HAMM BUILDING 

ST. PAUL, MINNESOTA 

Introduced in the 1890s, prismatic glass 

transoms were a popular and practical 

means of directing daylight into building 

interiors. With origins in sidewalk vault 

lights and glass panels used on ship 

decks, prismatic tiles had ridges or other 

raised pattems on their inside surface 

that refracted sunlight toward the rear of 

a building. The pressed tiles were usually joined together with zinc or lead in a 

process similar to that used to create 

stained glass windows (see figure 1). An 

alternate, less common approach was to 

bond the tiles to copper strips during 

immersion in an electrolytic bath, a 

process known as electroglazing. At the 

peak of popularity, over a dozen manufacturers offered varying tile patternseach "scientifically designed" to increase 

natural light levels and thereby reduce 

reliance upon light wells and artificial 

light sources. Prismatic glass tiles were 

used both in new construction and to 

update existing storefronts, until changing tastes and the dominance of electricity led to their functional obsolescence by 

the 1930s. 

Although prismatic transoms were 

seen most frequently above the display 

windows and doorways of modest main 

street buildings, they were also used in 

larger commercial structures. An example is the Harnm Building in St. Paul, 

Minnesota. This six-story structure, with 

shops on the ground floor, offices above, 

and a theatre on one side, was completed 

in 1920. The exterior of the building was 

especially admired for its cream-colored 

terra cotta with Classical and 

Renaissance Revival ornamentation. 

Another important feature of the 

Harnm Building exterior was the large 

band of prismatic glass located just 

above the storefront awnings (see figure 

2). Divided into groups of three and four 

panels separated by terra cotta pilasters, 

PRESERVATION 

Tech Rotes ef- Cultural 

~~Interlor Resou ..... 

HISTORIC GLASS 

NUMBER 1 

Repair and Reproduction 

of Prismatic Glass Transoms 

Chad Randl 

Technical Preservation Services 

National Park Service 

Deteriorated prismatic glass 

transoms should be repaired 

using historic tiles. When 

tiles are missing, the transoms 

should be replicated using glass 

that matches closely the 

appearance of the historic prisms. 
Prismatic Tile 

Refracted 

Sunlight 

\~ 

Typical Storefront 

Refracted Sunlight 

Transom 

'.... 

-::::::===r Prismatic 

of the exterior and 

natural light levels in 

the interior. 

Problem 

When the Hamm 

Building was rehabilitated beginning in 1996, 

plans were made to 

return the prism glass 

transoms to their hisPrisms~ 

Raised 

pattern 

Glass 

display ~ 

'A· wnmg 

toric appearance. At the 

time, all but sixteen of 

the panels were 

obscured behind plywood sheets (see figure 

4). Installed decades 

before, the covering was 

probably a response to 

window 

Interior Exterior Interior Exterior 

the gradual deformation 

of the assemblies and 

water infiltration that 

resulted from deferred 

Figure 1. A typical prismatic tile glazed with zinc came is shown on the left. Tiles on the Hamm Building had a pattern of 

raised circles on the inside surface. The drawing on the right illustrates a typical storefront with prismatic transom. 

Drawing: Chad Randl 

maintenance. The desire 

for a more contemporary appearance and 

their placement mirrored the fenestration 

pattern of the rest of the facade. Each of 

the fifty-six panels was approximately 4 

1f2' tall by 5' wide and contained 224 

tiles, each measuring 4" high by 4" wide 

by 1/4" thick. The tiles were made by the 

Manufacturers Glass Company, one of 

several prismatic glass producers based 

in Chicago. Two types of tiles were used 

in each Hamm Building panel. Most featured uniform rows of raised circles 

resembling bubbles on 

both sides of the tile. 

The other tile pattern 

had bubbles on the interior face only, with the 

manufacturer's trademark logo on the exterior side. (Both differed 

from a more common 

prism tile of the time 

that featured horizontal 

ribs on the inside surface). In the Hamm 

Building transoms, the 

trademark tiles were 

arranged in a square border along the edge of 

each panel (see figure 

3). The prisms were 

glazed with zinc came 

and set within a copper 

frame. A small pivoting 

ventilator was located in 

the center of each panel. 

