Anticipating Stained Glass

[NOTE: The following is an updated and expanded version of “Anticipating Stained Glass” which I contributed as the cover story for Construction Specifier Magazine. A “metricized”, “anglicized” edit of this new version appeared as “Envisioning Stained Glass” in Construction Canada Magazine.]

Having been involved in numerous and diverse stained glass commissions since 1974, I have learned that although specific solutions 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.

What is Stained Glass?

Before exploring the integration of stained glass into architecture, it is probably a good idea to define just what is meant by stained glass…

We might as well begin by acknowledging that the English term stained glass is actually a misnomer. Early in its history in mainland Europe, painting on glass had transformed what began as a transparent, three-dimensional experience into a two-dimensional, if glowing, derivation from easel painting. When English travelers first observed French glass painters creating their vitraux, they coined the English term stained glass to describe this novel medium. Rather than staining or painting color onto glass, each color in a stained glass window is actually cut from a separate sheet of glass. The color derives from unique chemical recipes prepared by the glass blower. Leaded glass would have been more accurate, but stained glass is now the irrevocably accepted term in the English language.

In any case, by the eleventh century stained glass had come of age in the Romanesque cathedrals of Europe. From the 12th through 16th centuries stained glass blossomed in the many windowed Gothic Cathedral. Modern stained glass is practically identical to its historical predecessors. Some of the tools have been electrified, mechanized or plasticized, but the basic technique remains the same as in ages past.

The fabrication of a stained glass panel begins with the enlargement of the stained glass design into a full-scale paper cartoon. Working directly on the cartoon, the stained glazier cuts the 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 are soldered together creating a continuous stained glass panel. Finally the spaces between glass and leading are caulked with glazing compound, the excess caulking is removed and the panel is scrubbed clean.

Other stained glass techniques have evolved since Gothic times. Louis Comfort Tiffany popularized the copper foil technique. Due to its relative brittleness, copper foiled stained glass is much less resilient than leaded stained glass and is seldom specified in large architectural applications. Another technique, called facetted glass, slab glass or dalle de vere, was developed in the early 20th century and is made from thick dalles or slabs of glass joined by a plastic resin matrix. In Europe this technique is often used as a structural, load-bearing element, but in North America it is used much like its leaded glass cousin. Other techniques that attempt to simulate stained glass include overlays and printed laminated glass, but, at best, these are only approximations that do not approach the sparkling lucidity and subtle, saturated color of true stained glass.

First Step

Now that we have a basic idea of what stained glass is, let’s get back to the nuts and bolts of incorporating stained glass 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 the building. Stained glass benefits from careful planning during all architectural design phases. Because stained glass is created for a specific building and client, with adequate planning, it can achieve its full potential and resonate beautifully with the architecture and client on many levels.

The Right Artist/Studio

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 project design team may prefer to investigate specific artists themselves, or they may decide to enlist the services of an art consultant. Regardless, 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 involved, but whose artistic style and sensibilities are compatible with both architectural concept and client expectations. The sooner a studio is retained, the sooner a meaningful collaboration can begin, thereby insuring that all options are considered and the best decisions are made.

Besides responding to the unique needs of the architecture and client, the artist/studio can provide useful perspectives in many areas that will affect the integration of stained glass into the building. An early collaborative starting point allows for a healthy cross-pollination between architecture, materials, furnishings, lighting, landscape and site. The artist/studio will provide specific recommendations on topics such as window detailing and sizing, solar orientation, view enhancement and screening, cost options and budgetary considerations.

Window Framing

The physical interface between building and stained glass is window framing. The framing system most widely specified in new construction is of the extruded aluminum variety, and will be the focus here. Aluminum framing is easily modifiable to accommodate stained glass thanks to ample space on its mullions to install stained glass. Other window framing materials (steel, wood, masonry) and systems (curtain wall, casement) can be similarly detailed to accommodate stained glass.

The sizing of structural members may need to be increased to accommodate additional loading from stained glass. Besides the normal weight of the exterior glazing (see Double-Glazing below), the window framing system must also accommodate the weight and thickness of the stained glass. The weight per square foot for stained glass will vary depending on design complexity (amount of leading). For most commissions, we recommend 4.25 pounds per square foot for calculating the weight of stained glass.

