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In my alternate history, during the Islamic golden age (in this case, 9th century Arabia - present day Oman), my characters are building a series of glass towers.

They are like lighthouses traversing from present-day Muscat to Niswah; 6 towers, one every five miles (total 30 mile journey). For aesthetic purposes, they want to make them out of glass with minimal stone support.

enter image description here

I'd like the six towers to be glass, and about 10 stories high; imitation of Western lighthouses. The foundation is sandstone jebloon (or little rocky mountains). What will the requirements and limitations be? Where should stone supports be?

There are no issues with security, or storms, and the engineers have been given the best engineering literature from North Africa to the Levant and Arabia (imagine they have access to the sum of the Islamic renaissance, for this question).

enter image description here

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    $\begingroup$ Wouldn't a glass tower in the desert be... hot? $\endgroup$ – Jane S May 18 '16 at 5:23
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    $\begingroup$ @JaneS - I hope so. $\endgroup$ – Mikey May 18 '16 at 6:15
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    $\begingroup$ @JaneS I would agree to some extent but Oman is not in the whole a desert and the wikipedia data about the climate differ, while the German page shows average temperatures based on the measured data records (and define the climate as semi-tropical), the authors of the English page have taken the NOAA data en.wikipedia.org/wiki/Climate_of_Muscat which have apparently higher values (they define the climate as arid)! $\endgroup$ – Medi1Saif May 18 '16 at 6:26
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    $\begingroup$ @Medi1Saif - agreed. The part of Oman I'm talking about is relatively moderated by the sea. One thing I'm worried about is that since it is extremely dry climate, it can be hot during the day and cold at night (expansion/contraction). $\endgroup$ – Mikey May 18 '16 at 6:38
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    $\begingroup$ I am not a physicist and I don't have the know-how to do the calculations to find out, but wouldn't this building act as a huge lens in places, potentially catching things inside of it or behind it on fire? Wouldn't that put a damper on the whole thing? $\endgroup$ – Kevin Wells May 18 '16 at 20:45
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You could do it, but you'd want to build your lighthouse out of small glass blocks.

Glass, even the sorts of glass available in the Renaissance, has a much higher compressive strength than brick. Something on the order of 1 GPa to brick's 7 to 70 MPa. Even substances like basalt and granite have lower compressive strength than glass, with strengths in the range of 100MPa or so. This means that a glass tower, build in a similar manner to a regular brick tower, will have no problem supporting its own weight.

Unfortunately, glass is liable to fracture and propagates cracks extremely well. If you want to build a tower out of it, you should use short, stout glass bricks instead of large panes. If you want your tower to look glass-like, you should look into dry stacking techniques, in which irregular, but closely fitted, blocks are stacked into walls without the use of mortar.

Also unfortunately, glass is far more difficult to work with than stone because of its tendency to fracture and crack. In addition to the cost of glass, which will be much higher than the cost of similar stone, your towers will take an order of magnitude more effort to build.

In short, your glass towers won't need any stone support, but they'll be incredibly expensive to construct.

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    $\begingroup$ This is awesome and I want to build one now... $\endgroup$ – James May 17 '16 at 19:42
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    $\begingroup$ @AndreiROM What rough weather systems do you think would be problematic? Glass is fairly resistant to weathering from rain and wind storms should just result in compressive force on the lee side of the tower. If there's sand storms in the area, the surface will become pitted, but that issue could be handled by adding a facing layer of non-structural thin glass bricks than can be replaced when they get excessively damaged. $\endgroup$ – ckersch May 17 '16 at 20:27
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    $\begingroup$ Stone lighthouses used dovetails to resist sideways force rather than rely on mortar, so there's no additional difficulty there with dry joints in rough weather. $\endgroup$ – Pete Kirkham May 17 '16 at 21:09
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    $\begingroup$ Glass gets LOADS of internal stress as it cools down. This means that if a solid block is cast too big - it'll spontaneously shatter during production. Even today we can't make a glass block larger than 30-50cm. Small blocks are not "want", they're a must. This also means that the 1GPa value will end up very different - either larger or smaller, depending on how stress sets. (please note that the common "glass blocks" or "glass bricks" are not solid but either thinned down to 1-2cms in the middle or glued from 2 separate pieces and hollow inside) $\endgroup$ – Agent_L May 18 '16 at 13:02
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    $\begingroup$ I feel like this answer needs to point out that glass bricks are not nearly as transparent. At best you get distorted, blurry images. And I don’t believe people of this time had particularly impressive glass-making skills which would lend itself to consistently achieving that best case. $\endgroup$ – KRyan May 18 '16 at 13:26
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A lighthouse contains around 2000 tonnes material ( 2 million kg )

