NOTE: The following is an updated version of “Anticipating Stained Glass” which first appeared in Construction Specifier Magazine as the cover story. A “metricized”, “anglicized” edit later appeared as “Envisioning Stained Glass” in Construction Canada Magazine. Recently, I was asked to revisit the article for the Summer 2023 issue of the Stained Glass Association of America’s Stained Glass Quarterly.
The original intent was to write an introduction for those who may be unfamiliar with stained glass, yet are faced with integrating it into architecture. Any experienced practitioner of stained glass could write uniquely valid versions themselves. I hope my newest version will continue to serve its original purpose while now also providing useful perspective for newer stained glass artists who find themselves at the opposite ends of their careers from where I now sit. Best wishes!
CLICK photos for captions. . . Technical notes and links in blue may be of interest to practicing stained glass artists.
ANTICIPATING STAINED GLASS
Having been involved in numerous and diverse stained glass commissions, I have learned that although projects and strategies vary considerably, there are recurring questions that usually crop up at the start of a project. Hopefully this article will help you the next time you are asked to anticipate stained glass.
I. What is STAINED GLASS?
Before exploring the integration of stained glass into architecture, it is probably a good idea to attempt to define just what is meant by stained glass…
HISTORY: The rich history of stained glass is long and fascinating. Following is an excruciatingly brief overview of a few developments:
The essence of stained glass is glass, lead and light. By the 11th century stained glass had come of age in the Romanesque cathedrals of Europe. From the mid-12th through 16th centuries stained glass flourished in the many-windowed Gothic Cathedral. During the 1840s the Gothic Revivalists nudged stained glass into more extensive use the public sector. (They also rediscovered mouthblown sheet glass, which they called antique glass.)
The mysteries of stained glass were protected and passed down through the centuries within the European guild system, and later through independent studios. The 1800s saw the stained glass aesthetic begin to reflect contemporary developments in architecture and the fine arts. Also at this time, once anonymous artisans began to leave their names to history. Seeds sown at the turn of the 19th century by Antonio Gaudi, Frank Loyd Wright, Johannes Thornprikker, Anton Wendling, Heinrich Campendonk and others eventually blossomed in a fresh, rethinking of the medium in post World War II Europe by artists including Wilhelm Bushulte, Georg Meistermann, Ludwig Schaffrath, Johannes Schreiter, Hubert Spierling, Paul Weigmann et alii who responded to the sad, gaping opportunities left by the war. Their work inspired a New Stained Glass movement in the 1960s and 1970’s that saw the practice of stained glass move beyond the traditional studio to be embraced by artists from diverse backgrounds and media. Like it or not, after centuries as a hard-won and carefully guarded secret, the stained glass cat was let out of the bag by artists on both continents including Josef Albers, Patrick Reyntiens, Paul Dufour, Peter Mollica, and Otto Rigan.
Other techniques have arisen since Gothic times. For better or worse, John Lefarge introduced opalescent cathedral glass in New York around 1875. In the late 18th and early 19th centuries Louis Comfort Tiffany popularized the copper foil technique, especially useful for 3-dimensional objects such as lampshades for which his studio became famous. Another innovation, variously called faceted glass, dalle de verre, or slab glass was developed by Jean Gaudin in Paris in the 1930s, wherein 1”-thick± dalles or slabs of glass are cut, sawn or chipped (faceted) into pieces. The pieces are then arranged on a thin bed of sand in a tray-like mold into which opaque epoxy resin is poured between (but not under or on top of) the glass chunks. Once cured, this produces a monolithic panel that functions like a thicker, heavier version of its leaded glass cousin.