How Big?

When installed in a structurally sound framing system, a stained glass window can be any size. However, large stained glass windows must be subdivided into lites that are smaller than with conventional window glazing. This is because, unlike conventional glazing, stained glass is not inherently as rigid or structurally sound. As a rule of thumb, individual panels of stained glass should not exceed 6 to 8 square feet. Stained glass designs bisected by numerous lead lines (such as grid or diamond motifs) contain potential “hinge” lines and usually must be divided into relatively smaller component panels than does a design with interlocking linework.

It is important to acknowledge stained glass’ size limitations when subdividing large windows with vertical mullions. As a general rule, vertical mullions in a window should be spaced on centers 42 inches apart. In extreme situation this may be extended to 48″ O.C. at additional expense. The placement of horizontal divisions in window framing is not critical. Stained glass panels can be stacked into vertical openings of any height that conventional glazing can span. Stacking is accomplished by installing saddle-bars such as steel or aluminum T, angle, or H-bar between stained glass panels. These bars connect the upper edges of lower panels to the lower edges of the panels above and are independent of the exterior glazing. This allows the weight of upper panels to be grounded directly into the mullions and not onto panels below thereby preventing buckling in lower panels. The exterior glazing will be continuous behind the vertical “stack” of stained glass panels.

Reinforcing

The size of a individual panels of stained glass can be increased towards 10 to 12 square feet with reinforcing. This reinforcing can take the form of galvanized steel bars soldered or wired to panels of stained glass. Although round bar has been the historic norm, we normally use 1/8″ by either 3/8″ or 1/2″ steel flat-bar for bracing. In either case reinforcement resists the forces of gravity, wind, air pressure variation, and thermal cycles which can cause buckling, bending and eventually failure of the stained glass panel. Steel reinforcing can adversely affect the design, installation and cost of stained glass.

Double-Glazing

Historically stained glass was the only glazing within masonry window openings and traceries. Only since the 1950’s has double-glazing become commonplace. Prior to this practice, exposure to the elements meant that stained glass windows had to be re-leaded and repaired from time to time, and were more susceptible to buckling. For these as well as general maintenance reasons, just about all modern installations rely on double-glazing. An exterior glazing of plate glass, tempered glass, laminated and/or or insulated glass units protects the inner glazing of stained glass from the elements, birds and B-Bs. Double-glazing with Lexan® or Plexiglas® is not recommended due to the inevitable “bulged out” look that detracts from exterior views. In the past such acrylic glazing was easiest to retrofit over existing stained glass windows, but other options now far outweigh it for new construction. As an added benefit, the airspace between the exterior glazing and the stained glass creates a storm-window condition providing additional thermal insulation.

Stop It!

The thickness of a stained glass panel is approximately 5/16″. We typically request that the framing system provide a 3/8” thick installation pocket to allow for glazing tape, shims and setting blocks. [NOTES: 1) For taller lites this pocket should be 1/2″ to accommodate horizontal saddle bars between stacked panels; 2) If protective glazing is required, the pocket-thickness will need to be increased accordingly.] See Typical Installation Detail.

For aluminum framing systems, this pocket can easily be created with the addition of two sets of stops (sashes) to the normal window framing. These stops should be aluminum U-channel or square tube ranging from 1/2 x 1/2” to 3/4 x 3/4” and match finish and color of the framing.

Both sets of stops are continuous around the perimeter of each window lite where stained glass is planned. The first set of stops will act as a spacer between the exterior glazing (and associated gaskets or seals) and the stained glass. This spacer stop can be located anywhere between the exterior glazing and the inner face of the window mullions. We usually prefer to recess the stained glass as close to the exterior glazing as possible thereby creating a pleasing reveal. The second set of stops will hold the stained glass panels (and horizontal saddle bars if required) within the frame against the spacer stops. Specify that screws securing this second set of stops are either perfectly perpendicular to the framing or angled slightly away from the plane of the glazing, but never angled into the plane of the future stained glass. Normally screws are spaced 12-18 inches apart and should only be loosely tightened by the frame manufacturer to prevent stripping out of threads during eventual installation of the stained glass.

