What is the purpose of the milled slot in Frank Lloyd Wright’s windows?

Our window restoration project took a major step forward yesterday with the installation of a prototype replica fabricated window in the back hall. It’s a simple cypress hinged sash fitted into the original pine framing, mating with birch interior trim, replacing a frameless double pane that had been installed sometime in the 1970s. The new window is built exactly to Wright’s dimensions and directions for mortising and gluing and the hinges (and eventually the latches and stays will) match originals found elsewhere in the house.

There must have been powerful drafts in this space at the time of the 1970s window replacement. It took us the better part of two hours to cut away tens of pounds of caulking that held the massive pane snugly in place. The previous owners seem to have tired of high heating bills and hired a window installer who promised no leakage.

We realize that we may not enjoy the same degree of energy efficiency by reverting to vintage technologies. For now, our prototype has a single pane of glass, but we have ordered a double pane that will replace the single when it arrives.

But we remain perplexed about one detail. Part number B9 in Wright’s plans show a slot milled in the mating edge of both the jams and sills. Here is the drawing and the slot:

We wonder if this slot was to have carried a sealing gasket – perhaps of rubber or waxed rope – to ensure a tight fit between the casement and the jams. Or was there was another intended purpose? We have consulted other American System-Built Home-owners to see if they have found evidence to support the theory, like actual gaskets or residue from them, and so far, no one has.

So what do you think? Why is there an invisible slot milled in Frank Lloyd Wright’s windows? Should we fill it with something, and if so, what? Architects, homeowners, historians and restorers are welcomed to weigh in, either below in the comments, or via private message.

Here is a 1970s window verses our new prototype.

Track all our our work to restore and protect this home here and follow us on Instagram to keep up with this and future projects.

About this blog: The Elizabeth Murphy House is a 103 year-old American System-Built Home and the protagonist in the book “Frank Lloyd Wright’s Forgotten House.”

Image Credit: American System-Built (Ready-Cut) houses for The Richards Company, Floor Plan, Frank Lloyd Wright Foundation Archives: architectural drawings, ca. 1885–1959. The Frank Lloyd Wright Foundation Archives (The Museum of Modern Art | Avery Architectural & Fine Arts Library, Columbia University, New York) 1506.656

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