by Sarah Marie Jackson, Tye Botting, and Mary Striegel
The results of the abrasion and adhesion
testing before and after artificial
weathering are presented in this section.
Solids tests were compared for each of
the limewash samples. In addition,
changes in appearance of the limewashes
were recorded by colorimetry
and photographic documentation.
Test results were represented as an unweighted
average of the results from the
individual samples for each wash.
For each test except the artificial
weathering, three replicates were prepared
of each wash and the results
averaged. Due to space limitations in the
QUV, the artificial weathering was
performed in duplicate. A ranking system
was devised to evaluate the results
of each test, and each limewash was
ranked from best to worst for relative
change in appearance, adhesion, and
abrasion for samples both before and
after weathering. Depending upon the
number of limewashes for the sample,
the rankings varied from 1 (worst) to 10
to 13 (best) (Table 2). The ranking was
based on the unweighted averages of the
result from each test. The ratings where
two washes are grouped together are
representative of washes with the same
overall rating.
Table 1. Limewash Recipes Showing Ingredients Used |
|
Handmade Brick |
|
Modern Brick |
|
Weathered and Rough-sawn New Wood |
|
Epoxy |
Best 13 |
Wash A |
Best 9 |
Wash B |
Best 12 |
Wash E |
Best 3 |
Wash E |
12 |
Wash K |
8 |
Wash D |
11 |
Wash G |
2 |
Wash D |
11 |
Wash M |
7 |
Wash A & K |
10 |
Wash D |
Worst 1 |
Wash G |
10 |
Wash D |
6 |
|
9 |
Wash A & I |
|
|
9 |
Wash C |
5 |
Wash E |
8 |
|
|
|
8 |
Wash B |
4 |
Wash F & M |
7 |
Wash B & H |
|
|
7 |
Wash G |
3 |
|
6 |
|
|
|
6 |
Wash L |
2 |
Wash G |
5 |
Wash F |
|
|
5 |
Wash I |
Worst 1 |
Wash C & I |
4 |
Wash C |
|
|
4 |
Wash H |
|
|
3 |
Wash L |
|
|
3 |
Wash F |
|
|
2 |
Wash M |
|
|
2 |
Wash E |
|
|
Worst 1 |
Wash N |
|
|
1 |
Wash N |
|
|
|
|
|
|
The results of abrasion testing on all
limewashed samples of handmade brick
are presented in Figure 3. These results
compare abrasion testing before and
after artificial weathering. In all cases
the limewash performed better before
artificial weathering except for wash M,
which performed better after artificial
weathering. Washes that include a salt
additive required the most volume of
sand to abrade through to the substrate
before artificial weathering. After artificial
weathering, all limewashes performed
markedly worse, with the exception
of wash M, which performed more
than twice as well. Washes A, B, L, and
M required similar volumes of sand
abrasion after artificial weathering.
Limewashed modern-brick samples
performed similarly to handmade brick.
The results of abrasion testing on
limewashed wood samples, including
both weathered and rough-sawn new
wood, are presented in Figure 4. It
should be noted that limewash was
flaking off samples of washes C, D, and
E before testing began. All limewashes
were poor performers on wood substrates
both before and after artificial
weathering. None of the washes withstood
more than 5 liters of sand abrasion.
Several samples from washes L, M,
and N retained insufficient limewash
after artificial weathering to perform
abrasion testing. The samples that were
tested from washes L, M, and N took
less than 250 milliliters to abrade to the
wood substrate. Epoxy samples had
results similar to the wood in the abrasion
tests.
Figure 5 presents adhesion results on
historic handmade brick. All limewashes
performed similarly before and after
artificial weathering except wash M,
which performed better after artificial
weathering. Before artificial weathering
washes D, F, and H were well rated.
After artificial weathering, wash M
was the best rated, followed closely by
washes C, E, G, and K. Washes D
through G had powdering surfaces that
made it more difficult to perform adhesion
tests. In many of these adhesion
tests there was little consistency between
replicates, leading to a large standard
deviation.
On the wood samples all washes
performed similarly in the adhesion
testing before and after artificial weathering.
On most samples the limewash
was beginning to flake off before testing,
and there was not a solid, cohesive coat
to remove with the tape. The best performers,
washes E, F, and G, had a
rating average in the middle of the scale
and experienced between 1⁄16 inch and 1⁄8
inch of loss along the incision. Before
artificial weathering wash A received an
average rating and ranked with the best
performers. After artificial weathering
wash A was rated significantly lower,
near the bottom of the group. The rest
of the washes averaged a rating between
0A and 1A both before and after artificial
weathering. The migration of salt
through the brick samples during artificial
weathering is one likely cause for
the poor performance of wash A in tests
after artificial weathering.
Handmade-brick and modern-brick
samples performed exceptionally well
during artificial weathering. All recipes
on brick samples were rated 4A or 5A,
the top rankings, and had an excellent
appearance after artificial weathering.
However, a marked difference could be
seen in the performance of all washes on
the adhesion and abrasion tests before
and after artificial weathering.
There was a noticeable failure of the
limewash on numerous weathered-wood
and rough-sawn new wood samples
during artificial weathering (Fig. 6).
Washes D, E, and I were the only recipes
that had a rating average above 4A.
Washes F and G had the next highest
average ratings but a large standard
deviation. For all recipes applied to
wood samples, artificial weathering
generally removed limewash from the
peaks of the grain on the weathered
samples, leaving limewash remaining in
the valleys. This may be a result of the
valleys in the grain being created by the
less dense spring growth that erodes
faster than the harder, denser summer
growth. 14 The lower density wood in the
valleys in the grain or the valley itself
may have provided assistance in the
adhesion of the limewash.
On all materials washes A, B, and C
showed the highest solids deposit, which
may be a result of the salt additive (Fig.
