EVALUATION OF COATING PERFORMANCE ON SILVER
EXPOSED TO HYDROGEN SULFIDE
CAROL A. GRISSOM, NICOLE GRABOW, COREY SMITH RILEY, AND
A. ELENA CHAROLA
Museum Conservation Institute, Smithsonian Institution
Midwest Art Conservation Center
Caryatid Conservation Services
Twelve coatings applied to sterling silver coupons were subjected to high levels of hydrogen sulfide to test their effec-
tiveness in preventing tarnish in indoor environments. Silver coupons were assessed over the course of the exper-
iment using visual observation, image analysis of digital photographs, gloss measurement, and colorimetry.
Acrysol WS-, Agateen Air Dry Lacquer No. , HMG Heat and Waterproof Adhesive, and PVAc AYAT gener-
ally performed best. Mowiol - performed very well on some areas but was mediocre otherwise. Butvar B-
and Acryloid B-N were generally less effective in retarding tarnish. Acryloid B- and Butvar B- performed
less well. Aquazol , Aquazol  topped with Renaissance Wax, and Renaissance Wax were least effective.
Coating thickness proved to be a significant factor in coating performance.
KEYWORDS: silver, coatings, hydrogen sulfide, thickness, gloss, colorimetry
. INTRODUCTION
The common pollutant hydrogen sulfide (HS) is well
known to be the primary cause of tarnish on silver
objects, occurring at concentrations as low as .
parts per billion (Franey et al. ). Passive methods
of sulfide mitigation for storage and display environ-
ments are the goal of most museums. When these non-
invasive methods are not feasible, however, conserva-
tors have applied a limited number of organic coatings
to surfaces of silver artifacts. On account of excellent
appearance, ease of application, and tarnish preven-
tion, cellulose nitrate lacquers have been the most
popular coatings for this purpose in the United States
and United Kingdom. In particular, Agateen Air Dry
Lacquer No.  has been used in the United States
(Heller ; Reedy et al. ) and H. Marcel
Guest’s HMG and MacDermid’s Frigilene FR
lacquer in the United Kingdom (Plenderleith and
Werner , ; Thickett and Hockey ). Cellu-
lose nitrate coatings are broken down by ultraviolet
light (Selwitz ), however, and reapplication is
necessary after – years (Luxford and Thickett
). The acrylic resins Paraloid (formerly Acryloid
in the U.S.) B-, B-N, and B-, which are so
often used by conservators on other types of artifacts
because of their stability, are increasingly applied on
silver (Argyropoulos et al. ; Degrigny ),
although two studies have shown these acrylics to be
less effective in preventing tarnish from HS than cellu-
lose nitrate lacquer (De Witte ; Reedy et al. ).
Microcrystalline waxes have also been used for coating
silver, especially when ventilation is poor, since they do
not require the quantities of volatile organic solvents
necessary for resin-coating formulations (Argyropoulos
et al. ). Few scientific studies comparing the effec-
tiveness of coatings on silver have been published.
The goal of this study was to assess the effectiveness of
 coatings in preventing silver from tarnishing in the
presence of HS. Five of the coatings tested are among
those just described that are widely used by conservators
on silver, although each is known to have drawbacks.
The remaining seven materials, in use by conservators
for other purposes, were selected for comparative
testing as a first step in identifying possible alternatives.
Preliminary data for eight of the coatings were presented
at the Metal  conference (Grabow et al. ). This
paper provides a more comprehensive evaluation of all
 coatings using four assessment techniques.
. EXPERIMENTAL
. INTRODUCTION
The  coatings represented a range of materials
from several major classes of polymers commonly
used in conservation (table ). Cellulose nitrate,
acrylic resin, and Renaissance Wax coatings have
© American Institute for Conservation
of Historic and Artistic Works 
DOI: ./Z. Journal of the American Institute for Conservation , Vol.  No. , –
been commonly used on silver, while use of the remain-
ing seven coatings has been limited to other purposes.
Coatings were prepared as listed in table . Acrysol
WS-, HMG Adhesive, and Renaissance wax were
applied as supplied by their distributers. Since prep-
arations of these pre-mixed solutions were identified
as optimal by the manufacturers, they were not
adjusted. For other coatings, the most appropriate
solvent was selected for each coating based on strong
solvent-solute interaction and, whenever possible, a
low evaporation rate in order to improve leveling and
even film formation. Fifteen percent solids were used
for each coating as far as was practical, but in several
cases the solutions were adjusted to achieve coatings
with similar viscosities. The Butvar resins mixed at
% were too thick for the draw-down application,
for example, and were therefore thinned to  and
%. The final solutions of the  coatings seemed to
vary only slightly in viscosity.
