COLOR AND
LONG-TERM COLOR RETENTION IN COMPOSITE PATCHING SYSTEMS FOR STONE AND MASONRY
Summary Statement:
Precise measurement and understanding of composite patch color and color change are prerequisites to the optimization of long-term repair aesthetics.
Abstract:
Anecdotal and photographic data have limited value in assessing color and long-term color retention in composite patching systems for stone and masonry. Observer subjectivity and process inaccuracies are inherent in these methods. Modern spectrophotometers and color management software provide objective tools for precise measurement and communication of color and color change data.
Objective color measurement tools were used to define color changes in brownstone repair mortars exposed to accelerated weathering in the laboratory and to natural weathering over a twelve-year period. The data derived describe the extent and nature of the changes in color, and are the basis for discussion of the role of efflorescence as a contributing cause.
COLOR AND LONG-TERM COLOR RETENTION IN COMPOSITE PATCHING SYSTEMS FOR STONE AND MASONRY
The intended aesthetic outcome of employing
composite patching systems in architectural masonry restoration is a repair
that is visually indistinguishable from the host substrate. Close matching of
composite repair mortar color to the original stone or masonry is fundamental
to the fulfillment of that objective. The long-term retention of closely
matched color is a more complex challenge, and ultimately may be the more
decisive determinant of aesthetic success or failure.
Long-term color retention of composite repair
mortars has assumed increased significance as a conservation issue as a result
of prolonged service life expectations for composite repairs. No longer relegated
to the role of short term, temporary or minor repair, some composite patching
products are now routinely providing decades of service on repairs of sizable
scale.
The work undertaken to date in this ongoing
study has been aimed at gaining a better understanding of how composite repair
material color responds to extended periods of both natural and accelerated
weather exposure, how to catalogue and communicate color changes, and how to
apply this understanding to the objective of achieving aesthetically successful
restoration work. It is part of a broader, continuing effort, aimed at making
incremental improvements in various aspects of repair mortar performance. Such
development work is multi-faceted and constantly evolving, rather than narrowly
focused with clearly defined beginning and endpoints. The objective is not to
develop “perfect materials”, which may never be realized, but to move existing,
effective materials further along the path toward that ideal. Opportunities for
improvement are part of the intimate knowledge that can be gained by a
material’s manufacturer over a long period of both ongoing development and
large-scale commercial use.
In the course of pursuing such advances a
knowledge base is developed that is not particular to any product. Although
most of the work performed in this study employed cementitious, latex-modified
brownstone repair mortars manufactured by Edison Coatings, Inc., the research
concepts, methods and direction have broad relevance to color-matched repair
mortars in general, whether field-mixed or produced by any of a number of
manufacturers. Objective definition and communication of repair mortar color
and color retention represent a common interest.
The
Role of Anecdotal Evidence
The determination as to what constitutes
aesthetic success or failure is, today, largely a matter of personal judgment.
Sites may be revisited periodically after an intervention, and individuals may
observe and judge how well repair materials and their
color are withstanding the test of time. The record may be augmented through
the taking and cataloguing of photographic images.
There are times, particularly in extreme cases
where the evaluation of relative aesthetic success or failure is not terribly
difficult, when these methods may be sufficient. For example, the following
cases of two restored 19th Century brownstone churches, situated
just a few streets apart in
Figure 1
The First Church of Christ, on the left side of
the figure, was restored in 1996 using three standard colors of a commercial,
prepackaged brownstone repair mortar.
The extensive five-year-old repairs at
While anecdotal data of this type can be useful,
there are limits to the value of this methodology for observing and reporting
on color and color retention. If aesthetic success or failure is to be assessed
on a subjective basis, then it must also be recognized that reports by
individual observers are inherently disposed toward inaccuracies and
distortions, however unintentional they may be. Whether a particular repair
represents a “good” color match is subject to a complicated set of personal
values, expectations and perceptions.
On a perceptual level, one basic obstacle to
accurate anecdotal reporting is that not everyone “sees” color in the same way
or with equal accuracy. At one extreme, approximately one in 10-12 men and one
in 165 women exhibit some degree of colorblindness1. At the other
end of the spectrum, only a small percentage of individuals exhibit sufficient
visual color acuity to accurately distinguish between multiple close shades of
the same color, as assessed in color perception tests. Such tests have been
used by some paint manufacturers to evaluate an individual’s potential capacity
for performing color-matching work. Most observers fall somewhere between the
two extremes, with levels of perceptual acuity that vary accordingly.