7). Washes from the same recipe tended
to have similar solids deposit regardless
of the lime used. Washes G, H, and I,
with the acrylic-emulsion additive,
showed the lowest solids deposit. Washes
D, E, and F, with the casein binder,
had the second-highest solids deposit.
Washes L, M, and N, which did not
have additives, had solids deposits similar
in amount to washes G, H, and I.
The total color difference was calculated
based on data from the Minolta
colorimeter before and after artificial
weathering. 15 Results were similar for all
samples where limewash remained after
testing. Samples where limewash remained
after artificial weathering often
showed lighter results than before
weathering. One sample from wash G
on the wood substrate had a drastic
color change that was the result of a
tan spot that developed during artificial
weathering. It is unclear whether the tan
spot was the result of tannins in the
wood migrating through the limewash
or a reaction of one of the additives.
During the visual inspection and
documentation of samples before testing,
crystallization was observed on
washes A, B, and C. These washes had a
salt additive. Samples were examined
under a Leica MZ 8 stereomicroscope to
confirm the crystallization (Fig. 8). In
the stereomicroscope photographs, the
crystals are readily apparent as raised,
discolored material differing in composition
from the limewash itself. These
crystals could be either salt or sugar
(from the molasses additive), since those
were the only constituents capable of
such crystallization. Their presence suggests
that the additive did not become a
cohesive part of the limewash matrix
upon drying. Furthermore, since both
salt and sugar are highly soluble, the
crystals would be lost upon exposure to
water, disrupting the matrix significantly
and weakening the limewash.
During the artificial weathering, a
white residue began to appear on the
unexposed back of the modern brick
samples from washes A, B, and C
(Fig. 9). Using a Keymaster TRACer III
Portable XRF for X-ray fluorescence
analysis, the residue was studied. The
results showed that the residue contained
chlorine. The limewash on the
surface, which was tested after artificial
weathering, had almost no trace of
chlorine. Thus, the chlorine on the
backside of the samples suggests that the
salt migrated from the limewash through
the modern brick samples.
Originally published in APT BULLETIN: JOURNAL OF PRESERVATION TECHNOLOGY / 38:2-3, 2007
Notes
1. Laura Soulliere Gates, email to author, Aug.
17, 2006.
2. National Park Service Technical Information
Center, 'Class C' Cost Estimating Guide: Historic
Preservation and Stabilization (Denver:
Denver Service Center, 1993), 18.
3. Colin Mitchell Rose, Traditional Paints,
available from http://www.buildingconservation.com/articles
/paint/paint.htm.
4. Abbott Lowell Cummings and Richard M.
Candee, "Colonial and Federal America:
Accounts of Early Painting Practices" in Paint
in America: The Colors of Historic Buildings
14 (New York: Wiley, 1994), 14.
5. Scottish Lime Centre, Technical Advice Note
15: External Lime Coatings on Traditional
Buildings (Edinburgh: Historic Scotland, 2001).
6. Ibid.
7. John Ashurst and Nicola Ashurst, Mortars,
Plasters, and Renders, vol. 3 of English Heritage
Technical Handbook (Great Britain:
Gower, 1995), 47.
8. Roger W. Moss, "Nineteenth-Century Paints:
A Documentary Approach" in Paint in America:
The Colors of Historic Buildings (New
York: Wiley, 1994), 55.
9. ASTM Subcommittee D01.24, Standard Test
Methods for Viscosity by Ford Viscosity Cup,
ASTM D 1200-94 (West Conshohocken, Pa.:
ASTM, 1996).
10. Marcy Frantom, email to author, Sept. 12,
2005.
11. ASTM Subcommittee D01.23, Standard
Test Methods for Abrasion Resistance of
Organic Coatings by Falling Abrasive, ASTM
D 968-93 (West Conshohocken, Pa.: ASTM,
1996).
12. ASTM Subcommittee D01.23, Standard
Test Methods for Measuring Adhesion by Tape
Test, ASTM D 3359-95 (West Conshohocken,
Pa.: ASTM, 1996).
13. ASTM Subcommittee D01.27, Standard
Practice for Conducting Tests on Paint and
Related Coatings and Materials Using a Fluorescent
UV-Condensation Light- and Water-
Exposure Apparatus, ASTM D 4587-91 (West
Conshohocken, Pa.: ASTM, 1996).
14. Pete Sotos, conversation with author, Nov.
15, 2006.
15. Ruth Johnston-Feller, Color Science in the
Examination of Museum Objects: Nondestructive
Procedures (Los Angeles: Getty Conservation
Institute, 2001), 35.
16. L. Franke and I. Schumann, "Causes and
Mechanisms of Decay of Historic Brick Buildings
in Northern Germany," in Conservation of
Historic Brick Structures, ed. N. S. Baer, S. Fitz,
and R. A. Livingston (Shaftsbury: Donhead,
1998), 26-34.
SARAH MARIE JACKSON joined NCPTT in
2005 as a graduate intern to continue the testing
for the limewash study. In 2006 she accepted
a permanent position with the Architecture
and Engineering Program at NCPTT. She
received a master’s degree in historic preservation
from the Savannah College of Art and
Design.
TYE BOTTING is a research staff member at
the Institute for Defense Analyses. He served as
the NCPTT/NSU joint faculty researcher for
three years. He holds a PhD in nuclear chemistry
from Texas A&M University, where he did
post-doctoral work in nuclear engineering.
MARY STRIEGEL is responsible for NCPTT’s
Materials Research Program, focusing on
evaluation of preservation treatments for
preventing damage to cultural resources. She
also directs investigation of preservation treatments
geared towards cemeteries and develops
seminars and workshops nationwide. She holds
a PhD in inorganic chemistry from Washington
University in St. Louis.