We do not recommend window framing systems with vinyl and/or pressure-applied (snap-in) stops/sash. Advertised as being created specifically for stained glass, snap-in stops/sash can be problematic during the budgeting, installation, maintenance and conservation/repair of stained glass. During an installation in New England we were nervous wrecks after having to resort to hammers to seat the stops/sash around each of our stained glass windows.

In double-glazed installations, the exterior glazing serves as the weather-tight seal against the elements. It is not necessary to hermetically seal the stained glass into the framing system. Doing so can increase chances of condensation. From Fairbanks, Alaska to Kona, Hawaii, we have avoided condensation problems simply by allowing the air space between exterior glazing and stained glass to breathe or vent to the interior side. Never specify silicone, double-sided glazing tape, etc. to adhere stained glass into window openings. Finally, whenever possible, schedule the installation of the stained glass at the end of the project after other/most trades have finished.

One final note about stops: We often create stained glass for windows in previously completed buildings. Even when future stained glass has been planned from the start, the necessary stops often aren’t included as part of the window framing at the time it is manufactured and installed. Later, the avoidable expense of retrofitting window framing with stops can take a huge bite out of the stained glass budget. This added cost can be inexpensively avoided if stops are detailed and built into the framing during initial construction. Such planning and forethought also assures a precise factory fit and color match between stops and framing. When included as an integral part of window framing, stops/sash can save up to 90% of the cost of retrofitting stops into the framing after the fact.

Safety Glazing vs. Protective Glazing

There is a difference between safety glazing and protective glazing even though there is sometimes a functional overlap. Safety glazing is used to protect the public from harm due to stained glass. Protective glazing is used to protect stained glass from damage as well as to minimize cleaning and other scheduled maintenance. Double-glazing as mentioned above is a type of protective glazing that in certain cases has safety benefits.

Safety glazing is typically mandated by building codes. 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 (see www.iccsafe.org), local or other authority, specific code implications need to be researched for every project. We have seen building code requirements specific to conventional, commercial glazing applied inappropriately to stained glass. It is worth noting that in 1978 the U.S. Consumer Product Safety Commission, upon which much of the code related to safety in building components is based, adopted guidelines directly addressing stained glass. Specifically, these guidelines exempt stained glass from most of the limiting requirements imposed on conventional glazing materials. See www.cpsc.gov and reference Title 16, Chapter II Part 1201.

Even when codes do not mandate safety glazing, stained glass should be evaluated for its potential for being damaged by crowds, children, 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 inches of finish floor level or otherwise within arms’ reach), protective glazing can minimize exposure. By simply installing tempered or laminated glass over stained glass, most risks will be minimized. Such protective glazing can be installed as a sandwich with the stained glass in the middle simply by widening the installation pocket to accept the extra thickness of tempered or laminated glass.

Where interior installations are exposed on both sides (doors, partitions, etc.), triple glazing is a simple way to protect stained glass and people from each other. This can be accomplished by sandwiching the stained glass between two layers of tempered or laminated glass. Double- and triple-glazing also reduces cleaning and maintenance of stained glass to that of conventional windows. On a few occasions we have had our stained glass encapsulated within the void-spaces of insulated glass units (IGU’s). This approach can be problematic since IGU manufacturers typically do not assume responsibility for stained glass breakage during encapsulation, the warranty on the IGU itself is voided and IGU failure rates sometimes exceed 5%.

Summary

Time is important. Retain a stained glass artist/studio as soon as possible to allow full collaboration with the rest of the design team. By asking the right questions early, a smooth integration of the stained glass art into the architecture will be assured. Quickly identify locations where stained glass will be required, including future phases, and specify their windows to be “stained glass ready”. This includes spacing vertical mullions no more than 48 inches apart and detailing appropriate stops (sash) to receive the stained glass

When a proactive approach is taken, stained glass can be fully utilized as an important contributor to the architectural whole. Whether stained glass is conceived of 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.

Thanks to the Stained Glass Association of America for information regarding building codes and stained glass safety.

 

Jeff Smith was part of the groundbreaking stained glass program at Louisiana State University in Baton Rouge in the mid 1970’s and 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 Corning Museum’s New Glass Review designation on two occasions. After 24 years in Dallas, Smith and his studio relocated in 2000 to the Davis Mountains of west Texas. For an overview of Smith’s work, please visit www.archstglassinc.com.