Stained glass continues the march from its Romanesque and Gothic origins into the 21st century. Much of the current evolution has been led by Derix Glasstudios in Germany. Here in the US, studios including Judson Studios, Devereaux Architectural Glass, Stanton Studios, and others are pushing this technical envelope. In these latest explorations, traditional stained glass’s reliance on lead calme and linework is often minimized or even eliminated. Instead, mouthblown antique glass is etched, stained, and/or painted before being laminated or fused onto large glass “canvasses”. Both in-house design, and collaboration with outside artists have led to a dramatic new aesthetic that often exhibits an airy kinship with painting and other of the graphic arts. In this way, centuries-old stained glass tradition has evolved its way into the most cutting-edge architecture in which more traditional leaded stained glass finds diminishing acceptance. For a dazzling example of this newest direction in stained glass, see Guy Kemper’s “Becoming” cover story in the Fall 2022 issue of Stained Glass Quarterly.
NEVER FEAR: The preceding notwithstanding, stained glass of the leaded variety is still very much alive and kicking in 2023 as is reflected regularly here in the pages of SGQ. No other medium relies on “line” as both dynamic art element and as functional necessity quite like leaded stained glass.
This is probably a good time to point out that the English term stained glass is somewhat of a misnomer. During the 16th century, when English-speaking travelers observed French artisans applying silver stain to their vitraux, they mistakenly named this “new” medium stained glass. By that time staining and painting on glass had indeed transformed a more transparent medium into a wonderfully glowing, if increasingly 2-dimensional, derivation from easel painting.
However, rather than staining glass, each color of glass seen in a non-painted stained glass window is cut from a differently colored sheet of glass. The colors of each sheet are achieved through unique chemical recipes painstakingly developed by glass blowers over the centuries. Leaded glass would have been more accurate, but stained glass is now irrevocably the more accepted term in the English language.
In light of recent developments that embrace surface treatment while eschewing the lead line, the term “stained glass” has come full circle since those 16th century travelers coined it in France. Maybe it’s time to distinguish leaded stained glass from newer permutations. Maybe we should be calling leaded stained glass… well… simply LEADED GLASS. No worries though – at least for the rest of this article stained glass will be understood to mean leaded stained glass.
SO, HOW’S IT MADE? Technically today’s (leaded) stained glass is virtually identical to its historical antecedents. Some tools have been electrified, mechanized or plasticized, and lead calme is no longer cast in molds, but the basic technique for making stained glass windows remains very similar to that practiced in Gothic times. Read Theophilus Presbyter’s description from around 1122 AD for an eerily familiar stained glass “how-to”.
The fabrication of a stained glass panel begins with the enlargement of the design into a full-scale paper cartoon. Working directly on the cartoon, the stained glazier cuts each piece of glass and connects adjacent pieces with strips of lead called calmes. These calmes resemble tiny “I-beams”, and the pieces of glass fit into their grooves. Once a panel has been cut and leaded, all connections or joints between calmes can then be soldered together creating a continuous stained glass panel. Finally the spaces between glass and calme are caulked/puttied/cemented. Many studios rely on their own tried-and-true putty formulas. ASG prefers C.R. Laurence’s CRL’s 528GL or DAP’s 1012 Gray Glazing Compound which work great straight out of the bucket. They’re formulated for a glass-to-metal bond, and are reliably long-lasting. In any case, after excess caulking is removed, the panel is scrubbed clean with whiting (powdered CaCO3) which oxidizes and darkens the lead while polishing both glass and solder.
II. QUICK! . . . Find the Right Artist/Studio
Now that we’ve hit some of the high-points in the history of stained glass and its making, let’s get back to the nuts and bolts of incorporating it into architecture…
The first step is arguably the most important: the early selection of an artist/studio. The sooner an artist/studio can be brought into the process, the more seamlessly and sensitively stained glass can be integrated into a building. Stained glass benefits from careful planning throughout the architectural design process. Because stained glass is created for a specific client, building and site, with adequate planning it can achieve its full potential and resonate beautifully within its architectural context.