The
Need for Objective Color Measurement & Communication
Color tolerance determinations based exclusively
on subjective individual observations represent undefined standards. Although
the ultimate goal may be to satisfy the aesthetic sensibilities of a particular
individual or group of individuals, undefined standards leave ample room for
disagreement and have the potential to engender conflict. Except for the most
obvious cases, terms like “aesthetic success” and “aesthetic failure” become
arbitrary, due to the lack of standardized definitions, guidelines or
practices.
Based on the illustrative cases of the two
churches in
While photographs have some value in allowing us
to compare patch appearance to the adjacent stone, at a particular instant,
they do not provide an objective way of evaluating how or precisely how much
these materials are changing in color over time.
OBJECTIVE
COLOR MEASUREMENT & COMMUNICATION
There is essential knowledge and experience to
be gained by studying color and color changes over time in a precise and
objective manner. Such assessments can provide critical insight into the nature
of color change as well as implications as to its probable causes. The data
also provides a basis for further optimization of long-term color retention of
composite repair systems. Prerequisite to any such studies, however, must be
the definition of a common language for precisely defining and communicating
color information.
The Munsell color system2, developed
in 1905 and still in use to some extent today, was an early attempt to
catalogue colors in an objective and quantitative manner by creating a series
of fixed color standards. The colors of the spectrum are divided into ten color
groups or “hues”, organized around a vertical axis called “value”, which runs
from light at the top to dark at the bottom. “Saturation”, or how pure the
colors appear, runs from grayer/”muddier” at the center to more
saturated/”purer” at the outer perimeter. The problem with the Munsell system
is that it still relies on human observers to compare colors to a particular
Munsell color standard, and the number of Munsell standards with which colors
can be compared is generally limited. The Munsell tree shown in Figure 2
contains fewer than 400 standards, and even the most extensive sets of Munsell
standards typically provide no more than 1550 colors3.
The development of the spectrophotometer,
twenty-three years after Munsell’s system, opened the door to more precise and
objective color measurement. Mathematical models were developed for describing
color and they have evolved over time, facilitating more precise color
communication.
Figure
2
The most widely used model for measuring and
communicating color today is the 1976 CIE L*a*b* sphere4, which
provides a means of precisely describing every possible color in terms of
geometric coordinates within its three-dimensional “color space”.
As in the Munsell tree model, the Vertical axis,
“L”, is lightness and darkness, with White at the top and Black at the
bottom. If the sphere is bisected along its equator, the two-dimensional,
horizontal cross-section is described by two perpendicular axes: “a”,
the red-green axis and “b”, the yellow-blue axis. Colors become grayer
or “muddier” as we move toward the center and cleaner or more saturated as we
move toward the edge. Alternatively, the same color space can be described in
polar coordinates. This is the basis for the “L C h” model (not shown).
Stated simply, the spectrophotometer measures
reflected light across the spectrum of visible light wavelengths. These
reflected light measurements are converted into numerical values, which
describe the color of an object in terms of a 3-dimensional set of coordinates
in color space. Today’s color software can rapidly calculate color differences
in comparison to an established standard, and can report those differences in a
number of different ways. These include both numerical and graphic reports.
The combination of modern spectrophotometer and
commercial color software provides a means of measuring and communicating
millions of colors with objective precision. That capability cannot be equaled
by human observers working with limited numbers of “hard” printed standards.
Coatings manufacturers utilize
spectrophotometers and color management software to aid in their daily color
matching and batch color correction work. These tools provide the means for quickly
matching and correcting colors of such products as breathable masonry coatings,
adhesives, sealants and repair materials. They replace an otherwise tedious,
time-consuming, trial-and-error process requiring a skilled and experienced
operator.
