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Analysis and Comparison of Old and New Lead CameThis article originally appeared in the Spring 2005 issue of The Stained Glass Quarterly and is reprinted here with permission. For the complete illustrated article in Acrobat (pdf) format, please click this link.
History and Discussion
Post-Industrial Revolution lead is a
thoroughly modern substance, with a purity and control of
composition that medieval glass window artisans could not begin
to imagine. In fact, at the time that many of the famous
European windows were created, the producers of their lead cames
did not possess the ability to determine what other metals were
alloyed with the lead, let alone refine the lead to modern
standards or produce consistent alloys. Literature searches
revealed that analyses of medieval came indicated that the lead
of this time contained silver, antimony, copper, tin, etc., in
varying amounts.
By the mid-nineteenth century, modern refining
processes were developed that enabled the extraction of these
extraneous metals from the lead. Unfortunately, removal of the
alloying elements resulted in a much weaker came. The unrefined
medieval lead was much better at handling the loading imparted
by the glass it contained and the wind forces to which it was
exposed. Modern “restoration quality” lead is reported to be
based upon analysis of some medieval cames. As the chemical
analysis performed in this investigation shows, “restoration”
lead contains a higher percentage of elements known to produce
solid-solution hardening of lead than the older, late
nineteenth- and early twentieth-century lead. While this means
that the restoration lead is stronger than the older lead of
higher purity, even this lead, and its medieval counterpart,
will eventually fail in service.
The reason the lead will eventually fail in
service is due to the nature of the substance. Lead is
unresponsive to heat treatment and can spontaneously
recrystallize at room temperature, making work-hardening for any
useful period of time impossible. Due to its low melting
temperature, lead is subject to “creep” at the temperatures in
which it is normally used. Creep is a slow, plastic (i.e.,
permanent, doesn’t return to its shape once stress is removed)
deformation of materials under constant stress, such as lead
came supporting glass in a window. This means that the buckling
and came cracking exhibited by many aging nineteenth- and
twentieth-century windows is an inherent and unavoidable
structural failure of the lead came resulting from the
combination of modern refining processes and the nature of lead
itself. While medieval and restoration lead will be better at
handling service stress due to their different chemical makeup,
even these leads will eventually fail in a similar manner.
As our laboratory testing showed, old cames do
not solder as effectively as new cames. Therefore, once a window
experiences structural lead failure – broken joints, cracked
cames, stretched and collapsed sections – the most effective
structural solution is complete replacement of the failed lead.
This is true regardless of the reason behind the failure, even
lack of support bars. As our testing confirmed, methods that
retain the cracked and/or sagging lead – flattening, resoldering
or adding support bars – will not produce a structurally sound
window. There is no “CPR” for lead that has torn, stretched,
ruptured or otherwise deformed.
Some practitioners have expressed concern that
replacing the entire lead matrix negatively affects both the
aesthetic and the historic value of a window. However, a failed
structural support system is in no way comparable to the
cosmetic scratches, dents, or finish cracking of desirable
antiques. No reputable antique furniture dealer would suggest
repairing a splintered, sagging bureau leg with plywood braces
and nails to “preserve the history” of such structural damage.
This is analogous to the “flatten, resolder and reinforce”
method advocated by some.
Releading a window does not alter the artwork
of the glass. It merely replaces failed framework. A reputable
restorer will seek to preserve the original glass reveal and any
special effects by utilizing accurately replicated lead profiles
and preserving any irreplaceable appliqués.
Glass is a brittle substance and breaks
easily. Once a window has buckled, its glass panes are subjected
to loads never intended by the original window designer. The
cracked and stretched cames can no longer bear their original
loads, and these loads are then transferred to the glass. This
is a recipe for the destruction of the glass.
In an effort to preserve “authentic” lead, a
window owner or repair facility using the flatten technique
greatly increases the likelihood of damage to the glass. This
appears to be a classic “throw-the-baby-out-with-the-bathwater”
response. Releading provides the opportunity to preserve the
artwork in a stained glass window by transferring the loads back
to a new lead framework. This allows us to view the artistry of
that window as originally intended: flat, structurally sound,
and with the original glass preserved.