During the early stages of the architectural design process, as soon as it becomes apparent that stained glass will be needed, a search should be undertaken to locate a stained glass artist/studio. The client or building committee may undertake a search on its own, or the project design team may be asked to investigate artists and make recommendations, or the services of an art consultant may be enlisted. In any case, care should be taken to identify an experienced, qualified artist/studio that has demonstrated not only the ability to professionally execute the scope of work required, but also one whose artistic style and sensibilities resonate with both the client’s expectations and the architectural vision.
The sooner an artist/studio is retained, the sooner a meaningful collaboration can begin, thereby insuring that all options are considered and optimal decisions are made. An early collaborative starting point allows for a healthy integration of program, architecture, materials, furnishings, lighting, site and budget with the eventual stained glass. The artist/studio can contribute useful perspectives in many areas such as window layout, placement and sizing; solar orientation; view enhancement and mitigation; pricing; and more.
III. Window Framing
The physical interface between building and stained glass is the window framing. The framing system most widely specified for new construction is of the extruded aluminum variety, and will be the focus here. Aluminum framing is available in several anodized finishes, or can be custom powder-coated in any color. Aluminum framing is easily modifiable to accommodate stained glass thanks to the normally ample room on its mullions for the installation of stained glass (see Stop It? below). Other window framing materials (steel, wood, masonry) and systems (curtain wall, casement, etc.) can be similarly detailed to accommodate stained glass.
The architect, civil engineer &/or glazing contractor may need to tweak the sizing of window framing to accommodate additional loading from stained glass. Besides the normal weight of the exterior glazing (see Double-Glazing below), the window framing system also needs to accommodate the added weight of stained glass, whose weight increases with design complexity (more lead-work). For most projects, ASG calculates the weight of stained glass at 4.25 pounds per square foot. If interior projective glazing will be installed over the stained glass, its weight must be accounted for as well (i.e: allow 3.3 pounds per square foot for 1/4” tempered glass).
The following section provides suggestions regarding the sizing of individual stained glass panels and the placement of vertical mullions in large windows.
IV. How Big?
As a general rule, individual stained glass panels should be no more than 10 square feet in area. An interlocking design is usually more rigid than one bisected by straight or nearly straight lead lines that create potential “hinge” lines (grid or diamond designs being extreme examples). To compensate for less stable patterns, individual panels can be made smaller and/or reinforcing can be added (see Reinforcing below).
To determine the exact size of a stained glass panel or panels, start with the actual “see-thru” dimensions of the window frame not including stops. Make sure the window or lite is “square” (90˚ corners with equal diagonals). Take measurements at top, center and bottom; left, center, and right. If measurements vary, use smallest height and/or width measurement.
[Note: Determining the size panels is often easier when done graphically directly on the cartoon.] You don’t want to build stained glass to the exact size of the opening – subtract 1/4” to 3/8” from the height and width of each window/lite opening (1/8’ to 3/16” allowance for all four sides). Assuming 1/8” setting blocks will be used under the (bottom) stained glass panel, subtract another 1/8” from the bottom panel. Finally, if multiple panels will be “stacked” into a larger window/lite, you’ll also need to make room for the H-channel (described below). Allow 1/8″ – 3/16” for the heart of the H-channels which includes a small allowance.
After all the subtractions, what’s left are the exact dimensions of the stained glass panel(s) itself/themselves.
Large windows must be subdivided in order to accommodate stained glass. Their exterior glazing, usually I.G.U.s (see below), is separate from, and continuous behind the stained glass. When installed into a structurally sound window frame, a stained glass window can be just about any size. Still, it is important to acknowledge stained glass’s limitations – it is not as rigid or structurally sound as commercial glazing. When subdividing large windows, whenever possible, space vertical mullions a maximum of 48” center-to-center (O.C.). The placement of horizontal mullions within a large window frame is not as critical. Stained glass panels can be stacked into vertical openings of any height that commercial (exterior) glazing can span.