The particular challenges of color control and retention in brownstone
patching compounds was the basis for selecting deep brownstone colors for
evaluation in Edison Coatings’ study of composite patch color and color
retention. Slight color changes in deep brown-colored patches tend to be more
noticeable and objectionable than in lighter patches, such as those matched to
typical
Studies performed in the Edison Coatings, Inc.
laboratory utilized a Dataflash 100 spectrophotometer and Colortools v.1.3
Quality Control software to measure and report changes in brownstone patching
material color over time and exposure. The exposure was provided by the use of
a QUV accelerated weathering apparatus.
The Dataflash 100 dual beam spectrophotometer
utilizes a pulsed xenon light source and dual 128-element diode arrays to
measure reflected light in the 400 to 700 nm wavelength range. It can measure
reflectance percentages from 0 to 200%. A small area view aperture was used to
allow careful selection of particular “spots” for color measurement, when
required. The area measured in each reading was 5 mm in diameter. Each reading
reported was the average of three measurements of the same 5 mm “spot”. The
instrument is calibrated and tested at the beginning of each eight-hour
session.
For the work involving measurement of patching
compound color change, sample panels that had been properly cured and aged were
measured and designated as the “Standard”. Panels exposed to accelerated
weathering were measured and compared to that Standard to assess color changes.
It is understood that color change is a
consequence of weathering, and that the changes in many building products are
closely associated with exposure to ultraviolet radiation and wet-dry cycling5.
The QUV apparatus, which is operated in accordance with ASTM G536,
produces alternating cycles of ultraviolet radiation and hot water condensation
to simulate the stresses of natural weathering. A variety of ultraviolet
sources may be chosen, and operating temperatures and cycle lengths can be
varied as well.
The Edison Coatings laboratory began use of its
QUV apparatus in 1983, and has since logged tens of thousands of testing hours
for many types of architectural and industrial coatings, cementitious and
synthetic repair materials, weatherproofing treatments, stains and sealants.
Specimens tested have included the Company’s existing product formulations,
potential ingredient substitutions or changes to those formulations, potential
new products and a wide variety of competitive products. The objective is to
observe and compare how the various materials perform when tested under the
same conditions, and to predict whether proposed changes will enhance or diminish
weather resistance.
Over the course of this testing, and through
correlation with observations made under natural weathering conditions, the
laboratory has defined ultraviolet sources, operating temperatures and cycle
times which correlate well with natural weathering. Hundreds of laboratories
have similarly developed empirical “rules of thumb” to permit rough estimation
of the correlation between hours of accelerated weathering exposure and years
of natural exposure5. This method has proven extremely useful in
ongoing product development and evaluation work.
For the accelerated weather exposure of
brownstone patching compound samples, UVA 340 lamps were used. The apparatus
was operated in alternating 4-hour cycles of ultraviolet radiation (dry) and hot
water condensation (wet) at 60 and 50 degrees Centigrade, respectively.
Test panels were prepared by mixing the patching
compound, designated as 701R, as specified by the manufacturer, and filling
into 3” (7.6 cm) x 6” (15.2 cm) x ¼” thick (6.4 mm) open aluminum frames.
Hardened panels were removed from the shallow, open molds after 72 hours, and
initial color was measured. They were then allowed to air cure for 28 days.
Following this curing period, panels were re-measured for color, prior to the
start of the 800-hour accelerated weathering test. Some of the panels were
exposed to accelerated weathering, while others were stored as Controls under
ambient laboratory conditions.
Figure 3
Figure 3 shows two pairs of test panels,
representing two different brownstone repair products. 701R is on the left. The
product on the right is a commercial cementitious mortar that is no longer
being produced by its manufacturer.
The two upper panels are the unweathered
Controls for each product. The two lower panels have been exposed in the QUV
apparatus for 500 hours. Although there is very little visible difference
between the weathered and unweathered 701R panels on the left, the product on
the right exhibits severe white streaking after exposure, suggesting a deficiency
in this product’s weather resistance.
QUV exposure for the 701R mortar (left) was
continued for a total of 800 hours, at which time the spectrophotometer was
used to measure the precise changes that occurred. Color of an aged,
unweathered Control was also measured at the conclusion of the 800-hour test
period. This measurement was designated as the “Standard”.
The unexposed Controls were measured yet again
after a total of six months’ aging to aid in estimating the color changes
unrelated to weather exposure that are to be expected with time.