Building panels were covered over during a mid-twentieth century renovation. 

Only partially intact when the building 

was rehabilitated in the 1990s, the 

prisms were uncovered, removed, 

cleaned and reglazed. New glass that 

approximated the historic prisms was 

used to replicate several missing panels. 

With modifications that improved their 

structural strength, the transoms continue to contribute to the visual interest 

more prominent sign 

space may also have been factors. 

After removing the plywood, the 

individual panels were found to be in 

various states of disrepair. In approximately 20 percent, the tiles were either 

completely missing or had fallen to a 

pile at the bottom of the frame. Most 

panels that survived were bulging or 

sagging and had severely pitted and corroded came and cracked solder joints. 

Like many historic 

transoms today, a large 

number of the Hamm 

Figure 2. The Hamm Building featured fifty-six prismatic panels set directly above the awnings. Each panel contained 

over two hundred individual tiles. Photo shows completed project. Photo: Chad Randl 

2 
Figure 3. Two tile designs were used in the Hamm Building transoms. Most featured rows of raised 

bubbles. A second type, with the manufacturer's trademark pattern, formed a border near the edge 

of each panel. Photo: Oertel Architects. 

Numerous paint layers covered the exterior surface of the panels and several had 

holes cut to accommodate the addition 

of air conditioning or ventilation ducts 

(see figure 5). 

Repairing the transoms required a 

knowledgeable craftsperson experienced 

in specialty glass assemblies and exterior 

installations. A source for replacement 

tiles was also necessary. Both presented 

challenges, as the skills and materials 

developed for the prism glass industry 

became scarce when the popularity of 

the panels faded. As is the case with 

many twentieth century materials from 

plastic laminate walls to Vitrolite storefronts, finding suitable replacement 

material was a primary obstacle to 

restoring the transoms successfully (see 

sidebar). Prismatic tiles had not been 

mass-produced for sixty years and all of 

the manufacturers had either folded or 

were involved in completely different 

markets. Finding the right craftsperson 

or workshop also proved difficult. The 

work required an understanding both of 

zinc glazing and the structural and environmental demands that accompanied an 

installation the size of the Hamm 

Building panels. 

Solution 

Repair of the prismatic glass transoms 

involved removing, disassembling, 

cleaning, and reglazing the surviving 

panels with original glass tiles, some 

replacement glass, and new zinc came. 

Rehabilitation of the transoms also provided an opportunity to increase the 

strength of the original assembly. Simply 

rebuilding the panels according to the 

original design would establish the same 

conditions that contributed to their initial 

failure. With this in mind, the rebuilt 

panels incorporated unobtrusive changes 

that provided additional reinforcement. 

All of the surviving prismatic tiles 

were used again in the rebuilt panels. 

The number of surviving tiles, however, 

was sufficient to rebuild only forty-six of 

the fifty-six panels. Since the project 

team was unable at the time to locate an 

affordable source for newly cast replacement tiles, a textured, or patterned, glass 

served as a substitute in the remaining 

ten panels. Purchased in sheets from an 

art glass dealer and cut to size in the 

workshop, the appearance of the replacement glass was a close approximation of 

the original bubble design. 

The contract for rebuilding the 

Harnrn Building transoms was awarded 

to a local stained glass studio. Their previous work with leaded glass windows 

demonstrated an understanding of structural load issues, expansion and contraction characteristics, and other concerns 

relating to glass assemblies in exposed 

locations. The studio owner also had 

considerable experience repairing and 

rebuilding smaller prismatic glass installations. After meeting with the studio 

team and viewing examples of their earFigure 4. When the Hamm Building rehabilitation began, most of the prismatic tiles 

(above the awnings) were hidden beneath plywood panels. Photo: Oertel Architects. 