Stacking can be accomplished with H-channel or T-bar “saddle-bars” between vertically stacked panels. My studio uses aluminum H-channel (1” x 7/16” overall) available from C.R. Laurence. The dark bronze duronodic finish (CRL H14DU) works best with darker glass palettes and frames, while the brushed-silver (“clear”) anodized finish (CRL H14A) works well with lighter palettes and framing.
Because both ends of each H-channel are connected to the window frame, the weight of stained glass above is grounded directly into the window frame and does not bear on stained glass below. This effectively eliminates any tendency for lower sections to warp and buckle as is sometimes seen in older stained glass installations.
During installation of stained glass, the ends of the H-channel are connected to the window frame with hex-head sheet-metal screws (#8 x 1/2”) screwed into the vertical mullion at appropriate heights within the installation pocket (see Stop It! below). First mark screw locations (heights) centered in the installation pocket, drill a 1/8” pilot hole, and screw in the screws. Then cut the H-channel 1/8” to 3/16” narrower than the width between mullions to allow for screw heads. Before setting any stained glass, test-fit all H-channel by sliding their lower “notch” down over the screw heads. Too tight? Trim the H-channel. Too loose? Back screws out a half turn.
V. Stop It!
For aluminum windows, the “installation pocket” for stained glass can be easily created by adding two sets of stops (sashes) to the window framing. The addition of stops to window frames is best handled by the glazing contractor when the are installing the window frames. Stops should be aluminum U-channel or square tube ranging from 1/2 x 1/2” to 3/4 x 3/4”. They should match both finish and color of the window framing, and be attached with screws spaced 18” to 24” on center. See Typical Stained Glass Installation.
Both sets of stops should be continuous around the perimeter of each window (or each component lite comprising larger windows). The first (outer) set of stops acts as a spacer between the stained glass and exterior glazing (see Double-Glazing with I.G.U.s below). This outer, spacer stop can be located anywhere between the exterior glazing and the inner face of the window mullions. We prefer to center the stops within the available depth of the framing, thereby creating a pleasing reveal. Besides, it is difficult to install stops perfectly flush with the face of mullions. The second (inner) set of stops completes the “installation pocket”, and will secure the stained glass panels (and, when needed, the H-channel described above) into the window frame. Screws for inner stops should be loosely hand-tightened by the glazing contractor to prevent snapping off of heads or stripping of threads when removed for the eventual installation of the stained glass.
My studio normally requests that the stops create a 1/2” thick installation pocket which will accommodate a 5/16” stained glass panel plus glazing tape, as well as aluminum H-channel when required (see How Big? below). When protective glazing over the stained glass is required, the pocket-thickness needs to be increased accordingly. When the Artist/Studio arrives to install the stained glass, the inner set of stops are removed, and single-sided glazing tape applied to both stops (1/8” x 1/2”: CRL 74418x12BL or 1/4” x 1/2”: CRL 74814x12BL). The stained glass panels can then then centered within its window (or lite within a window) using neoprene setting blocks of varying thicknesses (i.e. CRL SB562), and spaced every 6” to 12”. Finally, the stained glass is secured in its window or lite by reattaching of the inner set of stops.
ASG prefers to avoid window framing systems with integral, pressure-applied (snap-in) stops/sash. These have been designed specifically for stained glass, yet we have found them to be problematic. During an installation in New England we were nervous wrecks after having to resort to hammers to seat the sash around four 8’ diameter windows 24’ above the floor. This is only one opinion – many studios swear by these, so do some checking and decide for yourself. YMMV. . .
Most new windows are glazed with Insulated Glass Units (I.G.U.s – see below). I.G.U.s, or any other exterior glazing, provide weathertight protection against the elements, making it unnecessary to hermetically seal stained glass into a window frame. Do not use silicone, double-sided glazing tape, etc. to seal stained glass into window openings. Doing so increases chances of condensation. From Texas to Alaska and beyond, my studio has avoided condensation problems simply by allowing the void space created between stained glass and exterior glazing to breathe or vent into the building interior. When applying foam glazing tape to the inner faces of stops, we intentionally leave 2”-3” gaps in the tape every 24”-48” to allow venting between the void space and interior air. Setting blocks along the bottom edge of the bottom panel allow for additional breathing.