As tensile-strength testing indicates, lead is
a weak, low-strength material. Buckling of the lead framework
and associated overload cracking of the came walls is typical of
structural failure of a load-bearing member. Given the
traditional came profiles, this appears to be inevitable, and a
window owner will eventually have to decide whether to save and
preserve the old lead or to save and preserve the glass panes.
Background
Stained Glass Resources, Inc., of Hampden,
Massachusetts, requested that Massachusetts Materials Research,
Inc. (MMR), West Boylston, Massachusetts, compare and evaluate
lead cames from various sources and document any age-related
differences that may exist. A representative of MMR visited the
Stained Glass Resources facility to observe the various steps
involved in the releading process. During this visit, there was
discussion of common practices other than complete releading,
including: cleaning and recementing, resoldering of cracked
joints, partial releading, and flattening with impact, applied
weight or heat.
As a result of this visit and the information
provided, MMR developed a testing and evaluation plan that
included the following:
• Binocular microscope examination of the came
samples
• Scanning electron microscope (SEM)
examination of fracture surfaces present on old cames
• Energy dispersive x-ray spectroscopy (EDS)
analysis of fracture surfaces present on old cames
• Comparative tensile strength testing of old
and new cames
• Metallurgical analysis of old and new solder
joints and old and new cames
These analyses were chosen as the best ways to
present any differences noted between the old cames and new
cames, and to evaluate these differences with respect to the
structural integrity of a window.
The term “old” as used in this report refers
to cames produced from the mid-nineteenth to mid-twentieth
centuries. Documenting the effects of time, stress, and
atmospheric exposure, as well as the quantifiable differences
between partial repairs and total lead replacement can help
develop a more scientific way to evaluate window conditions.
This analysis was a first step toward that goal.
Result of Technical Investigation
Visual Examination
Old cames from two windows were chosen for
extensive evaluation and comparison with new cames. Resoldered
joints from a repair performed in the mid-1970s were chosen for
evaluation with respect to a new joint. Resoldering cracks and
joints is reportedly a common practice in window repair.
Therefore, comparison of the joints produced is key when
evaluating the different remedial practices listed previously,
especially the effects of repair or partial releading versus
complete releading. The windows chosen for testing were provided
by Stained Glass Resources, but the actual samples were selected
by MMR. These samples are listed in Table I (page 55).
Visual examination of the older cames revealed
a multitude of fine cracks extending into the came from its
outer edges. Figure 1 (page 53) shows several of these cracks
along a one-inch length of Sample A. The older joints revealed
widespread cracking as well.
Figures 2 and 3 (page 53) show joint cracks in
Sample A. The crack in the lower joint shown in Figure 2 was
later examined with EDS analysis. For comparison, Sample E, a
new joint on new lead is shown in Figure 4 (page 53). Visible
cracks were common to all older samples and not present on new
samples. Since the cracking visible with the naked eye is not
necessarily the only cracking present, further microscopic
examination was performed.
Binocular Microscope Examination
A binocular microscope is a light microscope
of the type commonly pictured when the word “microscope” is
mentioned. Another term for this piece of equipment is stereo
microscope.
This examination was conducted to allow
inspection of the subject cames at magnifications up to 50X.
Selected cracks were carefully broken open to reveal their
fracture surfaces and examined with this method as well.
This examination did not reveal any new
information with regard to the came surfaces. The fracture
surfaces, however, were obviously different in appearance from
the bright, shiny laboratory-created surfaces formed upon
exposing the cracks. When a fracture is opened for inspection,
metal that was still intact nearby the crack in question
produces a new fracture. This is the laboratory-created
fracture. While it is not related to the initial crack, it can
provide information about the base metal to compare with the
crack in question.
Figures 5 and 6 (page 53) show the fracture
surfaces of cracks in the came Samples A and B, respectively.