Figure 4:
Color and Color Difference Data for Various Controls and Weathered Panels of
Composite Brownstone Patching Compound
Specimen
|
da Red/Green
|
db Yellow/Blue
|
dL Lightness
|
dE Overall
|
|
Demolded
panel, 72 hrs. old |
0.39 |
0.08 |
-0.56 |
0.69 |
|
Panel
1, 28 days ambient cure |
0.06 |
0.32 |
-0.14 |
0.35 |
|
Panel
2, 28 days ambient cure |
-0.06 |
0.01 |
-0.21 |
0.21 |
|
CONTROL,
800 hours lab air |
0.00 |
-0.01 |
0.02 |
0.02 |
|
Duplicate
Control, 800 hours lab air |
0.23 |
0.21 |
0.01 |
0.31 |
|
Triplicate
Control, 800 hours lab air |
0.34 |
0.39 |
0.11 |
0.53 |
|
CONTROL,
6 months lab air |
0.44 |
0.29 |
0.45 |
0.69 |
|
Duplicate
Control, 6 months lab air |
0.09 |
-0.02 |
-0.42 |
0.43 |
|
Panel
1, 800 hours QUV Exposure |
-0.12 |
-0.16 |
3.91 |
3.92 |
|
Panel
2, 800 hours QUV Exposure |
0.59 |
0.04 |
4.09 |
4.13 |
Note:
All values shown are averages of three measurements. Larger positive or
negative values represent greater change. (No change = 0). dE
is overall color change and is calculated as the square root of the sum of dL2
+ da2 + db2 .
The
significance of positive and negative data values are as follows:
|
Data |
Interpretation |
|
+da |
Sample redder (or less green) |
|
-da |
Sample greener (or less red) |
|
+db |
Sample yellower (or less blue) |
|
-db |
Sample bluer (or less yellow) |
|
+dL |
Sample lighter |
|
-dL |
Sample darker |
Figure 4 lists the results obtained for each of
the sample panels measured. The unexposed panels exhibited relatively small
color differences over time, with dE ranging from 0.21 to 0.69. The weathered
panels exhibited dE of around 4.
The graphic output shown in figure 5 is the
color difference plot for the brownstone patch after 800 hours of accelerated
weathering. The color difference plot has two segments: The circular region at
the center of the plot represents Chromaticity, or the degree to which the
balance of red-green and yellow-blue are the same when comparing the weathered
test panel with the unweathered control. The vertical bar at the right side of
the plot is Value, expressed as dL, the difference in lightness-darkness
between the two measured panels. The circle in the center of the Chromaticity
segment of the figure represents a tolerance range of da, db = 1.0, an arbitrary color difference
limit often used in paint color matching. Color differences of 1.0 or less are
generally considered acceptable as paint color matches, although the eye can
often detect some color difference below this limit.
There are no specific standards governing
acceptable levels of color change in composite repair materials. The color
changes or deviations observers consider acceptable can only be ascertained
through testing of statistically significant numbers of human observers.
Defining the means of developing such tests and standards are beyond the scope
of this study, but ultimately the aim would be to correlate particular dE
values in a number of different color ranges with human responses as to the
acceptability of those color differences.
Color control of cementitious materials is more
challenging than for typical architectural coatings such as latex paints.
Cementitious products incorporate much lower levels of pigment than typical
architectural paints and appearance is more strongly influenced by the binding
matrix. While latex paint undergoes relatively simple drying and film formation
after application, the process of cement hydration is more readily influenced
by cure conditions and several other factors.
Depending on the temperature of cure, cement
will form different crystal shapes, affecting color7. The amount of
mixing water added to cementitious materials (water-cement ratio)8 and the rate at which the mortar mixture dries
can also have an influence on color shading. Accordingly, colored cement
products cannot be expected to maintain the same degree of color consistency as
paints, and any eventual standard established for mortar color change would
necessarily be less stringent as well.
The change in Chromaticity (da, db) of the 701R
patching compound tested over 800 hours of QUV exposure in this study is less
than 1.0 (See Figure 4). The dL data, the change in Lightness/Darkness,
provides nearly all of the overall color change, dE, in this case.