Creating glass takes up to 35 MJ per kg

The discovery of Arabian mineral oil for fuel didn't happen until the 20th century.

Charcoal gives 30 MJ per kg.

You get 3 tons wood per acre, and around 40% charcoal yield, so 1.2 tons per acre.

So each lighthouse requires felling about 2000 acres of forest.

Six towers is 12,000 acres of woodland. Not absurdly impossible - 36,000 tonnes of hardwood could have been 60 warships and some navies were bigger than that - but possibly significant given the location isn't very wooded. Trying to build one huge ship in the 16th century cost Scotland much of its oak forests, though of course there's a difference in yield of wood good enough for ships vs wood good enough for charcoal.

Your tower builder may go down in history as a great feller.

Solar power is around 1 kW/m², so 1 m² running for 10 hours per day gives 36 MJ, enough to create 1 kg of glass. So with a perfect solar furnace you would need 12 million square-metre days to create the towers, about 8 acres running for a year. Efficiency is probably at best 1% with the available materials, so 800 acre-years of mirrors. A square metre of 0.5 mm bronze would weigh about 4kg and require 42 MJ/kg, so 800 acres of mirrors is 13 million kg or another 500 million MJ energy on top of the 450 million MJ for the glass. One square metre produces energy to create 0.01*36/(4*42) = 0.002 square metres of mirror per day; at 0.2% growth per day it would take 8.5 years to get to 800 acres, five years if you start with 10 acres and so on. There's probably a formula for the shortest time based growing enough mirrors vs. using them for the glass. You also could use thinner bronze, but then need wood or something to support it, so still be dependent on felling trees.

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    $\begingroup$ I hope "great feller" is also a play on words. $\endgroup$ – Mikey May 17 '16 at 21:11
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    $\begingroup$ I recommend the use of solar mirrors to heat the ingredients to fusing temperature. A large array of small flat mirrors, with plenty of semiskilled labor to keep them pointed, should be able to melt at least 500 lbs of silica into glass. Of course batch sizes would be restricted to the amount that can be fused in twelve hours. The mirrors and support scaffolding can be moved from one tower site to another. $\endgroup$ – A. I. Breveleri May 18 '16 at 1:11
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    $\begingroup$ @A.I.Breveleri Could the people of this time have created such mirrors, and would they have known to do so? Optical glass is really precision stuff; I know in Europe it wasn’t really explored as an art until much later than this, and then independently—doesn’t rule out Arabia having figured it out, and the knowledge lost, earlier, but I’d want to see evidence that they did. $\endgroup$ – KRyan May 18 '16 at 13:28
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    $\begingroup$ @KRyan: Optical glass is not necessary for mirrors -- any flat shiny material will do. Possibilities include obsidian (even easier to make and cast than transparent glass), polished bronze, tin-plated iron or copper, or even silver (as this relatively expensive metal could be returned to the treasury upon project completion). $\endgroup$ – A. I. Breveleri May 18 '16 at 14:14
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    $\begingroup$ Oooh, solar mirrors... this took a while to find... From Volume 3 (downloadable) of the Surangama Sutra, online.sfsu.edu/rone/Buddhism/Shurangama/Shurangama.htm, the words of the Buddha... “Ananda, fire, which has no nature of its own, depends upon various causes and conditions for its existence. Consider a family in the city that has not yet eaten. When they wish to prepare food, they hold up a speculum to the sun, seeking fire." (and the Chinese commentary translates "Speculum" as fire-mirror) ... so well enough known to be used in an examination of causality. $\endgroup$ – Brian Drummond May 18 '16 at 14:38

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