Both photographs were taken with the same settings under the
same lighting conditions within minutes of each other. Note that
the fracture surface of the Sample A crack is noticeably darker
than that of the Sample B crack. A portion of the
laboratory-created fracture is visible in Figure 6. This
laboratory-created crack is knife-edged and shiny. Contrast this
bright, shiny appearance with the older fracture surfaces. The
darker fracture surfaces are likely the result of greater
oxidation. To verify that greater oxidation is the cause of the
difference in appearance, these fracture surfaces were examined
in a scanning electron microscope.
Scanning Electron Microscope (SEM) Analysis
Scanning electron microscope (SEM) analysis
was used for two reasons in this investigation: to reveal crack
fracture mode, if not too heavily corroded, and to analyze the
surface for differing oxygen levels to see if there was a
detectable difference between samples of different ages. A SEM
is different from a binocular microscope in that it uses an
electron beam instead of light to form an image of the surface
being analyzed. This means that the resolution and
depth-of-field is greatly increased. SEM analysis provides for
viewing of samples at much higher magnification than binocular
microscopes.
The surfaces of the cracks shown in Figures 5
and 6 were examined in both the as-received and cleaned
conditions. The oxide layer present on both surfaces obscured
the fracture features in the as-received condition, so a light
cleaning solution of a substance known as Alconox was used to
remove it. After cleaning, both fracture surfaces exhibited
ductile dimple rupture fracture mode with extensive stretching
and tearing, Figure 7 (page 55). This indicates a very ductile,
or deformable, metal. This is the same fracture mode that most
ductile metals exhibit under tensile testing, except that the
test specimens typically lack the tearing features. It
represents exposure of the metal to a force beyond its physical
capabilities to withstand. Such tearing could occur from
unusually high wind gusts, undersized cames, lead creep,
out-of-alignment panes, or the weight of the glass over time.
The flat surfaces of the Sample A came were
also examined to check for cracks not visible to the naked eye.
Several randomly selected regions were examined and
approximately one third of them possessed a crack. Several of
these cracks are shown in Figures 8 through 10 (page 55). Note
that the magnifications in these figures range from 50X to 500X.
None of these cracks was visible to the naked eye, and only one
was visible at 15X (shown in Figure 10 at 50X for greater
clarity). Figure 11 (page 55) shows the region where the crack
pictured in Figure 8 was located. Note that it is not visible at
15X. This means that any repairs carried out on visible cracks
leave a multitude of cracks untouched and unremedied.
The oxide layer itself and any differences it
might exhibit on cames of different ages were also examined.
This examination occurred prior to cleaning. To analyze this,
energy-dispersive x-ray spectroscopy, or EDS, was used to
analyze the two fracture surfaces in question along with
baseline laboratory-created fracture surfaces. EDS analysis uses
equipment attached to a SEM to reveal the elements present in
the analyzed region based upon characteristic x-ray emissions
from the specimen. This is a qualitative microchemical analysis
technique, meaning it detects relative amounts of elements. It
cannot detect compounds (i.e., it will detect sodium and
chlorine, but not sodium chloride) or determine percent
composition. It will produce graphs, called spectrograms, that
show peaks of various heights that correspond to an element’s
relative abundance in the analyzed region. In this way, it
becomes easy to see in a graphic manner which region possesses
more oxygen.
Figures 12 and 13 (page 56) are the
spectrograms for Sample A old fracture (present when sample was
received) and new fracture (laboratory-created). The difference
in oxygen levels is readily apparent with the old fracture
possessing an oxygen peak approximately three times as high as
the laboratory-created fracture.
The difference is a little less striking in
Figures 14 and 15 (page 56), which show the old and
laboratory-created fracture oxygen levels of Sample B. The old
fracture oxygen peak is approximately half again as high as the
new fracture peak. Recall that Sample B is younger than Sample
A, so age-related cracking would likely occur later in Sample B,
assuming similarity of stresses and environment. This translates
into less oxidation time for the Sample B crack than for the
Sample A crack.
