Sash windows are a style of window that's been around for several hundred years, first being developed in the 16th century. Throughout that time there have been a variety of styles, varying every aspect of the design from the way it was set into the wall, to the ways the muntins are positioned within the individual sashes. Thicker muntins, recessed-wall models, concealed sash-box designs, even variations in the material the muntins were made of have all been innovations made throughout the centuries. It should come as no surprise, then, that the types of glass used for the panes is a detail just as clearly varied.
One of the most frequent decisions being made today in regards to traditional style sash windows is whether or not to reconfigure their existing windows for a more energy efficient set-up. When the time comes to start thinking about improving the energy efficiency of your home, then it's time to start thinking about adding panes to your windows. Double pane windows provide an extra layer of insulation in the form of the airspace between the panes, and remain clear so long as the sealant remains in place. Triple pane windows, as you might imagine, provide even more insulation, and prevents heat from transferring either in or out through the window.
Float glass is a type of glass created by pouring molten glass onto a surface made of molten metal, typically tin, but lead and other metals with a low melting point have been used. It is from these processes that one gets the large smooth sheets of glass with a standardized thickness. This is the standard form of glass used in most modern construction, and is the most likely default for modern replacements.
Some of the fancier designs of Sash Windows have utilized the beautiful artistry that is stained glass. While it's entirely possible that these colored panes may be simply be cut to fit into standardized square panes, but this isn't their most common application. Using muntins cut into different intricate shapes, stained glass panes are inserted into these designs creating beautiful patterns that cast their light onto the floor within during daylight, or the ground without during evening hours when lit from within. Some of the most intricate versions of this style will have thin metal muntins holding together the pattern, and require special equipment to replace panes that become broken or dislodged.
If you're looking for a way to provide some privacy, such as sash windows found in front doors or on bathroom windows, there are a variety of frosted glass patterns available. All in all they serve largely the same purpose, obscuring what's within from those trying to view through the window. There are multiple patterns providing larger and smaller degrees of obfuscation, and some that are purely decorative in their design.
These are just a few of the types of glass that could be used in your sash windows to create a custom look and style that fits your home. If you're looking for something a bit unusual that holds onto the traditional old world values of the sash windows, this is another way to personalize them!