“There is no escape from matter.”
–Robert Smithson
Our range of vision has something to do with the wavelength of light, but vision also depends on cognition. We filter much of what we see, editing incoming spectral signals to suit our purposes. Many aspects of the built environment within the visible spectrum are imperceptible to us, camouflaged by preoccupation and habits of seeing. Analogues in architectural design are represented by the things we often take for granted when we detail buildings–concrete topping, double glazing, door closers, mortar, thresholds, and the like. The hollow metal doors that you opened today–leaving your office building, walking into your apartment, exiting the parking garage–exist in that ambiguous visible-invisible zone of comprehension and interest–the property of door schedules, and a means to an end. Taking advantage of the mandate of the Centre for Architectural Structures and Technology (CAST) as a site for experimental construction and material investigations, researchers at the University of Manitoba’s Department of Architecture initiated a study of alternative exterior door finishes.
Hollow door manufacture
The material typically used in hollow metal doors is 18-gauge cold-rolled A40 sheet steel, wipe-coated with zinc. This product is corrosion-resistant and paint-ready, and does not require priming. The steel is “tension-leveled” to reduce oil canning. There are three available cores: kraft paper 1* cell size honeycomb; polystyrene insulation (R7); and isocyanurate (R11).
The sheet metal is prepared on a Computer Numerically Controlled (CNC) punch press. Its automatically-selected 19 interchangeable dies are fast and accurate, and minimize set-up time. The steel is then brake-formed with folds of approximately 1/16* radius for interlocking assembly. Inner and outer sheets are glued to the core with a water-based water-activated polyurethane adhesive, and require three hours under pressure for bonding and setup.
Door frames are similarly sheared and die punched, and prepared for hardware, end miters, knockouts, hinges and lock strikes. The frame components are also brake-formed, reinforcements are attached using projection welding, and the components are snapped and welded together prior to shipping.
Metallizing
Metallizing is an electric arc process that uses electric current to super-heat metal to between 7,000 and 12,000 degrees. This atomized metal is sprayed onto a substrate using compressed air. The metal particles lose a considerable amount of heat and bond with the substrate at less than 300 degrees. The metal can be applied in thicknesses from three mils (3/1000ths of an inch) to more than 100 mils. The resulting bonded metal can be polished, buffed and coated with secondary protective layers and paints.
Our terms of reference included both aesthetics and durability. There are at least six types of corrosion: uniform corrosion of the surface (most common); pitting, where small random holes allow corrosion to develop beneath the surface; crevice corrosion, a concentration of electrolytes in a small space (usually a seam); poultice corrosion, generated by debris trapped against the metal causing variations in electrolyte concentration; galvanic corrosion, from contact between dissimilar metals; and intergranular corrosion, where active elements migrate across grain boundaries, causing selective corrosion.
For the CAST doors we selected an aluminum, bronze and nickel alloy, notable for its low cavitation, durability, and high corrosion resistance. Its normal use is in highly corrosive environments–for example, saltwater pumps in power generation plants and natural gas compressor pistons. The high bond strength of this metal is enhanced by the nickel content, with increased hardness and resistance to thermal and mechanical shock. Not all thermal spraying machines are equal. The Tafa 8830 electric arc machine is designed for a uniform and smooth spray pattern, and therefore an ideal choice for architectural finishes.
To test the viability of this application, we engaged in a scaled series of experiments in collaboration with the supplier and the applicator. First, two 300 300 mm samples with a build-up of 45 mils on 1/4* steel plate were prepared to confirm colour, pitting size, and workability. One of the samples was polished to a highly reflective finish. A 600 600 mm sample of A40 steel was coated with 10 mils dry film thickness (DFT). Finally, a one metre wide section of a polystyrene-insulated hollow metal door was tested. Forensic examination of this sample revealed no visible heat damage to the insulation and there was no de-lamination between the core and the steel.
Having confirmed the viability of the application we specified a three pass/8-10 mil application, adding approximately 12 lbs. of metal to each of the large 3,150 mm high 900 mm wide doors. Following the advice of the applicator, we specified a spray application of two coats of water-based Varathane Satin Sealer. Alan Schmidt of Carlson Decorating Ltd. notes that “sealers should always be specified when the total dry film thickness (DFT) of metallizing is relatively thin–i.e., less than 30 mils DFT, or when the metallized surface is not machined or buffed to close porosity.”
The final application process involved four steps: a solvent wipe; a light brush blast with fine silica sand at low pressure in order to preserve the zinc coating; the metallizing of 8.0 to 10.0 mils DFT; and a coating of clear water-based Varathane. The installation of the doors was reviewed to insure minimal contact between the metallized surface and other metals and materials. Stainless steel hinges and hardware were installed, and the zinc-coated astragal and weather-stripping were fastened to the inside of the door in order to prevent galvanic corrosion.
In the context of day-to-day architectural practice, we buffer ourselves from matter with abstract working drawings, shop drawings, specifications, and cost calculations. CAST is intended to transcend this barrier of abstraction by providing a platform for in-depth material investigation and new product development in collaboration with the building industry. The hollow metal doors served as a case study to test the feasibility of this prospect, shifting the balance from abstraction (the realm of the mind) to material experiment (the realm of matter).
To paraphrase Robert Smithson, matter matters.
Herb Enns is a Professor in the Department of Architecture, University of Manitoba, principal of Herbert Enns Architect and a contributing editor to Canadian Architect. Thanks to Mark West (Director, CAST); Edward Redekopp, David Sargent, Jacobus (Jack) Poortenaar, (Greensteel Industries Ltd.); C.J. Bruce Merwart (Eutectic Canada Inc.); Alan Schmidt, Ray Pelletier (Carlson Decorating Ltd.).
Below left: an aluminum, bronze and nickel alloy was selected for the CAST hollow metal doors for its low cavitation, durability and high corrosion resistance. Below: Metallizing involves the use of electric current to super-heat metal so that it may be atomized and sprayed onto a substrate using compressed air.