Prism glass: the stained glass used in storefront windows historically found in commercial buildings on main street. The shimmering, textured surfaces of original prism glass transom windows — running the width of a historic storefront above the main display windows — can be a rare treat when they’re intact.
Sometimes referred to as clerestory windows, traditional transom windows on main street were sometimes equipped with special prism glass: sheets of glass composed of small panels, each roughly 15 cm square, featuring a distinctive prismatic or wedge-like pattern cast into the interior-facing surface (see photograph below). Some panels also featured patterns cast into the outer face too. Assembled into panels using cames, or narrow bars of lead or zinc, prism glass transom windows were popular in pre-World War I commercial buildings when interior electric lighting was relatively weak and inefficient. With sunlight free and plentiful, especially in Alberta, the prismatic panels were oriented in such a way as to capture incoming light and refract or deflect it deep into the store within.
Given their relative rarity, the analogy to stained glass windows of churches and other buildings isn’t entirely out of place. Like stained glass, the glass itself is often coloured: manganese added during the manufacturing process to make the naturally green glass clearer often turned a purplish colour after decades of exposure to ultraviolet radiation in sunlight. (The colour also appears in some old telegraph line insulators and the now almost-vanished sidewalk “skylights” — which are a story of their own). You can spot prism glass in certain parts of Calgary and Edmonton and on some of Alberta’s historic main streets, although it is easily mistaken with so-called “reeded” glass, similar glass panels impressed with narrow ridges rather than the unique prismatic profile specially oriented to pull sunlight into store interiors. Sometimes only close inspection can distinguish the two.
Why is prism glass so rare now? Like many features of historic buildings, particularly in the commercial district, these components were subject to changing fashion, retailing trends and, of course, the rigours of our climate. The benefit of prism glass and transom windows declined as electric lighting improved in efficiency and affordability, particularly with the mid-century introduction of fluorescent lighting. Prism glass transom windows were traditionally paired with retractable awnings, which allowed the store’s proprietor to regulate the amount of light coming into the store. As awnings went out of fashion and were removed, the transom windows often pulled in so much sunlight that stores were often too brightly lit, hot and stuffy. The unrelenting sunlight would also fade display items.
Physical deterioration of course also played a part: decades of thermal expansion and contraction, exposure to moisture, and wind loading took its toll on the delicate assemblies, causing cames and soldered joints to fatigue or break, with the eventual result that many transom windows were removed or covered with painted plywood sheets. Changing retail fashion assisted the process: as automobile use increased throughout the twentieth century, so did the need for larger, illuminated signs. Prism glass and transom windows, having outlived their original purpose, were covered over or removed with “slip cover” remodeling of main street storefronts in the 1930s onward.
Today, historic transom windows, and especially those of rare prism glass, are coveted historic building elements that many building owners want to restore. Sometimes intact original windows are preserved behind later materials; often, however, they need extensive repairs. Where that’s the case, there are excellent resources to assist in restoration.
For information on conservation prism glass or other elements of a historic resource, contact your heritage conservation advisor.
Written by: Fraser Shaw, Heritage Conservation Advisor.