Figure 5. Transoms hidden beneath the plywood covers 

were in varying condition. Tiles had been removed from the 

panel above to fit a ventilation duct. When the panels were 

later rebuilt new material was needed to replace missing 

tiles. Photo: Oertel Architects. 

3 
Replacement Glass for Historic Prism Transoms 

Whether a project involves minor repairs or recreating an entire installation that 

had been removed and destroyed, work on historic transoms often requires 

replacement material. Zinc came is still commonly in use, and can be supplied 

by some of the same manufacturers that produced came a century ago. The 

ingredients of waterproofing grout, likewise, have changed little. Prismatic 

glass tiles, however, are harder to come by. There are generally three replacement options: custom cast new glass tiles, textured glass and salvaged historic 

material. Deciding which approach is most appropriate is dependent upon the 

amount of glass required, the degree of accuracy desired, and the project 

budget. 

Several companies are capable of producing new pressed glass tiles. Some 

have historic molds bought from defunct prismatic glass companies; others use 

computer design software and laser cut graphite dies to create patterns that are 

identical to the historic material. Though this approach results in the most accurate reproduction material, the expense of custom casting may be prohibitive for 

smaller projects that require few tiles. 

Textured, or patterned, glass is sheet glass upon which a pattern is rolled. 

Several patterns especially those with linear ridges (such as narrow reeded or 

ribbed glass) may be acceptable substitutions for original prism tiles. Textured 

glass can be ordered in sheets and cut into tiles in the glazing workshop. 

Advantages of textured glass over custom pressed tiles include cheaper costs, 

virtually immediate availability, and a larger number of suppliers. 

Reputable specialty glass collectors and salvage companies may be a source 

for replacement historic prism tiles. The profusion of manufacturers and tile 

patterns once available limits the likelihood of locating an exact match. Those 

working with collectors should ensure that original materials on offer were not 

removed from existing installations purely for salvage and resale. 

The color of the replacement glass may also have to be decided. Originally, 

prismatic tiles were clear. Until World War I, manganese was added to the 

process to decolorize the otherwise green glass. Decades of exposure to ultraviolet radiation, however, can cause a reaction in the manganese that then imparts 

a purple or pinkish tone to the glass. It may be possible to find patterned glass 

or salvaged tiles in a color that approximates that seen in solarized historic tiles. 

Exact matching is not likely, or necessarily desirable, considering the fact that 

the replacement material and the historic tiles would continue to change color at 

different rates. Because the historic tiles on the Hamm Building had not experienced significant solarization, the architects and transom contractor chose a 

clear patterned glass. 

See http://www2.cr.nps.gov/tpslptn44/material.htm for a list of possible 

replacement material sources. 

lier work, the architects decided that the 

company would provide the level of 

expertise and quality required for such a 

historically significant and highly visible 

project. 

Rebuilt Transom Panels 

Like most prismatic transoms, the zinc 

came and the tiles themselves provided 

the only structural strength to the assembly. Manufacturers figured that zinc was 

rigid enough to not require additional 

support. In some cases, such as with the 

Hamm Building panels, time and weather have proven otherwise. Periodic high 

winds in downtown St. Paul tended to 

push the large transoms inward, while 

unequal atmospheric pressure between 

the building interior and exterior exerted 

an outward force. The resulting distortion cracked solders joints and separated 

came and glass from the waterproofing 

grout, causing or exacerbating leaks. 

To resist these tendencies, the architect and glass contractor attached four 

vertical bars to the interior face of each 

panel (see figure 6). Flat steel bars I f2 " 

deep were spaced three or five tiles apart 

and soldered on edge at points where the 

zinc came intersects. Applying continuous solder along the length of the reinforcing bar is not recommended as it 

may impede future repair efforts. The 

ends of the bars were then secured to the 

edge of the frame so that each bar supported sections of less than eight perimeter feet. Because of their thin, 1/8" profile and placement on the inside of the 

panel, the braces are invisible from the 

exterior. A similar approach is often used 

with leaded glass, where horizontal or 

vertical saddlebars are tied to the came 

in order to assume the structural load of 

the windows. 

