Taxes and Technology Change Sash Windows
On the technology side, industrial style processes meant the size glass panes were steadily increasing.
So bigger windows with fewer glazing bars were possible.
Along came the window tax (1696). This was a flat rate tax on houses with windows, initially if you than more than 10 windows you got charged more and another band was applied if you had more than 20. Now, if you’ve got to pay a fixed charge for having windows you might as well have the biggest windows you can afford that let in as much light as possible. So the demand was there for stronger, larger panes of glass, this allowed glazing bars to got thinner and there fewer of them to each sash.
On top of the Window Tax, In 1746 the Glass Tax was levied, by weight on the actual glass. So the thinner Crown glass, although comparatively expensive to produce, became became a cheaper option. However keeping it as thin as possible, it needed more support from glazing bars, so multiple pane windows remained popular.
The taxes wouldn't have bothered rich however who liked to show off. Their demand and that of commercial glazing kept developments in glass production going.
Georgian style 8 over 8 panes
At the beginning of the 18th century, twelve panes in each sash was common. Eight over Eight then Six over six soon became the norm until the mid 1800’s.
6 over 6 Sash Window with Slim Glazing Bars and Meeting Rails
The Window Tax was dropped in 1851 and the Glass Tax Dropped in 1845.
Prior to the 1850's it was quite technically possible and affordable to have fewer panes of glass in each sash. Without the Glass Tax it became an economically viable as well.
When the number of panes came down to 2 per window it was common to see “horns” on the bottom of the Top sash styles. As glazing bars where removed and the glass got heavier, extra timber on the top sash, horns, meant a full tenon could be used to support the extra weight.
One of many variants of sash horn details
Two over Two Sash Window with Horns
Let know what you think about this analysis.
(Thanks to David who commented, pointing out a few errors on the original version of this page, your input is much appreciated.)