In my previous post I wrote of the convergence of social, economic, and technological forces that lead to the greenhouse’s emergance during the 18th century in Great Britain, and that the Volunteer Park Conservatory (VPC) inherited this lineage. Today, I will probe into some of the tectonic characteristics of the greenhouse.
Iron and glass were certainly around prior to the invention of the greenhouse. The Iron Age pre-dates much of recorded history, and glass was present in ecclesiastical buildings and homes of the nobility. The industrialization that began in Great Britain in the 18th century made available glass and iron in greater quantities and at lower expense. Although their industrial uses (especially that of iron), were well established when greenhouses emerged, with their previous deployment primarily in bridges and machines. In the case of bridges, iron replaced stone or wood framing, and in machines, wood framing. Iron (unlike the later developed steel that is almost exclusively used in contemporary structures) has relatively low strength in tension and in that respect is similar to stone and masonry; however, iron is stronger and more flexible than stone yet it still shares with stone the characteristic that it ideally suited to be used compression. What it also has in addition to its great compressive strength is the ability to be cast with precision, and is better in that regards than steel as is iron’s resistance to oxidation. These qualities made it an ideal partner in the tectonic and special development of the greenhouse, which is made up of many small parts, connected by precision cast parts fastened together. Greenhouses are also high humidity environments, making corrosion important. While steel has replaced iron in structures, glass manufacturing and uses in building was different than it is today. Modern float glass, where molten glass is cast as liquid on molten beds of tin, was not developed until the 20th century. Prior to that, glass was hand (and breath) forged, by blowing through tubes, making spheres that were later flattened by rollers (a process still used today for artisan glass). This blown process limited the size of individual panes of glass to that which could be handled by this hand and breath forged process. I also suspect that the manufacturing process used prior to float glass introduced many imperfections in the thickness of glass (do to rolling), as well as the inclusion of air bubbles (from blowing) and other impurities and thereby reducing its inherit strength and spanning abilities.
In order to use these two very appropriated to the task (of constructing greenhouses) materials, designers had to incorporate forms and tectonics that worked within their respective limits. For iron, many of the forms used were based upon those from stone or masonry construction (namely the arch), although the dimensions spanned by iron and the size of the members are both greater and smaller respectively. This does not represent a stasis is tectonic evolution from stone to iron, as iron spanned farther and with smaller members. Iron also had the previously unknown quality of being able to be cast into any number of repetitive pieces, making pre-fabrication and its economies of manufacture and speed of assembly possible, both necessary properties for the ever increasing sizes of greenhouses. The arch appears in the roof form of many early greenhouses, including that at Volunteer Park. Infinitely lighter, more delicate, and longer spans were possible than those previously obtained in stone, and these iron arches with their slenderer members also allowed for a previously unseen amounts of glazing to structure, with buildings for the first time dominated by glass (a non-structural element), rather than being dominated by a building’s masonry structure. A building dominated by glass was, or course, perfectly suited for providing habitat for a host of tropical and other exotics on the princely estates in England. In addition to its obvious property of allowing for daylight to enter and promote photosynthesis, glass also has another key characteristic – it traps the infrared radiation in sunlight, warming the air temperature within the greenhouse.
This phenomena occurs because the incident wavelength of solar radiation is such that it passes through glass, but the reflected radiation is of a different wavelength and is unable to reflect back through the glass, resulting in a concentration of infrared radiation and a warmer environment (sometimes getting too warm, as noted by the white washing of the glass of the VPC during the summer months).
With a new type of structure in place, a new way of glazing had to be pioneered to exploit it. To best utilize the modular benefits of iron construction, glass became modular itself, and to a certain extent self supporting by following many of the same geometric rules that its supporting iron structure took: repetitive, small pieces laid out in geometries that suited their strengths and made installation quick and efficient. The glass of these early greenhouses was manufactured within the limits of the available technology, and was small and thin in its dimensions, and its installation reflects these limitations. Installed as one would install shakes on a roof (a traditional construction technique), glass was shingled within the iron frame that supported it. Oftentimes, the glass was installed in a folded plate configuration as in The Great Stove (reference Building the Future, page 23)., similar to say a folded plate in concrete construction (think of a sheet of paper with a crease along its length, and how that adds rigidity to the paper were you to hold it on its short end. These folds also created channels to allow rainfall to drain off the building. Without the convenience of modern sealants, glass house pioneers used the overlap of the glass provided by their shingling to keep out water, with the bottom edge scalloped to ensure the water droplets fell from one piece of glass to another as they cascaded down the glass skin. The shingling, and its open joint, may also have provided ventilation and aided in temperature regulation of the greenhouse. This shingle approach can be seen today in the works of such prominent architects such as Peter Zumthor in his Art Museum in Bregenz (1990-97), albeit on a different type of building, and with the benefits of modern engineering and manufacturing.
All images from the VPC, taken this September.
To find out more about the Volunteer Park Conservatory: http://www.volunteerparkconservatory.org/
To find out how you can support Seattle Parks: http://www.seattleparksfoundation.org/
Next time: The social impacts of this new architecture end its enduring impacts.