TL;DR: How early can windows made of transparent glass (can look through) be readily available (not a rare luxury) to "common" citizens (at least top 10% of population)?

Disclaimer: I know very little about the history of glassmaking or the modern processes involved. I dabbled a bit looking for an answer, and keep doing so, but maybe someone is an expert and can provide a better answer.

Glass was known to humans for quite some time, and as early as Romans (and probably earlier) it was used for windows. However, for a long time produced glass had poor optical qualities, so while it would let the light pass, it wouldn't allow you to see through a window. There is some evidence pointing towards transparent glass in the Ancient Rome and Greece, including references to magnifying glasses, but nothing consistent. After the Dark Ages, it took some time to recover glassmaking knowledge, and even by the 18th century glass windows were a luxury even for the highest society.

As with many things, what was a luxury, now costs less than the same amount of bread or vegetables. The question is, is it purely our technological superiority that allows for this? Or are there some glassmaking methods that would allow, if one were to know how, to make transparent glass relatively easily and cheaply? If this is merely a matter of knowledge and invention, then how far back can we go and still have good enough technologies to make glass for the masses? Can we have it in early Industrial age? In the Middle ages? Back in Roman times?

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    $\begingroup$ What do you mean by transparent many early windows were transparent they were just not completely flat, so they let in plenty of light but you could not see out of them with clarity. $\endgroup$
    – John
    Aug 15 '18 at 18:45
  • $\begingroup$ @John I'm not sure, really, what would be the correct word. Clear? What I want is being to see through those windows with decent clarity at least to a distance of a few hundred meters. $\endgroup$
    – Alice
    Aug 16 '18 at 9:24
  • $\begingroup$ Medieval churches had large windows each filled with many small panes of colored glass. Beginning in the 12th century gothic style churches were designed to have vast windows and little wall. Hardwick Hall built 1590-97 is know for having "more glass than wall". Shakespeare's house at Statford on Avon has many windows, each with many small glass panes. The upper classes had glass windows even in late medieval times, and I guess that to see outside clearly one would merely put their eye very close to one of the small panes. $\endgroup$ Aug 16 '18 at 16:48

~ 1900 technology

A cylinder of glass is pulled by a machine arm from a circular glass glob. The glass is then annealed, cut, reheated, and flattened resulting in a flat plate. This method will definitely meet your requirements.

~ 1600 technology

Pour molten glass onto an iron table, roll it flat, then polish it. Historically, this was done by hand, and was not cheap. Mechanization can be achieved in both rolling and polishing. Rolling was not widely applied until the Industrial Revolution, but there are examples of rolling mills on metals in Europe from 1590. William Herschel built a mirror polishing machine sometime in the 18th century, but Leonardo had sketches for something similar much earlier. I can't find a good source, but I recall reading that Milanese armorers used water-driven polishing machines to prep their wares for sale in the 15th century. A fully mechanized processed realized with ~1600 technology could meet your specifications.

~ BC technology

A rolling machine needs a power source and a gearing configuration that will allow it to roll two sections at the same speed. This can be accomplished by using a watermill (available 3rd century BC, in Greece) and a belt drive attached to both ends of the rolling machine (available 1st century BC, in China). Polishing requires complex gearing to increase rotational speed. This gearing was probably available by 1 AD in Rome, as the Romans had evidently built both kneading and fulling machines using water power. Combine these technologies and, optimistically, glass is as cheap as it was with the 1600s technology...or maybe even cheaper with all the slave labor.

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    $\begingroup$ You're probably not going to manage it in the Greek/Roman era. Yes, the technologies were (sort of) known, but execution is a problem. You're going to have trouble building a rolling machine that can survive the heat of molten glass (large pieces of iron were incredibly expensive, if they could be made at all), and a polishing machine is likely to break down too frequently to be practical. $\endgroup$
    – Mark
    Aug 15 '18 at 19:47
  • $\begingroup$ I'd go with the 1600 option, but I presented three possibilities of descending believability. $\endgroup$
    – kingledion
    Aug 15 '18 at 19:54
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    $\begingroup$ Clear glass was first invented about 1450 by Angelo Barovier. If you couple this with pouring it out on a tin plated "table"/cast and floating it, then polishing off the tin oxides, you would get some relatively nice glass. en.wikipedia.org/wiki/Cristallo $\endgroup$ Aug 15 '18 at 21:20
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    $\begingroup$ @Keeta The Romans had clear glass from ~100 AD. $\endgroup$
    – kingledion
    Aug 15 '18 at 22:20
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    $\begingroup$ "but Leonardo had sketches for something similar much earlier" - Leonardo had sketches for everything early. That does not mean anyone was prepared to build such a thing at the time. $\endgroup$
    – Kevin
    Aug 16 '18 at 2:16

Flat glass is made by pouring the molten glass on a bath of molten metal, usually tin. For this reason it is also called float glass.

