This is the first of a three-part series on Usonian concrete floors. Links to the other parts are at the bottom of this page.
Starting in the mid-1930s, concrete floors were an important component of a large majority of Wright’s subsequent buildings, both residential and non-residential. In the Usonian house, its concrete slab served as the the medium through which the house’s radiant heat flowed, it was the base for installation of the wood walls of the house and it was the finished surface of the floor. Wright’s practice of making construction elements serve multiple purposes is both a manifestation of his genius and a challenge to stewards as they maintain their buildings.
It cannot be overstated that each restoration project is different. There is no one solution that fits all projects. The information given here is to provide building stewards an understanding of the original, typical process of floor installation, show examples of restoration and an give an overview of common maintenance protocols. It is hoped this information will help inform building stewards as they plan their own restoration and maintenance strategies.
Feel free to share your comments and questions about this information through the link at the bottom of the this page.
Background and Original Installation
In The Natural House, Wright advocates for “trench” foundations and “slab-on-grade” construction:
One of the best foundations I know of, suitable to many places (particularly to frost regions), was devised by the old Welsh stonemason who put the foundation in for the buildings now used by Taliesin North. Instead of digging down three and a half feet or four feet below the frost line, as was standard practice in Wisconsin, not only terribly expensive but rendering capillary attraction a threat to the upper wall, he dug shallow trenches about sixteen inches deep and slightly pitched them to drain. These trenches he filled with broken stone about the size of your fist. Broken stone does not clog up, and provides drainage beneath the wall that saves it from being lifted by the frost.
While, Wright cautions, “No one foundation, then, is suitable for all soils,” this type of foundation was typically used for the Usonian house, particularly the early Usonians. This foundation supports a concrete floor slab, or “mat” as Wright often referred to it. The slab is typically specified to be 3 1/2″ thick and rests on a bed of crushed rock. For buildings with radiant floor heat, heating pipes lie on the crushed rock, just below the slab. This configuration is further described in our Radiant Heat 101. In execution, the slabs were often thicker than specified and were poured in two phases. Lawrence Tarantino, of Tarantino Studio, who has done extensive research and carried out several restorations of Wright’s concrete floors, has noted this. In a presentation at the 2012 Building Conservancy conference he noted,
Many of the FLLW floors investigated through a cross-section prove not to have been placed as per the FLLW Drawings. Instead of a 3 1/2” thick Mat many have a thick base slab with a 1 1/2” topping.
This can be seen in the fragment of the Sweeton House slab below. The fragment is approximately the specified 3 1/2″ depth, but the upper, approximately 1 1/4″ was poured separately, with a finer aggregate. This process had multiple advantages. It allowed the heating pipes to be covered by a rough pour, whose upper surface could be exposed as work proceeded on the construction of the house. The upper layer could be poured after much of the construction work was completed and its finer aggregate facilitated achieving a smooth finished surface.
It can also be seen from the Sweeton fragment that the term “integral coloring,” which has been used in reference to the tinting process, is relative. The pigment is not surface-applied, like paint, but is shaken on, after the final surface is achieved. The pigment is also not mixed in with the entire pour. Doing this results in a pale, pinkish color, as was discovered by one disappointed homeowner, when his restoration contractor did not follow the specified shake-on protocol. The process of shaking on the pigment, achieves two results: the intensity of the tinting will vary slightly: it is not perfect, and not meant to be. Some have likened the effect to that of “aged leather.” It also leads to variation between installations, depending on the tradesperson applying the tint. This is one of the reasons why when restoring or repairing tinted concrete floors it is best to match existing conditions, rather than trying to discover an “ideal” color.
While Wright and others have written about construction of these slabs and the virtue of radiant heat, few, including Wright, have directly discussed the finish of the slabs. Instead much of our knowledge about finish has come from architects and building stewards who have done research into the the color and maintenance of these floors. According to research by Penny Fowler and John Eifler at the Frank Lloyd Wright Foundation Archive, the practice of tinting the concrete slab appears to have developed from a comment by Katherine Jacobs, that an un-tinted concrete floor would be “too ordinary.” The color choice seems a logical extension of the “Cherokee red” rubber floor tiles installed in the Johnson Wax Administration Building. This tile may have been planned as early as the winter of 1936-37, when the Jacobs House was under construction. The evolution of Wright’s thinking regarding concrete coloring can be seen in a note on a drawings for the Florida Southern College Esplanades: “‘Dust on’ type of cement coloring will probably be found easiest and most economical.” At this point (c. 1938), “dust-on” is suggested as an option, it is not a requirement.
Steve Sikora, who owns and restored the Willey House in Minneapolis has provided an extensive examination of the concept of “Cherokee Red.” The specific color, he notes, is elusive. The term Cherokee red is misnomer when applied to the Usonian concrete floors as the term’s origin was a paint color. After experimenting with various custom methods of tinting the concrete floors, Wright discovered “Colorundum” a concrete hardener by the A.C. Horn Company. The Colorundum color most used for Usonian floors was “Tile” red, but even this color could vary from installation to installation. Sometimes, as at the Reisley House, an alternate color was used at the client’s request.
The process of shaking on the pigment, achieves two results: the intensity of the tinting will vary slightly, it is not perfect, and not meant to be. Some have likened the effect to that of "aged leather." It also leads to variation between installations, depending on the tradesperson applying the tint.