This is graphically illustrated by the
Reflectance vs. Wavelength Plot in Figure 6. The curves indicate the percentage
of reflected light across the visible light spectrum for both the Control and a
weathered specimen. Overall, the curves have nearly the same shape, with a
difference on the order of two to three percent across the entire spectrum. The
upper curve, the weathered specimen, is reflecting more light across the entire
spectrum than the Control, accounting for its relative lightness. This provides
an important clue as to why and how the material changed, slightly, over the
course of the exposure.
HOW
AND WHY DOES COLOR CHANGE?
While it is informative to describe the extent
and nature of color change, it is even more useful if we can expand upon this
information to gain an understanding of how and why composite patching systems
change in color over time.
The potential sources of color change in
cementitious repair systems include fading of colorants, chalking of the
matrix, yellowing associated with the aging of Portland cement, surface soiling
and development of efflorescence.
It is commonly assumed that when patches become
lighter in color, the pigments are “fading”. This is unlikely, however, when
using high quality iron oxide pigments meeting the requirements of ASTM C9799,10. The nature of the measured color change as
illustrated in Figure 5 further discounts the assumption of pigment fading. Had
the saturation of the pigments in the patching compound been reduced by fading,
we would expect to see corresponding changes in the Chromaticity data (da, db).
In particular, “fading” patches would be expected to become “greyer” or lower in
saturation, rather than lighter or higher in Value, due to the increased
influence of the grey Type I Portland cement upon which they are based.
Chalking of the patching compound matrix is
related to mechanical breakdown of the cementitious binder, which relates to
surface erosion.
The influence of a light-colored aggregate could
also predominate in a much later stage of weathering, when there is pronounced
erosion of the cementitious binder. There was no indication, however, of any
such erosion at this stage of exposure.
Color changes associated with the aging of
Portland cement have been reported to trend towards a more yellow hue11,
a development that is not evident in this case. Neither of the two 800-hour
weathered test panels showed a significant positive db, which would have
indicated increased levels of yellow. It should be noted that pigments used
with Portland cement tend to mask yellowing associated with cement aging11.
While surface soiling can have a dramatic effect
on naturally exposed surface colors, the enclosed QUV system offers little or
no opportunity for soiling to develop. Whereas the system produces wetness by
condensation of vapor on test panel surfaces, the water deposited on the panels
is free of contaminants that may build up as surface soiling. Soiling can
therefore be dismissed as a factor contributing to color change, in this case.
This leaves efflorescence as a final potential
cause of color change. For the purpose of illustration, let us assume that
instead of describing the change in color of the patching material over time
and exposure, the data in Figures 4 - 6 had been describing an attempt to
color-match a brownstone patch. The formula for that brownstone patch might
incorporate a number of different color pigments, possibly including white,
carbon black, red iron oxide, yellow iron oxide and perhaps some others. In the
course of adjusting the color of that patch, if we were to obtain readings
indicating the same color differences shown in Figures 4 - 6, color differences
which are almost entirely in the dL (Lightness/Darkness) portion of the data,
we would recognize that the need for color adjustment was in the level of the
white pigment. Since in this case dL >0, indicating that the color is
lighter, or “whiter” than the original brownstone standard, the data would be
indicating that a little too much white pigment has been added.
Of course, there could have been no
actual addition of white pigment to the patching compound over time in the
laboratory accelerated weathering exposure tests. An alternative source must
therefore be sought for the elevated whiteness level that has developed over
the 800 hours of accelerated weathering.
Measurement of dL variations over time have previously
been used to describe corresponding changes in levels of efflorescence.11
Efflorescence is known to be a common cause of short to medium term color
change in cementitious compositions12 and is an obvious potential
source of increased whiteness over a particular range of times and exposures.
Understanding this phenomenon is essential to understanding color and color
change in composite repair compounds.
Efflorescence
is the deposition of soluble salts on masonry surfaces as moisture evaporates.
The salts can originate from multiple sources in the building envelope,
including the masonry itself, masonry mortar, back-up materials and ingredients
in the patching compounds. To some extent, all materials containing Portland
cement or lime incorporate constituents capable of being transported to the
surface and deposited as efflorescence. While aesthetically objectionable,
efflorescence is generally not considered harmful in and of itself, but it can
be an indication of continuing moisture infiltration problems that may be
damaging if left unresolved.