Now for the stained glass! Drum roll please. . . With inner stops still removed, once the bottom panel of stained glass is set and centered on setting blocks snuggly against the outer stops, the bottom “slot” of the pre-fit H-channel can be slid snuggly down over its screw heads and previously set stained glass panel below. The upper edge of the next stained glass panel can then be slid down into and centered in the upper “slot” of the H-channel. Repeat the until the window or vertical lite is filled with stained glass. Screw the inner stops back in (non-adhesive side of foam tape against the stained glass) for a snug fit. Voilà.
One final note about stops: Even when future stained glass is identified at the start of a project, future stops often aren’t specified as part of the initial window framing. Later, the expense of retrofitting windows with stops can take a bite out of later stained glass phases. This can be avoided if future stops are specified and installed during the initial construction phase. Beside cost savings this also assures a precise contractor-fit and color-match with the window frames.
The size (area) of individual panels of stained glass can be increased beyond 6-10 square feet with reinforcing. Historically reinforcing was achieved with steel round-bar wired to the back of stained glass and dowelled into the sides of the framing. Today, 1/8″ by either 3/8″ or 1/2″ galvanized steel flat-bar soldered to lead on the back of the panel is more common. Reinforcing counters the forces of gravity, wind, air pressure flux, and thermal cycling which can cause buckling, bending and lead to the failure of stained glass. If reinforcing is an integral part of the stained glass design, rebar can be shaped to follow strategically placed lead-lines and effectively camouflaged. Another approach is to space rebar at 1/2 or 1/3-2/3 points across panels which makes the rebar somewhat less obtrusive. Plan on additional time and packing when crating reinforced stained glass. Corrugated cardboard or foam sheet can be layered between panels and rebar to offset the depth of rebar.
VII. Double-Glazing with I.G.U.s
Until the 20th century, stained glass was the only glazing in windows. The exposure of stained glass to the elements accelerates weathering of glass and oxidation of lead leading to more frequent repairs, re-leading and leak control. By the 1950s acrylic sheet (Plexiglas® and later Lexan®) became available for protecting stained glass from weather, birds and B-Bs. Acrylic sheet was easy to retrofit over existing stained glass, but also tended to yellow and scratch, and often has a “bulged out” look.
Today just about all stained glass for new commercial projects is installed into window framing with an exterior glazing of I.G.U.s (or other commercial glazing). I.G.U.s are comprised of two (or more) panes of glass separated by an insulating void space. When stained glass is installed behind the I.G.U.s, etc., a “double-glazed” condition is achieved. Double-glazing not only protects the stained glass from outside threats, but creates an additional dead-air space with increased thermal insulation (higher R-values).
Architects and engineers usually specify coatings and tints for I.G.U.s. It is critical for the stained glass artist/studio to determine early during a project what coatings have been specified for exterior glazing. If highly a reflective coating or dark tint have been specced for the I.G.U.s, the fact that it will significantly obscure outside visibility of the stained glass should be brought to the attention of owner and architect. Sometimes it is hard to get the exterior glass spec changed – the glass coating is often an integral part of the architectural aesthetic as well as critical to the building’s energy efficiency and HVAC calculations. On the other hand, mechanical engineers may be able come to your aid once they appreciate the insulating property of the void space between the I.G.U.s and your stained glass.
VIII. Protect Who from What?
There is a difference between safety glazing and protective glazing even though there is usually a functional overlap. Safety glazing is intended to protect people from harm from stained glass and is mandated by building codes. Protective glazing is used to protect stained glass from harm from people and is dictated by common sense. Both can reduce cleaning and scheduled maintenance, and minimize repair and conservation expense.