For those trying to match the original materials specified by Wright, a quick history of A.C. Horn Manufacturers may be helpful. The company was started in 1897 by Aaron C. Horn (c 1876-1952). In 1945, A.C. Horn merged with Sun Chemicals and Aaron Horn became president of Sun Chemical. A.C. Horn become a division of Sun Chemical and the brand was retained. in 1962, A.C. Horn was purchased by W.R. Grace.
For many years, Lithochrome, by L.M. Scofield, has been used as the replacement product for Colorundum. According to Architectural Advisory Committee chair Dan Nichols:
Recently (January 2023) I spoke with Mike DeCandia, the technical representative for Sika Group USA’s color concrete products. Mike is located in Rutherford, NJ and came into the Sika fold a few years ago when Sika acquired L.M. Scofield, where Mike was a division manager. L.M. Scofield has made the shake-on/ trowel-in powdered concrete color hardeners that are, according to Mike, “essentially the same” as those produced by A.C. Horn in the early to mid 20th century and specified by FLW. Mike noted that Sika is gradually transitioning their acquired companies branding to become SikaColor. The Scofield Lithochrome which has been used for years as the successor to A.C. Horn’s Colorundum will now be marketed as SikaColor-200 Color Hardener.
The process for installation
The Sweeton House specification is a typical example:
All poured concrete shall be mixed in proportion of one part Portland cement (approved), two parts clean, sharp sand & four parts hard, broken stone or gravel.
All concrete floors & steps shall be topped & finished monolithically with red “Colorundum” (as mfg’d by the A.C. Horn Co, Long Island City, N.Y) (or approved equal) & shall be steel trowelled to a dense, smooth surface. After floors are laid & well set they shall be treated with one coat of A.C. Horn “Colorundum” glaze coat (or equal).
Entire concrete mat shall be marked with lining tool 1/2″ “V” joint on unit lines.
Entire floor slab to be 3 1/2″ thick over 5″ broken stone (by others)
Mat reinforced with 6″x6″ welded wire mesh 1″ clear bottom.
Below are a series of photos from various installations, collected as a part of the “Usonians, show us your floors!” project. While inconsistency of lighting and camera equipment should be taken into account, these images show the variation that can occur:
Deviations from the typical process
Aside from expected variation stemming from the shake-on process, original installations may have deviated from the typical specifications. This could have happened for reasons that include cost, product availability or contractor experience.
In her book, Growing up Wright, Lonnie Lovness provides valuable description of her parents’ efforts in creating the original concrete floor at the Lovness Studio. According to Lonnie’s mother Virginia:
The worst thing of all though was pouring the cement floor. We didn’t know how to lay cement that would be slick. So we found an old German cement finisher who had retired, and he showed us ho to di the scoring and slicking. We divided the house into three sections and then the logia. Early in the morning, we would pour the cement, adding iron oxide, so it was an integral part of the cement. But you have to keep trowelling [sic] with boards until until you bring the water up to the surface, so you get the smooth effect, and then you have to score it. Meanwhile you’re standing on narrow planks on top of this. This all went well. But one day we started to do one section and the humidity was high and it didn’t set up. We started early in the morning, and at midnight or so we were still trowelling trying to get the thing right. That was the trickiest thing.
Lonnie Lovness continues:
In the end they [Virginia and Don Lovness] had to break up that section of the floor and start over. Since the red floor was being done in section, they had to be careful to mix the colorant evenly in different batches. Iron oxide is a fine powder, commonly used to tint paint, coatings and concrete. Its even used in cosmetics but in this case D [Don Lovness] bought it in bags of 50 or 60 pounds. As a tinting agent it’s extremely potent and will turn anything it contacts red: your clothes, hands, lumber. It must have been a mess to work with but ultimately they managed to produce a floor that was consistent in color and smoothness. Part of that success is due to D’s deviation from the specs, as explained:
(Mr. Wright) also specified using A.C. Horn . . . for the cement. And A.C. Horn stuff is very expensive. And being at 3M at the time we dealt with tones of iron oxide. So I took some iron oxide and some cement in the laboratory and I measured to see what the saturation point was. And I found it to be about between 2 and 3%. So I figured if I put 4% in it will be even all over the whole thing, which we did, and it worked like a million dollars. (Pages 96-100)
The Lovness process varies from the typical, both in the use of iron oxide and mixing the oxide in the full batch of concrete. As can be seen in the fragment from the Sweeton House at the top of this page, the Colorundum, when installed as specified was a relatively thin layer.
At the 1940 Baird House, the concrete floors did not receive integral coloring. As Theodore Baird writes in his essay “Building a House,” that on January 15, 1941, near the end of construction, “Floors being painted.” This cost-cutting measure has required that the floors be periodically repainted.
Please Note: Each restoration/maintenance/repair situation involving a Wright building requires analysis and research to identify the correct approach. The Conservancy is sharing the information so it may be of use to others as they evaluate their own specific situation and may consider these and other approaches. The Conservancy strongly recommends that owners consult with a design professional and an experienced contractor to determine which method is best for their specific projects.
Further, the provision of this information or mention of a specific product or products does not constitute endorsement, recommendation, preference or approval by the Conservancy.
Usonian Concrete Floors 101 - Part 2 Repair and Restoration
Usonian Concrete Floors 101 - Part 3 Maintenance
Posted February 8, 2024