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.
RETROactive 
Another glass product used historically but rare these days were sidewalk prisms: Edmonton, Calgary, even Lethbridge allowed building owners to build “vaults” underneath public sidewalks adjacent to their buildings. To bring sunlight into these spaces, glass “prisms” – a solid glass pyramid usually about six inches on all sides – were installed into the sidewalk to light the basement vault.
While I haven’t seen any recently in Alberta, Victoria and Seattle both have tours of their underground naturally lit sidewalk vaults. New York City has an active program that encourages building owners to maintain and preserve their sidewalk vaults, as naturally lit sidewalk vaults are precious spaces in crowded cities.
Darrel, thanks for those observations. The last sidewalk vault I recall was beside the former Alec Arms Hotel in Lethbridge. I believe it was removed several years ago, but I understand some of the glass itself is conserved at the Galt Archives. I think there may still be one in the recessed entrance (technically not the sidewalk, I suppose) of the Carson Block in Inglewood in Calgary. Locating these would make a great scavenger hunt! — Fraser
There are sidewalk vaults extant in Edmonton, at the Gibson Block, and at the MacLeod Block. In both instances, while the sidewalk vaults remain, the sidewalk prisms do not. Indeed, these would make for a tremendous scavenger hunt. Cities like Victoria and Seattle have turned sidewalk vaults into tourist attractions.
That’s interesting to hear what other cities are doing for their tourists, thanks for sharing your info with us Darrel!
It is a shame that this “technology” of the past is rare to find. I actually collect these tiles and have just started a web page geared toward people who also collect (I only know a few). I have been so impressed with these simple glass panes I actually started rebuilding them (in smaller scale) for fun, more for hanging in windows then actually being a true weather tight window pane. The old zinc came usally gets very brittle (rotten) after 100 years, but ones made with copper came obviously have done way better. Once the window putty (which is oil based) dries out the windows start leaking water and letting in drafts. Then the whole window has to have the old putty cleaned out and re-applied. I’m not sure if this can be done in place or if the whole pane has to be removed. Reguardless, I’m sure a lot of building owners just didn’t want to deal with it. And, as stated in the original post, electricity moved in, dropped ceilngs for air conditioning and heater ducts hid them on the inside and big storefront signs on the outside. When I look at old buildings now, sometimes I can tell there are covered windows, if they are plain glass or prismatic is unknow. I would also guess many building owners don’t actually know if these windows are in there buildings or not. If anyone is interested in my site it’s – http://luxferprismglasstilecollector.weebly.com/
I have links to some restoration work and to information on the tiles themselves. I welcome anyone to share pictures of intact windows or individual panes. There are a lot of great geometric designs.
Jeff, thanks for your comments and the link to some great examples of prism glass. The repairs I’ve seen have all been in a shop, where it’s easier to work on a flat surface in controlled conditions. This also allows reinforcement to be added if needed. Southern Alberta in particular gets frequent high winds which contribute to fatiguing of these types of window assemblies. — Fraser