A
common assumption is that if patches discolor or efflorescence develops on
patch surfaces, that this represents a problem with the patching system. As we
have seen in Figure 3, sometimes that is the case, as some patch formulations
demonstrate strong tendencies to “whiten” under both QUV and natural exposures.
The presumption of patch instability is not valid, however.
Figure
7 shows a restored building section exhibiting some moderate to heavy white
efflorescence. In this case, the efflorescence cannot be a sign of patching
compound instability or sensitivity, because the depicted section is not a
patch, it is new replacement limestone. The source of the salts in this case is
the backup masonry, which has remained saturated for many years and has built
up a high level of dissolved salts within a wet zone behind the surface. As the
building dried out following restoration, the water migrated to the surface and
evaporated, leaving behind the salts.
The
mechanisms by which efflorescence forms are related to the way in which masonry
assemblies which are wet go about drying. The extent to which efflorescence
develops is highly variable, and will depend on such factors as the
concentration of available soluble salts within the masonry, the volume and
pattern of water infiltration and flow through the building envelope, and the
length of time over which the process has been working. Efflorescence may be as
light and unobtrusive as to be unnoticeable, may appear as a slight surface
haze, or may be distinctly white and sufficiently heavy to measurably alter
surface profile. The point is that many things in the building envelope
can be sources of efflorescence, and repairs or coatings can alter the way in
which a masonry assembly dries. In some cases, this will induce the formation
of efflorescence.
Studies
of efflorescence and color in concrete products have suggested that most of the
color change associated with efflorescence will diminish with time and weather
exposure11. Some level of permanent lightening has been reported,
however, and available data suggests that long term lightening on the order of
dL = 3 or more is to be expected9, 11. Efflorescence is more
noticeable over darker, more intense color shades. Experience with composite
patching of brownstone confirms this observation, as even very slight deposits
of efflorescence can create very noticeable changes in color.
Efflorescence
and its tendency to form have also been associated with the density and porosity
of cementitious materials. Generally, the lower the density and the higher the
porosity, the more freely liquid water can move through a material and the
greater will be the tendency for efflorescence to form11. Composite
repair mortars for soft sandstones are intentionally designed for relatively
high permeability in order to be compatible with the porous host stone.
Unfortunately, this also makes them ideal for the passage of salt-laden water,
which can lead to the deposition of efflorescence.
Change
in Lightness, dL, on the order of 3 to 4, as recorded in the 800-hour QUV test
for compound 701R, is unlikely to be objectionable. It is conceivable, however,
that more pronounced efflorescence (higher dL) may develop in any particular
case. If heavy efflorescence does occur, efforts should be made to determine
whether the causes are continuing moisture infiltration, repair mortar
instability, or a simple one-time event related to drying of the structure.
Once the causes are understood and addressed, if required, any remaining
aesthetic issues can be resolved by such means as removal, staining, or simply
allowing nature to weather it away in due course.
Correlating Laboratory Studies with Natural Exposure Data
Laboratory
methods for evaluating the various performance characteristics of repair
materials are only valuable if they correlate reasonably well with the actual
performance of the same materials under natural exposure conditions.
Accelerated weathering tests do not provide quantitative predictions of the
rates at which materials will change over time and exposure. Rather, their
objective is to evaluate relative resistance to weathering, and good
correlations are those that accurately predict the relative ranking of the
formulations or materials being tested and their resistance to change under
natural weather exposures. The mode of change to be expected, such as
yellowing, chalking, swelling or flaking are also
important observations. A reality check for the methods used in the
laboratory’s accelerated weathering study of color change involved the
evaluation of naturally weathered twelve-year-old brownstone patches on a
building in
In
1989, Christ Church Cathedral, a 19th century brownstone church
listed in the National Register of Historic Places, was patched using composite
repair compounds. In 2001, additional repairs were undertaken, affording an
opportunity to examine the twelve-year-old work at close quarters. Patch
fragments were removed to the color lab, and the spectrophotometer and color
software were used to compare the color of the twelve-year-old patch to a
Control chip of the same color, which had been deposited in the lab’s color
library in 1989. Results are listed in Figure 8.