Oxidation produces a layer of corrosion
product on the surface of a crack. As time passes, this layer
becomes thicker as more metal is consumed by the corrosion
process. To evaluate the thickness of this layer, metallurgical
mounts were created.
Metallurgical Analysis
Several samples were mounted in clear epoxy
and ground and polished to reveal the interiors of soldered
joints and profiles of cames. These resulting “mounts” were
examined in the as-polished condition to provide for the best
contrast between solder and came metal and any cracks, voids or
inclusions present. Figure 16 (page 58) shows a new solder joint
on new cames, Sample E, created for comparison. The cames joined
by the solder are marked “C1” and “C2,” and the solder is marked
with an “S.” Note
that there are no gaps between the cames and the solder and the
solder is solid with no inclusions (i.e., foreign particles), no
cracks, no porosity
(i.e., holes), or regions with lack of fusion. This was
consistent along the entire joint.
Figure 17 shows a joint, Sample F, that was
resoldered in the mid-1970s. Note the dark, round shapes
indicative of porosity and how the new solder appears from the
OD to join a much larger amount of metal than it actually does.
At higher magnification, the extent of the lack of fusion is
revealed to be even greater than it originally appeared in the
lower magnification view, Figure 18. Large regions of porosity
and lack of fusion such as this should not be present in a
structural joint. The smooth profile of the new joint and solid
fill of its solder provide a joint of greater soundness than the
material of the resoldered joint. Porosity and lack of fusion
represent discrete regions where gaps in the joint exist.
The jagged profile of the joint creates sites known as
“stress raisers,” or places where the stresses the joint
experiences are magnified due to geometry. Stress raisers can
accelerate joint failure.
Metallurgical mounts also reveal the depth of
any oxide layer present. Figures 19 through 21 show the profiles
of the came walls of the new sample, Sample C, and of older
cames, Samples A and B, respectively. As expected, the new came,
shown in Figure 19, possesses no visible oxide layer. Sample A,
Figure 20, possesses a well-developed, tightly adhered oxide
layer on the came OD. Debris visible on the came ID is caulking
remnant. The oxide layer is approximately 0.008 inch thick. Lead
is known to produce a protective oxide layer, so this very thin
layer is expected and normal, even after approx- imately 91
years.
Sample B, dating from the 1930s, is shown in
Figure 21. The oxide layer present on this sample is
approximately 0.0005 inch thick. The thickness difference is
negligible and the non-continuous layer of Sample B was very
likely caused by oxide spalling, or falling off, during removal
from its window.
In summary, metallurgical examination revealed
negligible oxide-layer differences between the two older samples
studied and a new sample. This is normal, as lead is known to
produce an adherent, protective oxide layer when exposed to the
elements. Once formed, a protective oxide layer greatly
decreases further oxidation, and a relatively stable condition
is achieved.
What this examination also revealed was a
notable difference between a new joint and an older, resoldered
joint. The new joint was solid, lacked porosity, and was well
fused to the cames. The resoldered joint possessed porosity,
lack of fusion, a jagged, stress-raising profile, and spotty
fusion to the came. All these make the resoldered joint a much
weaker construct.
The rationale behind resoldering old joints or
only partially replacing cracked cames assumes the resulting
joints are “good as new” if done “properly.” Properly generally
refers to adequate cleaning, temperature control, flux selection
and joint design.
However, as this and SEM examination showed,
came cracks possess a layer of oxidation. No matter how well the
flat came surface is scrubbed or cleaned, the crack fracture
surface oxide layer, due to geometry, will persist. Fluxes are
not substitutes for cleaning and cannot remove such persistent,
well-adhered oxide layers. They should not be counted on to do
so. Fluxes remove tarnish films from pre-cleaned surfaces,
prevent oxidation during the soldering process, and lower the
surface tension of the solder. Soldering over an oxide-filled
crack will not produce a bond that is metallurgically equivalent
to a new, uncracked length of came. It may even produce
undesirable brittle intermetallic compounds in and near the
solder joint that accelerate cracking of the joint.