By floating on the molten metal, the glass will acquire its flatness.

Until the 16th century, window glass or flat glass was generally cut from large discs (or rondels) of crown glass. Larger sheets of glass were made by blowing large cylinders which were cut open and flattened, then cut into panes. Most window glass in the early 19th century was made using the cylinder method. The 'cylinders' were 6 to 8 feet (180 to 240 cm) long and 10 to 14 inches (25 to 36 cm) in diameter, limiting the width that panes of glass could be cut, and resulting in windows divided by transoms into rectangular panels.

The problem with the above described process is that it has low throughput, influencing the final price of the product.

Before the development of float glass, larger sheets of plate glass were made by casting a large puddle of glass on an iron surface, and then polishing both sides, a costly process.

The only way to mimic the float process and allow mass production is to have molten metal and the ability to control its temperature (you don't want the metal to cool down and solidify with the glass) and composition (you don't want it to react with the oxygen). So, I am afraid you cannot go earlier than what we have had if you really want a large production. For limited volumes one can use the above mentioned techniques.

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    $\begingroup$ This was a great answer until the last sentence. There are many ways that anything we know of could be developed or mass produced sooner, including glass. All it takes is an earlier inventor or accidental insight. In this specific case, you seem to suggest temperature control to keep the metal liquid keeps it from being possible earlier, but there is a large gap between glass melting temperature and that of many metals, and many smiths were good at estimating/controlling temperature. Also, OP said "readily available/not luxury (to at least 10%)", not "Available to a modern extent". $\endgroup$
    – Loduwijk
    Aug 15 '18 at 18:28
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    $\begingroup$ Float glass is only necessary to make LARGE sheets of flat glass, the cylinder method can make glass you can see out of just fine. $\endgroup$
    – John
    Aug 15 '18 at 18:50
  • $\begingroup$ Tin melts at low temperatures of about 232 °C, glass transition temperature (the point at which it becomes a solid) is 573 °C for soda-lime glass so there is absolutely no problem with keeping the tin liquid. Pure metallic tin exists from 600 BC. The actual reason that this is not done in earlier times is that one needs to prevent tin oxides from forming, which requires an atmosphere with as little oxygen as possible (source: wikipedia articles). $\endgroup$
    – GretchenV
    Aug 15 '18 at 19:01
  • $\begingroup$ @GretchenV, good point, added $\endgroup$
    – L.Dutch
    Aug 15 '18 at 19:15
  • $\begingroup$ You can probably go somewhat earlier on float glass: around 1890, when various practical methods for liquefying gasses were invented. This gives you the argon or nitrogen atmosphere needed to keep the tin from oxidizing. Coal gas (to provide controllable heating) had become widespread half a century earlier, and you can usually substitute labor and inefficiency for modern industrial-control systems. $\endgroup$
    – Mark
    Aug 15 '18 at 19:58

If the Romans had the know-how to produce consistent results with their glass making, and someone showed them the cylinder method, they would have been able to produce plate glass in quantity. Enough that landowners could glaze the windows of their villas at least, whether they would have considered it worthwhile to do so in the core, Mediterranean, parts of the Empire? I don't think that glass windows would have caught on in Rome, it's too warm for too much of the year to get much advantage from glazing.

What we know of Roman glass working seems to suggest that they had good working techniques, like glass blowing, but poor production techniques so the glass artisans were working with was of unpredictable quality, colour and clarity.

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    $\begingroup$ The cylinder method is labor-intensive. If you consider the "top 10% of the population" specified in the question to mean "the top 10% of the non-slave population", then maybe this will work. Otherwise, it's too expensive. $\endgroup$
    – Mark
    Aug 15 '18 at 20:01
  • $\begingroup$ @Mark So the top 10% of the counted population of Rome then. They didn't count slaves as people after all. $\endgroup$
    – Ash
    Aug 19 '18 at 10:26

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