Figure
8: Color Difference Data, three sample points, 12-year-old naturally weathered
brownstone patches. All data are averages of three measurements.
|
Specimen |
da |
db |
dL |
dE |
|
2001-1 |
1.49 |
1.32 |
1.05 |
2.25 |
|
2001-2 |
1.50 |
2.37 |
2.74 |
3.92 |
|
2001-3 |
1.10 |
1.08 |
3.14 |
3.50 |
Overall
color change, dE, was of similar magnitude to the color changes recorded in the
laboratory study, typically on the order of 3 to 4. Chromaticity differences
were slightly higher in the naturally exposed material than in the 800-hour QUV
weathered materials. The difference in dL, Lightness/Darkness, was again a
significant element of overall color difference, and as in the accelerated
weathering study, colors generally became slightly lighter with time and
exposure. The naturally weathered patches in this case were also slightly more
yellow and redder than the control.
Figure 9
Visually, the most conspicuous difference
between the laboratory-weathered specimens and the naturally weathered
specimens was in the amount of mica evident in the naturally exposed twelve-year
old mortar. Mica flakes were incorporated in the brownstone patch mixtures to
replicate the mica found within the natural stone. Over the course of twelve
years of natural exposure, slight surface erosion occurred, exposing more of
the mica flakes. This may have influenced Chromaticity measurements of the
weathered material, potentially obscuring actual color change in the composite
patch matrix.
This observation relates to the
importance of considering the correlation between natural and laboratory
exposure times. Reference has been made to the common practice of employing
empirically derived rules
of thumb for
correlating accelerated and natural weathering exposures5. The
800-hour QUV exposure would correspond with a significantly shorter period of
natural exposure in
Additional research is required to
establish more precise correlations between natural and accelerated weather
exposures, and the extent of the color changes to be expected in composite
repair mortars over time. Ideally, the work would include greater numbers of
samples spanning a wider range of colors, and involving more frequent color
measurements over the course of both natural and accelerated weathering.
Generally, however, the extent of the overall color changes measured and the
changes in lightness (dE, dL on the order of 3-4) in the naturally weathered
materials corresponded sufficiently well with the laboratory weathered
materials to confirm the merit of the research concept and direction.
KEEPING
SIGHT OF THE LARGER CONTEXT
No
technical study, focused as it must be on the details of a component of a
problem, can hope to resolve all of the greater issues of which it is a part.
The work of measuring and optimizing color and color retention has linkages to
much broader issues and implications. While the work of a materials development
laboratory centers on formulation issues over which it has control, the
limitations of what can be achieved through formula optimization work must also
be kept in perspective.
In
particular, proper analysis and resolution of the underlying problems that
caused the damages being repaired is critical. Moisture infiltration is the
most common cause of damage to masonry. Control of moisture infiltration is
also critical to controlling efflorescence12. Controlling
efflorescence is critical to controlling color.
The
work also relates to the broader issue of composite masonry repair material
selection. There are no consensus standards addressing the various properties
of these materials or the priorities or process by which they should be
selected. Color and Color Retention must be kept in perspective as but one
aspect of what repair materials must achieve. Every material selection
inherently involves a series of prioritized compromises13, as
“perfect” repair materials do not exist. Engineering basics, including the
importance of such properties as Tensile Bond Strength, Drying Shrinkage, and
Modulus of Elasticity must be given due attention and priority in masonry
repair14. Material selection should be based on the best available
overall balance of performance and aesthetic properties, as required to meet
the specific needs of each project.
TOWARD
OPTIMIZATION OF COLOR AND COLOR RETENTION
The
optimization of composite patch color and color retention requires the integration
of design and workmanship elements that also contribute to the goal of
long-term color compatibility. On a basic level, there are available techniques
for improving long-term aesthetics for composite repairs.
The
use of multiple colors of patching compound, for example, makes it less evident
to the casual observer that slight color differences in the repairs represent
anything other than the natural variations typically found in building stone.
The deliberate selection of colors that are initially
slightly darker than the host substrate can counterbalance any tendency of the
material to become slightly lighter with age. The close matching of texture as
well as color should not be overlooked as a critical component of perceived
color. The use of compatible accessory products such as translucent breathable
stains to simulate aging, and consolidants to stabilize substrates that are
deficient in strength, and water repellent treatments for substrates with poor
weather resistance can also profoundly improve repair color and long-term color
retention.