As Figure 3 shows, cracking at resoldered
joints is a concern. In addition to the crack, note the jagged
came form and melt-through regions at this T-joint. These are
all hallmarks of a poor bond. The melt-through and jagged
eaten-away appearance of the came results from too high heat
and/or too long a contact between the soldering tool and the
came in these regions. All the stress-raiser issues previously
discussed regarding uneven geometry are illustrated here. Cracks
in the weld toe region, common in the samples examined from
different windows, are the result of the metal attempting to
accommodate strains induced by the soldering. This can be due to
excessive heat application, entrapped flux, creation of brittle
intermetallic compounds, or poor stress distribution elsewhere
along the came due to other repair work.
The prominence of such cracks in the samples
examined from different sources suggests that they are less the
result of the skill level of the person resoldering the joint
(although the overall quality of the Figure 3 joint is very low)
than of the difficulty in properly cleaning and designing a
repair joint involving old, oxidized lead.
Also, as noted in the SEM examination section,
the visible cracks are not the only cracks present on the came.
Many of the cracks present on the came surfaces examined were
visible only at magnifications over 100X. Even assuming that
they could be resoldered properly, locating all such cracks on a
sample intended for repair would require extensive microscopic
examination.
Tensile Testing
Tensile testing was performed on samples of
older cames and samples of new cames. Tensile testing was chosen
as a test for this evaluation because it can provide an
at-a-glance comparison between specimens. This type of testing
pulls a specimen in tension at a slow, controlled rate until the
specimen ruptures, or breaks. The sample cames, both old and
new, were pulled in tension “as-is,” or in their came
configuration rather than as a machined tensile-test specimen.
This provided a real-world comparison between
samples, as cracks present in the old cames were not eliminated
by machining. The results of this testing are summarized in
Table II. Note that the sample designations here are specific to
this testing and do not refer to Table I sample designations.
The new cames tested were chosen based upon size to compare with
older cames. This means that one new Sample B was the same size
and configuration as Sample A; and Sample D was the same size
and configuration as Sample C. This is shown in Figure 22. These
results indicate that the strength of a new came is a minimum of
two and a half times that of an old came. In other words, using
a new came provides 250% more tensile strength than the old
cames. Since the lead cames are the structural framework for the
glass, this translates into a much greater ability to withstand
the weight of the glass and the wind loads to which windows are
subjected. This is significant because SEM examination of an
older crack fracture surface showed a fracture mode consistent
with an overload failure, the same type of failure a tensile
test produces.
The practice of allowing a buckled window to
settle and pressing it flat again will not heal the cracks that
were instrumental in producing the lowered tensile strength of
the two older cames. In fact, attempting to press buckled and
distorted came walls back into position can extend cracks
already present, as well as cause new ones, when the stretched
metal is forced to lie flat again. This is a simple geometric
response. The lead came walls cannot “unstretch.”
Chemical Analysis
Chemical analysis was performed on came
Samples A, B, and C to determine if any compositional
differences existed between the older leads from the early
twentieth century and new lead ordered to “restoration quality.”
The results are summarized in Table III. These analysis results
indicate that the lead cames from 1913 and 1930 (Samples A and
B, respectively) are very similar to each other and are also
similar to two Unified Numbering System alloys: L52505
Lead-Antimony alloy and L52510, 99.8% Lead. This is consistent
with manufacturing efforts of the time to produce high-purity
lead for window cames.
The new “restoration lead” (Sample C) contains
a much higher level of antimony and tin than the older lead.
This alloy is similar to many UNS alloys, among them: L52560
Bullet Alloy, L52615 Lead-Base Die Casting Alloy, etc.
The new lead contains a larger amount of
elements known to produce something known as solid-solution
hardening effects (i.e., antimony, bismuth, arsenic, tin, etc.).
This means that lead with the chemical composition of the new
lead would be slightly stronger than lead with the chemical
composition of the old lead, even if both samples were in a new,
uncracked condition. A stronger alloy is capable of withstanding
service conditions better than a weaker alloy.
