The Glass Menagerie

Need to replace the glass in your windows? You have plenty of options that blend historical accuracy with modern technology.

By Clare Martin

	Demetra Aposporos photo

If you're like most old-house owners, your home's original windows are a point of pride. From their true divided lights to their counterweight-and-pulley mechanisms to their wavy, distorted mouth-blown glass, those windows are a huge part of the character of your house. But like most parts of your home, they need a little TLC to stay in top shape—in fact, they often need more of it, considering that they have to withstand harsh conditions like wind, sunlight and stray fastballs from your neighbor's back yard. Given all this, it's almost inevitable that you'll come to a restore-or-replace crossroads with a window at some point in the life of your house.

Unlike other components of historic windows (wood sashes that are prone to rot and decay, sticking counterweights that make the window difficult to open), the glass usually doesn't need as much attention. "We've taken glass out of windows that's 300 years old," says Marc Cleary, director of sales for Cleary and Son, a Massachusetts-based window restorer. "Unless something has caused it to break, an old window should last for a long time." Accidents do happen, of course—and there are a few other factors that might signal a need for glass replacement, including scratches, staining or a remuddling courtesy of a previous owner. Plus, with new technology for window glass (from impact resistance to energy-efficient coatings) popping up all the time, you might start to wonder if new glass is the way to go. The good news is that replacing the glass in your historic windows doesn't have to be an arduous task—and it's entirely possible to take advantage of new technology without sacrificing period authenticity.

The Match Game
Although glass has been made in some form or another since the time of the ancient Greeks and Romans, sheet glass for windows wasn't produced until the 11th century, when Europeans discovered that glass could be blown in a cylinder and then swung or rotated to create a flat surface—albeit one with a tell-tale circle in the middle that belied its origins. This type of glass (often known as crown glass) was common in palaces and luxury homes up until the 17th century, when French glassmakers patented a new process for creating glass. This method involved pouring molten glass onto a flat surface, then grinding and polishing it to produce a flatter surface with much higher visibility. As the Industrial Revolution began its march across Europe and America, processes for producing flat glass became more and more refined, and the glass became clearer and less distorted as the years went on.

Given this tie between history and glass distortion, it's important to take the time period of your house into account when looking for reproduction glass, especially if you don't have the original to compare it to. "Back then, glass was made to look as perfect as possible," explains Robert Jayson, president of Bendheim, one of the largest American suppliers of mouth-blown reproduction glass—in other words, don't assume that you need the most distortion possible to achieve the most authentic look. "Less is more most of the time," Robert says. "You don't want it to be overly distorted—it's difficult to look through and makes much more of a statement." With this in mind, Bendheim offers two different weights of its restoration glass—"full" is heavier and more distorted, designed for 17th- and 18th-century homes, while "light" has less movement and is recommended for 19th and early 20th-century homes. Marc says Cleary & Son typically uses Bendheim's light glass for buildings that are between 100 to 200 years old, and relies on a less distorted glass from a different manufacturer for homes less than 100 years old.

If you're lucky enough to have the window's original glass as a guide, determining the appropriate amount of distortion will be that much easier—but getting a perfect match could be tricky. "There were so many makers of historic glass that finding an exact match isn't always possible," says Robert. The best way to stack the deck in your favor, he suggests, is to either request samples from the manufacturer to try to find the best match on your own, or to send a piece of the original glass to a manufacturer and let him figure it out. "As long as the piece is large enough, we can make the determination as to which glass we have will work best."

New Meets Old
Glass replacement isn't just a matter of finding what looks best, though. You'll also want to take into account factors such as safety, energy efficiency and UV transmission when shopping around for reproduction glass—but you don't have to compromise today's standards in your quest for yesterday's aesthetics.

Take energy efficiency, for example: Older, single-pane glass is often cited as being wildly inefficient, even though a window's glass typically only accounts for a small percentage of the overall energy transmission of the window. Because of this perception, insulated glass (two panes of glass with air or gas sandwiched between) has become the new standard for modern windows. However, says Marc, "We stay away from insulated glass because it typically only lasts 10 or 20 years before the seal breaks and it starts to fog up." In addition, says Robert, you can get the same insulating effect while still using historic glass, by placing a pane of reproduction glass on the exterior and a pane of standard float glass on the interior. (He cautions against using reproduction glass for both panes, as that will make the level of obscuration too great—keeping the reproduction pane on the exterior will allow you to get a period-appropriate look from both angles, but still allow you to see out from the inside.)

For those who live in hurricane zones or other natural-disaster-prone areas, the same lamination that makes modern glass impact-resistant (involving a plastic interlayer that will stay intact when the glass itself is shattered) is also available on reproduction glass. Although this option can be substantially more expensive, says Robert, the benefits are numerous: In addition to providing safety from storms or break-ins, it also helps control sound transmission. Bendheim also offers the option of a UV-inhibiting coating on its glass (used on the restoration of the skylights at Monticello), which blocks 99 percent of fading UV rays.

Even without special applications or features, though, choosing authentic reproduction glass over some of the newer options available today will still be a substantial investment—Marc estimates that the cost for reproduction glass can be double what you would pay for standard float glass. However, he says, "When you have the chance to put in glass that will match the era of the house, you might spend more, but you'll get exactly what you want."

Start a discussion on this article in our old house forum!