The Farnsworth House, which Ludwig Mies van der Rohe designed in 1945 and built in 1951, sits in the floodplain of the Fox River in Plano, Illinois, about 55 miles west of Chicago. The house is a steel structure with concrete floors and travertine pavers, and its interior space is enclosed with large panes of glass held in place by simple steel frames.

In fall 2017 staff at the Farnsworth House noticed a crack in the corner of one of the windows. They observed the crack, which had been caused by corrosion in the glazing pocket, worsening in the following months. While losing historic material is never ideal, the cracked glass presented a risk to adjacent materials as well as a safety risk, so it required replacement. Fortunately, the project team was ready to implement a strategy they had tested during trial repairs in 2012–13 and implemented in emergency replacements in 2013–14. A generous grant from The Courtenay C. & Lucy Patten Davis Foundation (Denver, CO) covered a portion of the project.

The National Trust for Historic Preservation’s Graham Gund Architect Ashley Wilson and Silman Fellow for Preservation Engineering Allison Semrad discussed the repair process with the project coordinator, Joshua Freedland, architectural conservator from Wiss, Janney, Elstner Associates (WJE).

What kind of glass did Ludwig Mies van der Rohe use at Farnsworth? And how is it held in place?

With the exception of two small hopper windows on the east side of the building, the glass panes at the Farnsworth House are ¼" thick and more than nine feet high. The widest panes, located at the north and south, span 11 feet between vertical supports. The original glass and early replacements are polished plate glass, made by rolling molten glass into uniform sheets and polishing them until smooth. This was typical for window applications before the 1960s, especially for large storefronts.

The window panes are mounted in straightforward frames made of steel bar stock screwed to the structural steel channels at the floor and ceiling. The steel stops that hold the panes in place are spaced no more than 1/2" apart, leaving just enough room to accommodate the thickness of the glass and glazing material.

How many of the windows have been replaced?

Today, of the 17 windows, only five are the original polished plate glass. Since acquiring the Farnsworth House in 2003, the National Trust and Landmarks Illinois have replaced four panes and repaired six window frames.

Why has so much of the original glass been replaced?

Glass at the Farnsworth House has been replaced as needed since the house’s completion. While glass is generally durable, it is vulnerable to breakage and cannot be repaired—only replaced. And deterioration of the steel frames compounds the need to rehabilitate the Farnsworth House windows.

Given their large size and the rigidity of the materials surrounding the glass, the windows are susceptible to changes in the environment and the building itself. The steel components of the windows are prone to corrosion, which causes the jambs, frames, and stops to expand and move. Sometimes called “oxide jacking,” this movement puts additional stress on already fragile panes of glass. The corrosion is accelerated by flooding, which sometimes submerges the window sills. Cracks are also often caused by differential pressures, such as those produced by gusts of wind. Additionally, the repair process itself can be damaging. In 2013, when one pane of glass shattered in a windstorm, the adjacent pane was broken during an attempt to stabilize the broken window.

What kinds of replacement glass have been used?

Window panes at Farnsworth today are either the original polished plate glass, float glass, or tempered glass. In a perfect world, the original windows could be replaced in kind, but manufacturing technologies have shifted and public safety is a priority, so tempered replacement glass is now the best choice.

Both the original glass and early replacements are polished plate glass. Some later replacements are float glass. Float glass, which superseded plate glass around 1960, is made by floating molten glass over a bed of molten tin. While both plate and float glass are prized for their optical quality, both break into large, sharp shards. For this reason, more recent repairs use tempered glass, which is heat treated to increase its strength. Tempered glass breaks into many small pieces without sharp, jagged edges and is therefore much safer and code compliant. And, unlike laminated glass, it can be manufactured thin and flat enough to fit in the existing frames. The trade-off is that the tempering process often introduces noticeable optical distortion.

What window repairs have been undertaken during National Trust ownership?

In 2012–13, staff conducted a window investigation and trial repairs to study severely corroded window frames. They chose the two windows that were most affected by oxide jacking, but already had tempered glass that could be safely removed and reinstalled.

The trial repair proved timely: it provided the strategy for the emergency repairs that became necessary after a windstorm later that year. Three windows at the southeast corner of the house were replaced with newly manufactured tempered glass in 2013–14. The scope of work also included installing safety film on all non-tempered windows to stabilize any new cracks in the plate and float glass.

When a window developed a two-foot-long crack during summer 2017, the safety film stabilized the float glass window, allowing staff to schedule repairs for winter, when Farnsworth House is closed.

What is the strategy for replacing the windows?

The first step is making a temporary enclosure to protect the surrounding building fabric from weather and construction debris. Next, the project team carefully disassembles the window unit, which includes removing screws, steel stops, and sills as well as the broken glass. The frame elements, perimeter beams, and jambs are abrasively blasted, primed, and coated on site, including their concealed areas. When the bar stock cannot be salvaged, similar pieces are fabricated from new steel, then prepared and primed in the shop. In isolated areas, epoxy is used to fill pits that would otherwise retain water and lead to corrosion. The team sets the new tempered glass in place using setting blocks and shims and reinstalls the restored frame using stainless steel counter-sunk screws to match the original configuration and spacing. Joints throughout are sealed with elastomeric sealant to prevent water infiltration.

The project team works with to Agnora Architectural Glass in Ontario, Canada, because the company can guarantee tempered glass flat enough to meet Farnsworth’s stringent visual requirements. The installed glass must be ¼" thick to fit in the narrow glazing channel, and cannot warp more than 1/32" over any 12" span.

Who is on the project team?

The project team for the trial repairs and subsequent replacements includes conservator, engineer, and exterior envelope specialists WJE; restoration contractors Berglund Construction; metal work and glazing subcontractor MTH Industries/Illinois Bronze and Metal; and Agnora Architectural Glass. ATD performed the coating work. Krueck and Sexton Architects also worked on the trial repairs.

What’s next?

The next step for the Farnsworth House is fundraising for comprehensive restoration of the remaining window frames. This project is being coordinated with other preservation work, including travertine repair and flood mitigation. The character-defining simplicity of glass, steel, and travertine requires robust cyclical maintenance to keep corrosion and deterioration at bay, which is only possible with continued support from architecture enthusiasts.