Conclusion
Several conclusions can be drawn from the
analysis results and review of repair and releading techniques.
These are presented below as a bullet list. For greater detail,
refer to the History and Discussion section as well as
individual testing results.
• Modern refining techniques produced lead of
much greater purity for use in mid-ninteenth century to
mid-twentieth century windows. This lead is very different from
both medieval lead and its modern restoration-lead counterpart.
• Lead of greater purity is a weaker metal
than alloyed medieval lead and modern restoration lead. As a
result, the pure lead is less able to withstand glass weight and
wind loads than its alloyed relatives. Came-wall stretching and
cracking will eventually result.
• Pressing a buckled window flat does not
repair cracks in the cames. The pressing process is likely to
propagate existing cracks and create new ones.
• Resoldering old joints in old cames results
in poor joint quality and can induce further cracking at the
solder pool toe. This does not restore the window lead framework
to “good as new” condition.
• Window buckling due to lead framework
structural failure transfers loads that were previously handled
by the lead to the glass panes. This is a recipe for glass
breakage due to its inherent brittleness.
• Modern “restoration quality” lead came
consists of an alloy based upon chemical analysis of some
medieval leads. Use of this alloyed lead in restoration of
windows should result in the greater ability of the restored
lead framework to withstand service loads over the purer lead
used in the late ninteenth and early twentieth centuries.
However, as with all structural frameworks, even the restoration
lead will eventually require replacement.
• The cracks in cames visible to the naked eye
are not the only cracks present. Soldering over visible cracks
does not eliminate microscopic cracks. Cracking weakens cames
and reduces their ability to withstand service loads.
• Tensile-strength testing revealed new-came
strength to be a minimum of 250% higher than cracked-came
strength.
• Came cracking is an inevitable result of
service due to the inherent ability of lead to creep at normal
use temperatures and to resist heat treatment and work-hardening
procedures used regularly with other alloys. While the
solid-solution strengthening made possible with the use of
certain alloys makes stronger cames available, even these will
eventually experience structural failure due to the intrinsic
behavior of their lead base.
Finally, this analysis shows that the service
life of the lead-support system in a stained glass window is
influenced by more than age. While the testing indicates that
the older the lead is, the greater the likelihood of failure,
chemical composition certainly influences its life span.
Traditionally, we may think that lead should
be near 100 years old before considering replacement, but if it
is relatively pure, that time span may be greatly shortened.
Taking into account many other factors, including wind loads,
climate ranges, installation types and design style, the correct
response to the signs of structurally failing lead is complete
replacement with a new lead matrix.
About the Author
Veda-Anne Ulcickas is a Materials Engineer in
the Failure Analysis/Materials Engineering Consulting Dept. at
Massachusetts Materials Research, Inc. (MMR). She holds a BS in
Mechanical Engineering and an MS in Materials Science and
Engineering, both from Worcester Polytechnic Institute. Her
graduate work investigated the suitability of using
fractal-based advanced computational methods as a quantitative
surface-measurement tool.
Ms. Ulcickas’ work at MMR has included
product-design assistance for sporting goods and housewares,
material consultations for water treatment plants and other
severe-service environments, as well as a wide variety of
failure investigations. Her services are regularly requested for
investigations into natural gas fires and explosions, water main
breaks, and aircraft-industry product failures and quality
non-compliances. She has extensive experience evaluating brazed
and soldered joints for such diverse industries as electronics,
rock cutting, and aerospace component manufacturing.
Massachusetts Materials Research, Inc.
specializes in the practical application of advanced testing,
engineering, inspection and failure-analysis technology. They
have recently been involved in investigating the failures of
various aircraft engine components, inspection of observatory
azimuth gears and large, mobile tractor-trailer x-ray units, and
investigating several fires and explosions involving household
appliances and utilities. MMR also conducted the inspection of
infrastructure used to produce the Jeep™ Parking Only commercial
in New York City, which can be viewed on the jeep.com website. |
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