Repair Work 

After the transom contractor established 

a temporary workshop in one of the 

Hamm Building's vacant storefronts, 

each panel was removed and disassembled. Because the shop did not include 

cleaning facilities, the tiles were inventoried, packed, and sent to a furniture 

stripping company, where they were 

immersed in a solvent bath to loosen 

paint and dirt. The solvent used by the 

stripping company had been previously 

tested to ensure that it did not etch, 

cloud or discolor the glass. In the meantime, the transom contractor built plywood jigs and workbenches in the shop 

that would facilitate precutting came and 

assembling the panels. When the cleaned 

tiles were returned to the site, reglazing 

began. 

The majority of the rebuilt panels 

incorporated prism glass tiles that were 

original to the building (see figure 7). 

Both ventilator hardware and the historic 

trademark prism border were included in 

the rebuilt panels. The assembly process 

was similar to that used for stained glass 

windows. Individual prisms were set in a 

zinc H-came matrix that was gradually 

soldered together as more tiles were 

added. Reinforcement bars were then 

soldered to the panel. The last major step 

before reinstallation was to waterproof 

the assembled panel. A waterproof grout 

Reinforcing Bars 

/ ~ '- ~ 

.... I. i1f 

~ Zmc Came .l--"" 

Figure 6. The four vertical reinforcing bars were 

soldered to the inside face of the transom panel. 

Drawing: Chad Randl. 

4 
Figure 7. Rebuilt transoms featured prismatic glass tiles and vents that were 

original to the building. The decorative border of ''trademark'' tiles was incorporated into the reglazed assembly. Photo: Chad Rand!. 

Figure 8. Reproduction panels used a textured replacement glass and replicated vents. The panels were further differentiated from the those with historic glass by the absence of a decorative border. Photo: Chad Rand!. 

made from a traditional mix of putty, 

boiled linseed oil and lamp black was 

forced into the spaces between the came 

flange and the glass. 

The four reproduction panels containing new glass were reglazed in the 

same manner. A clear, patterned glass 

called "moss glass" that resembled the 

color and bubbled texture of the historic 

prism tiles was used as a substitute 

material (see figure 8). Where historic 

ventilators were beyond repair or missing, the transom contractor fabricated a 

simulated vent with strips of capping 

zinc (a material normally used to cover 

the edge of the transom frame). 

Reproduction panels also differed from 

the historic assemblies in that they did 

not have a border of trademark tiles 

along the perimeter. The resulting panels 

were distinguishable from the historic 

units up close, but when viewed from 

the street or the opposite sidewalk, 

seemed only slightly different from the 

original panels. 

5 

Evaluation 

The rehabilitation of the Hamm Building 

transoms included reassembling over 

12,500 prism tiles into fIfty-six panels. 

All of the surviving historic prisms were 

preserved in the rebuilt transoms. 

Replacement materials including zinc 

came and new patterned glass tiles were 

selected for their similarity to the original materials; modifIcations were developed that did not signifIcantly alter the 

historic appearance of the panels. 

It was important in this case, for the 

transom rehabilitation program to 

address long-term concerns over the 

structural strength and weather resistance of the original panels. The addition 

of reinforcing bars counters the tendency 

of wind and temperature extremes to distort the transoms. Applied on the interior, such a system has the added benefIt 

of being virtually undetectable. The 

longevity of the panels including their 

continued weather resistance is best 

assured through a process of regular 

inspection and maintenance and, when 

necessary, regrouting the panels before 

leaks develop. 

Though the Hamm Building transoms were deteriorated to a point where 

they had to be removed and taken apart, 

there are occasions where repairs may 

not require complete disassembly. In 

cases where the came is in good condition, but a single tile is missing or 

severely cracked, the panels can be 

removed to a workshop and the came 

flange adjacent to the damaged tile 

opened up with lead pliers. A replacement tile can then be fItted into the 

channels, and the came pushed back in 

place by hand and resoldered together. 

