How early can transparent glass windows be easily available?

Question

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?

Answer 1

~ 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.

Answer 2

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.

Answer 3

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.