Conclusion
The
overall quality of composite repairs has improved greatly over the past two
decades through greater development of the knowledge base, better training of
repair mechanics and advances in repair materials technology. While some
composite repair systems have proven their ability to withstand the test of
time for at least a decade or two, aesthetic expectations will and should
continue to rise. This provides the impetus for researching and developing
further improvements.
There
is a great deal more work to be done in the area of
understanding, controlling and optimizing composite patch color and long-term
color retention. The research is only at the beginning, because acceptance of
the notion that some composite patches are capable of enduring over the
long-term is a relatively recent development.
The
development of objective, acceptable color tolerances is an important long-term
goal. If conflict over subjective aesthetic judgments is to be avoided,
consensus guidelines must be developed. Eventual tolerance standards for
composite repair color and long-term color retention will have to balance what
is technically feasible, commercially available, economically affordable and
aesthetically desirable. Precise measurement and communication of color and
color change in composite repair mortars are important starting points. They
begin to define what is being achieved now, and that is the first step along
the path toward a higher future ideal.
The work will be multi-faceted and constantly
evolving, rather than narrowly focused with clearly defined beginning and
endpoints. Other work in progress involves measuring
how patching compound color responds to moisture. There is opportunity for
further aesthetic improvement if patch color can be controlled to match
substrate color not only when dry, but also when wet.
Michael
P. Edison, Chemical Engineer, is President and Founder of Edison Coatings,
Inc., of
Endnotes:
1. Drs. Jay and Maureen Neitz, Department of Cell Biology, Neurobiology & Anatomy, Department of Ophthalmology, Medical College of Wisconsin, Color Vision Basics, June 29, 1999, http://www.mcw.edu/cellbio/colorvision/index.html
2. Hunter Associates Laboratory,
3. Munsell Color Communication Products, www.munsell.com
4. Colorimetric
Fundamentals, CIE 1976 L*a*b* (CIELAB),
5. "Correlation Questions and Answers- A Discussion of the most frequently asked questions about accelerated weathering, Douglas M. Grossman, Q-Panel Co., January, 1984.
6. Standard Practice for Operating Light- and Water-Exposure Apparatus (Fluorescent UV-Condensation Type) for Exposure of Nonmetallic Materials, ASTM G53 - 88, American Society for Testing and Materials
7. “A Guide to Pigmenting Concrete”, p. 10, Bayer Corporation, Industrial Chemicals Division, Publication #52-B707(5)C, undated.
8. Ibid, p. 8.
9. Bayferrox Synthetic Iron Oxide Pigments, Specification Requirements for Coloring Concrete Products, Bayer Corporation, Industrial Chemicals Division, 09/97
10. ASTM C979, “Standard Specification for Pigments for Integrally Colored Concrete”, American Society for Testing and Materials
11. “A Guide to Pigmenting Concrete”, p. 12, Bayer Corporation, Industrial Chemicals Division, Publication #52-B707(5)C
Figure 1: Comparison of Composite Patch Color Retention on various
elements of two restored 19th Century
brownstone churches in Middletown, CT. Stone on the First Church of Christ
(left) was repaired with three standard colors of a commercial pre-packaged
repair mortar in 1996. Repair work on
Figure 2: The Munsell and CIE L*a*b* systems for cataloguing color
Figure 3: Two panels of the same brownstone patching compound, Custom
System 45 #701R; Upper panel is an unweathered control, lower panel is shown
after 800 hours’ accelerated weathering exposure. The section of panel exposed
to accelerated weathering shows slight change.
Figure 4: Color and Color Difference Data for Various Controls
and Weathered Panels of Custom System 45 #701R
Brownstone Patching Compound
Figure 7: Efflorescence formation is evident on a
recently replaced limestone element.
Figure 8: Color Difference Data, three sample
points, 12-year-old naturally weathered brownstone patches. All data are
averages of three measurements.
Figure
9.
Color Difference Plot for naturally exposed twelve-year-old brownstone patch
vs. a twelve-year-old control chip of the same color