Two years ago, I noticed the Heurich House has curved pane windows on the round corners of the house. Since then, I’ve noticed a lot of houses in Northwest D.C. that also have curved pane windows. I’ve really enjoyed my “treasure hunt” around town finding more examples. My question: In the old days, what sort of tool would glaziers carry to measure the curvature of the glass for replacements?

Dan Leather, Washington

No special tools required, just an understanding of geometry, said Dan Rudie, preservation manager at the Heurich House Museum, the handsome mansion built near Dupont Circle in the 1890s by beer baron Christian Heurich.

“You just need a tape measure that will bend around the curve,” Rudie said. “A sewing tape works well, as it is fabric and conforms tightly to the curves. That and a straightedge were the only tools needed then and now.”

Rudie explained it this way: Imagine you are looking down on a pane of curved glass, its top edge facing you. It resembles a frowning mouth.

With the fabric tape, measure the inside of the curve from edge to edge and the outside of the curve from edge to edge. The latter measurement will be larger, because the outside of a curve is larger than the inside. (It’s why jockeys want their horses racing on the inside rail.)

You also need to measure the depth at the midpoint of the curve. To envision this, imagine you are looking at a Quonset hut and are calculating the distance from the highest part of the ceiling to the floor.

Those measurements — plus the height and width of the window and the width of the glass — are all a glassmaker needs to whip you up a new piece of curved glass.

Now, this won’t come cheaply. Curved glass must be made to order. It’s at least five times as expensive as a similar-size piece of flat glass. But it’s so cool.

Before it can be cool, it has to be hot — very hot. Bending glass means heating it to around 1,000 degrees Fahrenheit, said Jon Johnson of Standard Bent Glass in East Butler, Pa., a company that has made some replacement panes for the Heurich House Museum.

One way to make a pane of curved glass is to put a flat piece in a curved mold, stick it into an oven and wait for gravity to do its magic.

“At a certain temperature, the glass will slump into the mold,” Johnson said.

A newer method involves putting glass into a mold, heating it and pressing on it to curve it.

You can do most of the things with concave glass that you can with flat glass, including tempering it, laminating it and increasing its energy efficiency.

Curved glass first became popular at the end of the 19th century. “It wasn’t really feasible until right about the time the Heurich House was being built, maybe a little before,” Rudie said.

Heurich’s house is in the Richardsonian Romanesque style. The Smithsonian Castle on the Mall is also in that style. It, too, has a turret, but the faces of that turret are flat, as are the panes in the flat windows.

After a few decades, turrets fell out of fashion — curved windows, too.

“A lot of times, you’ll see houses around D.C., and when the curved rooms were redone, they just put in a flat replacement sash unit,” Rudie said.

But curved glass is experiencing a renaissance, and not just as windows.

“In the last 15 years or so, curved glass has really come into its own, architecturally speaking,” Rudie said.

You see it used in interior walls, banisters, shower enclosures. . . . Answer Man understands the appeal: Glass looks fragile. Twisting it into interesting shapes seems to defy nature.

In 1893, the Evening Star wrote: “It is always a distinction when it is said of a private residence that it is the most costly in the city. The residence which Mr. C. Heurich is building at the corner of New Hampshire avenue and Sunderland place will be thus known.”

The paper estimated the cost of building the house would be close to $200,000.

It was built to last. There are 28 panes of curved glass in the Heurich castle, 14 of them in the windows of the turret. Rudie has been involved with the house since 2006. In that time, he has had to replace three curved window panes.

“All the sashes are still original, every one,” he said.

Questions, please

Have a question about something in the Washington area? Send it to answerman@washpost.com.

Twitter: @johnkelly

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