Requirements for safety glazing are found in building codes that vary from city to city, state to state, and country to country. It is beyond the scope of this article to address the specifics of building code requirements. Whether the prevailing code(s) are based on International Building Code, local or other authority, specific code implications for your stained glass need to be determined for every project. In the past, code requirements specific to commercial glazing was misapplied to stained glass. It is worth noting that in 1978, thanks in no small part to lobbying by the SGAA, the U.S. Consumer Product Safety Commission adopted guidelines directly addressing stained glass. In a nutshell, these guidelines exempt stained glass from many/most of the restrictive requirements imposed on conventional glazing materials.
Bottom Line: Unless you have experience researching building codes, leave code compliance to the engineers, architects and general contractors you work with. Simply making sure to ask them about any implications applicable building codes will have on your stained glass should get the ball rolling.
Whether codes mandate safety glazing or not, you still should evaluate your project’s stained glass for potential exposure to damage by patrons, cleaning crews, deliveries, vandalism, etc. Double-glazing as described above has the exterior covered, but what about damage from the inside? Where exposure is great (i.e.: in public spaces within 96” of finish floor level or within arms’ reach), protective glazing can minimize exposure. By simply installing tempered glass over stained glass, most risks can be minimized. After widening the installation pocket appropriately, and applying single-sided glazing tape around the perimeter of the protective glazing for cushion, it can be sandwiched against the stained glass during installation.
Where interior installations (doors, partitions, etc.) are exposed to people-traffic, etc. on both sides, triple glazing is a simple way to protect the stained glass and people from each other. This can be accomplished by sandwiching the stained glass between two layers of tempered, laminated or plate glass. Double- and triple-glazing also minimizes scheduled maintenance. Add single-sided glazing tape around the perimeter of both protective panes before sandwiching the stained glass between them.
On a few occasions we have had our stained glass encapsulated inside the void-spaces of I.G.U.s. This approach has the obvious benefit of “built-in” protection for the stained glass. But be forewarned: IGU manufacturers normally will not assume responsibility for any stained glass damage while it is in their possession, so make sure this risk is included in your Fine Arts coverage, or plan to self-insure. Encapsulation needs to be carefully scheduled and coordinated with the I.G.U. manufacturer. The precise size requirements for your stained glass must be determined with the I.G.U. manufacturer. It is critical to synch you schedule with that of the I.G.U. factory as well as other trades on the project. This is especially true for out-of-town projects. Finally IGU failure rates have been steadily improving, but still can approach several percent, especially when there’s a significant elevation change between the I.G.U. manufacturer and job site (micro-capillary tubes can help equalize air pressure).
Time is of the essence. Retain a stained glass artist/studio as soon as possible to allow full collaboration with the client or building committee and the rest of the design team. By asking the right questions early, a much smoother, cost-effective integration of stained glass into architecture will be assured. Quickly identify locations where stained glass is desired, including future phases, and make sure the framing for these windows is specified to be “stained glass ready”. This includes spacing vertical mullions no more than 48 inches center-to-center whenever possible, and the installation of required stops (sash) by the glazing contractor into their window frames.
When a proactive approach is taken, stained glass can be fully utilized as an integral part of the architectural whole. Whether stained glass is envisioned as an understated detail, or as a primary focal point, it can achieve its fullest potential when explored from the start while all options remain open.
Jeff Smith was part of the groundbreaking stained glass program established by Paul Dufour at Louisiana State University in Baton Rouge in the mid 1970’s. He later studied with German stained glass designers Ludwig Schaffrath and Johannes Schreiter. Smith founded Architectural Stained Glass, Inc. in 1977. His critically acclaimed stained glass can be found from Alaska to Florida and from New Hampshire to Hawaii. Past awards include AIA/IFRAA’s International Design Arts Award on four occasions and inclusion in Corning Museum’s New Glass Review on two occasions. In 2000, after 23 years in Dallas, Smith, his wife, Pam, and ASG relocated to the Davis Mountains of west Texas.