The rigidity of zinc and the difficulty of 

soldering on a vertical plane make in 

situ repairs to prismatic transoms 

extremely difficult. 
Figure 9. Panels with original tiles are shown on the left next to those featuring replacement textured glass. Photo: Chad Rand!. 

Conclusion 

The Harnm Building prismatic glass 

transoms are more than a tool for bringing daylight into ground floor shops and 

entrances; they are an integral feature of 

the building facade. The preserved 

prisms impart an openness and visual 

texture to the storefronts that were 

important to the building's design and 

historic appearance. Although the deteriorated condition of the seventy-five year 

old panels necessitated major repair, the 

work was planned to retain as much historic material as possible. Reglazed panels incorporated historic tiles and vent 

hardware while adhering to the dimensions, construction and overall appearance of the historic configuration. Where 

missing or damaged elements required 

replacement, new materials approximated the appearance of the original features (see figure 9). Lastly, modifications 

to the transoms improved the strength of 

the transoms but did not intrude upon 

the historic appearance of the prismatic 

tiles and the Harnm Building exterior. 

Additional Reading 

Neumann, Dietrich. "Prismatic Glass." 

In Twentieth Century Building 

Materials. History and Conservation. 

Thomas C. Jester, ed. New York, NY: 

McGraw-Hill, 1995. 

Vogel, Neal A. and Achilles, Rolf. 

Preservation Brief 33. The 

Preservation and Repair of Historic 

Stained and Leaded Glass. 

Washington, DC: US Department of 

Interior, National Park Service, 1993. 

Project Data: 

Building: 

The Hamm Building 

408 St. Peter Street 

St. Paul, MN 

Owner: 

Markham Company of Saint Paul 

St. Paul, MN 

Project Date: 

1996-1999 

Architect: 

Oertel Architects 

1795 Saint Clair Avenue 

Saint Paul, MN 

Transom Contractor: 

Palmer Art Glass 

6670 Anoka St. 

Fridley, MN 

Cost: 

The total cost of the transom restoration project including removing fiftysix original panels, cleaning the 

prismatic tiles, reassembly, fabricating 

replacement panels, and installation 

was $80,000. Panels containing original tiles were less expensive than the 

replacement panels because new glass 

was not required. 

THIS PRESERVATION TECH NOTE was prepared by the 

National Park Service. Charles E. Fisher, Heritage 

Preservation Services, National Park Service, serves as the 

Technical Editor of the PRESERVATION TECH NOTES. 

Information on the Hamm Building transom project was generously provided by Jeff Oertel of Oertel Architects, Al 

Palmer of Palmer Art Glass, and John Salisbury of Gay tee 

Stained Glass. Special thanks are extended to Al Husted of 

Albert Stained Glass Studio, Neal Vogel of Restoric, Inc., 

Deborah Slaton of Wiss, Janney, Elstner Associates, and 

Michael J. Auer, Sharon C. Park and Kay D. Weeks of the 

National Park Service's Heritage Preservation Services for 

their review and comments. Thanks also to Judy Randl for 

assistance in obtaining photographs. 

techniques for successfully maintaining and preserving cultural resources. All techniques and practices described herein 

conform to established National Park Service policies, procedures and standards. This Tech Note was prepared pursuant 

to the National Historic Preservation Act, which direct the 

Secretary of the Interior to develop and make available to 

government agencies and individuals information concerning 

professional methods and techniques for the preservation of 

historic properties. 

PRESERVATION TECH NOTES are designed to provide 

practical information on traditional practices and innovative 

6 

Comments on the usefulness of this information are welcomed and should be addressed to PRESERVATION TECH 

NOTES, Technical Preservation Services - NC200, National 

Center for Cultural Resources, National Park Service, 1849 C 

Street, NW, Washington, DC 20240. 

ISSN: 0741-9023 PTN44 September 2001