Chapter 10. Vitruvius (Lime)


Sand and its sources having been thus treated, next with regard to lime we must be careful that it is burned from a stone which, whether hard or soft, is in any case white. Lime made of close-grained stone of the harder sort will be good in structural parts; lime of porous stone in stucco. After slaking it, mix your mortar, if using pitsand, in the proportion of three parts of sand to one of lime; if using river or sea-sand, mix two parts of sand with one of lime. These will be the right proportions for the composition of the mixture. Further, in using river or sea-sand, the addition of a third part composed of burnt brick, pounded up and sifted, will make your mortar of a better composition to use.

The reason why lime makes a solid structure on being combined with water and sand seems to be this: that rocks, like all other bodies, are composed of the four elements. Those which contain a larger proportion of air, are soft; of water, are tough from the moisture; earth, hard; and of fire, more brittle. Therefore, if limestone, without being burned, is merely pounded up small and then mixed with sand and so put into the work, the mass does not solidify nor can it hold together. But if the stone is first thrown into the kiln, it loses its former property of solidity by exposure to the great heat of the fire, and so with its strength burned out and set free, and only a residuum of heat being left lying in it, if the stone is then immersed in water, the moisture, before the water can feel the influence of the fire, makes its ways into the open pores; then the stone begins to get hot, and finally, after it cools off, the heat is rejected from the body of the lime.

Consequently, limestone when taken out of the kiln cannot be as heavy as when it was thrown in, but on being weighed, though its bulk remains the same as before, it is found to have lost about a third of its weight owing to the boiling out of the water. Therefore, its pores being thus opened and its texture rendered loose, it mixes readily with sand, and hence the two materials cohere as they dry, unite with the rubble, and make a solid structure.

 — Vitruvius, The Ten Books on Architecture, ca. 40 BC [1]


You know, I've been sitting here real quiet while Lucifer and her descendants go on about the fire and all the stuff that comes out of it. But what about the stuff that goes into it, huh? You'd have no fire without wood, no pottery without clay, and no metal without ore. I figure that stone is getting the short end of the stick here. I've been waiting for Lucifer to talk about two stones in particular, since they come out of the fire, but he just whooshed right on past them. So let's just come on back to limestone and gypsum.

Of course, limestone's been used since God was a child to build pyramids and tombs and temples and such. And it's such a good building material that some of those buildings are still around, in spite of wars and revolution and time. To use limestone blocks, you have to cut them out of the quarry, chisel them to the shape you need, and haul them to where you need them. And I'm not saying it isn't worth the trouble for real important tombs and all, but it is a lot of work for ordinary houses and shops and offices. If you live where wood is expensive, these places are made out of brick.

Now, in the beginning, these bricks were made of sun-dried mud and they were just stacked up, and when the rain came, they got a little wet and stuck together, which was good. But over years and years of getting wet, they eventually crumbled and returned to the Earth, which was not so good. So folks started firing brick, like Athanor said, and the clay calcines into mullite and silica, which don't soften in water. But while a fired brick lasts a good long time, it doesn't stick to the others. All in all, it's just another brick in the wall with nothing to hold the wall together. That's where limestone comes into it again.

When you burn limestone in a kiln it just crumbles into a powder. And when you add water to this powder, it turns back into stone. Mix this powder with sand and you've got a real nice mortar for cementing bricks together. And there's another stone, gypsum, which works almost the same way; heat it up and it crumbles to dust; add water and it turns back into stone. Where limestone is good for making mortar, gypsum is good for making plaster and stucco. And folks have been using mortar and plaster since about 12,000 BC, so that's nothing new.

Long about 200 BC, my cousin Unktomi-Vitruvius was floating around in the head of a Roman builder and discovered that if you take burnt limestone, which is called lime, and mix it with a kind of volcanic sand, you get a kind of cement, hydraulic cement, which can even harden under water. Mix it with sand and pebbles and rubble and such and you get a brand spanking new material, concrete, that can be poured into the shape you want, which is a lot easier than cutting stone into that shape. Just build a wooden form to keep it in, mix up your concrete, pour it in, and once it sets up it's every bit as strong as if you had a limestone block of that same shape. Now my cousin moved down the line from Roman to Roman for a long time, but eventually the Roman Empire fell and my cousin went extinct along with it. So folks didn't know how to make concrete any more. All those Gothic cathedrals and such were made out of cut stone with no concrete at all, which is okay, but you can't afford to build too many fancy buildings like that.

Well, a long time went by. I was just a general-purpose inspiration at the time and I found myself living in a guy named Joseph Aspdin when all of a sudden in 1824, bam! I connectified lime and clay. That did the trick all right, made a hydraulic cement so I could get back to work concretizing the place. So when you're wandering around in all those skyscrapers and community centers and movie theaters and, well, most any building you ever go into, give old Unktomi-Aspdin a moment of thought. So I don't go extinct, you know.



Reference [28], Book II, Chapter 5, Section 1.