also probably saved considerable transportation costs. According to these scholars then, the burning of sculptural and architectural marble in limekilns set up within cities during Late Antiquity was primarily chosen for its productive efficiency: the product was superior and transportation more cost-efficient.

So for this particular type of "rock" it doesn't take very advanced technology ... they were doing it in the real world, in antiquity.

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    $\begingroup$ This isn't really an answer to the question being asked. The question wants to know if they can make rock out of rock scraps by melting and resolidifying it (using marble as an example) in order to have new material for sculpting. This is an answer to a question about whether scrap marble specifically can be put to another industrial use other than sculpture. $\endgroup$
    – Draco18s no longer trusts SE
    Jun 30, 2017 at 15:48
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    $\begingroup$ OP asked whether marble can be converted into bricks. Other answers suggested that that's difficult; whereas this answer suggests that something like that was done in the real-world, using antique technology, so maybe this answer does add something and was worth-while. $\endgroup$
    – ChrisW
    Jun 30, 2017 at 15:57
  • $\begingroup$ ("lime" being the key ingredient in cement, mortar, concrete) $\endgroup$
    – ChrisW
    Jun 30, 2017 at 16:06
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    $\begingroup$ This fails to answer the question. The OP wants to know if marble can be melted and reformed into marble. $\endgroup$
    – user64742
    Jul 3, 2017 at 4:04
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    $\begingroup$ @ChrisW No, it converts stone chips into a component of mortar: bricks are made of clay. And I commented as a direct response to your comment. (Also, I love the way that people who downvote without explaining get hit with "Downvoter, please explain" comments, whereas downvoters who do explain get hit with "Well, you could have just downvoted.") $\endgroup$
    – David Richerby
    Jul 3, 2017 at 16:39
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There are of course other ways of re-using or not-wasting or re-purposing stuff.

Rock pieces could be ground/crushed down very fine and then used to mix into some other substance to lend strength (think of making cement, or putting metal filings into something like JB Weld), or used to make other products (sandpaper is just fine ground rock/mineral of various types glued to paper)

And of course, there are always uses for just small pieces of rock - drainage systems, as part of a large natural water filter, paving, etc.

However, on a relatively small scale - like whatever leftovers there were after Michelangelo carved his David - wouldn't lend a significant enough of the left overs for economy of scale to kick in and make doing anything but keeping larger chunks to use for smaller works or practice, etc or tossing the smaller chunks into a French Drain.

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  • $\begingroup$ Actually, in the case of marble, I suspect that Michelangelo's scraps would have been burnt for lime - marble makes a high quality quicklime but is usually too valuable for that. $\endgroup$
    – Martin Bonner supports Monica
    Jul 3, 2017 at 12:52

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