. COUPON PREPARATION
Fifty-one silver coupons were cut to measure .
cm × . cm from -gauge (.-mm thick) sterling-
silver sheet; the relatively large silver coupon size was
chosen to avoid edge effects. Four silver coupons were
coated with each of the  materials, and an extra
three silver coupons were left uncoated as blanks.
Each silver coupon was wet polished using
progressively finer silicon-carbide abrasive paper and
micro-mesh cushioned abrasive cloths (down to  µm
abrasives) to match the near-mirror finish of a desig-
nated standard silver coupon. After thorough rinsing,
the silver coupons were air dried at ambient conditions.
This method of polishing differs from that commonly
employed on historic silver in the hardness of the abra-
sive used, but the end result is comparable. Two glass
microscope slides were coated with each material to
monitor any changes in the coatings themselves, and
two glass slides were left uncoated as blanks.
A random number was selected for each silver
coupon and glass slide, ranging from  to . The
number was scored into the lower right corner on the
back of each silver coupon and on the uncoated end
of each glass slide.
. COATING APPLICATION
Liquid coatings were applied to the silver coupons by
a draw-down method (ASTM ). Approximately
 ml of the coating solution was pipetted in a line
along the top edge of the silver coupon and drawn
down across the surface using a glass rod elevated a
fixed distance above the silver coupon. In order to
maximize measurable changes in the coatings them-
selves, glass slides were dipped in liquid coatings to
obtain thick films. Renaissance Wax was applied both
by itself and over Aquazol (one day after coating)
TABLE . TWELVE COMMERCIAL PRODUCTS USED IN THIS STUDY AND THEIR APPLICATION CONCENTRATIONS IN ORDER BY MAIN
COMPONENT
Main component Product name Tg°C Solvent or
diluenta
Conc. by
wt.
In use on
silver
Acrylic Acryloida B-N (now
Paraloid B-N)
 % xylenes/% acetone % x
Acrylic Acryloida B- (now
Paraloid B-)
 xylenes % x
Acrylic dispersion Acrysol WS- (Primal
WS-b)
 none %
Cellulose nitrate Agateen No.  % Agateen thinner – x
Cellulose nitrate HMG Adhesive none – x
Polyvinyl acetate PVAc AYAT  methanol %
Polyvinyl alcohol Mowiol – Boiling deionized water %
Polyvinyl butyral Butvar B-  -butanol %
Polyvinyl butyral Butvar B-  -butanol and acetone %
Oxazoline Aquazol   -butanol %
Oxazoline + wax,
microcrystalline
Aquazol + Renaissance Wax none –
Wax, microcrystalline Renaissance Wax none – x
aRohm and Haas’ Acryloid B- and BN, now Paraloid B- and B-N, were obtained in October .
bEquivalent product outside the United States.
EVALUATION OF COATING PERFORMANCE ON SILVER EXPOSED TO HYDROGEN SULFIDE 
Journal of the American Institute for Conservation , Vol.  No. , –
using a Kimwipe, and it was buffed after about 
minutes; the waxing process was repeated a second
time. Coated silver coupons and glass slides were air-
dried and stored in a fume hood for several months,
then in the conservation laboratory. Eight months
elapsed before exposure of the coated silver coupons
and glass slides to HS. This delay, while due to a per-
sonnel change, had the advantage of ensuring that the
coatings were completely dry when exposed.
Coating thickness on the silver coupons was
measured just before HS exposure using an Elcometer
 thickness gauge, calibrated using a .-μm stan-
dard. The -mm-diameter measurement head was cen-
tered over each of  test areas (see next paragraph) on
the four coupons coated with each material. Three
readings were averaged in each area.
The  test areas selected for measurement per
coating included  areas (five each) on the three
silver coupons that would be exposed to HS and five
areas on the fourth silver coupon that would be the
unexposed coated silver control. For each silver
coupon, the five . cm test areas were chosen ran-
domly from  squares in an imaginary grid over the
silver coupon, excluding a .-cm border to avoid
edge effects. Paper templates were custom made to
mask off the outer border and other  squares for
each test area, ensuring consistent positioning during
measurement. The same  test areas on each set of
coated silver coupons were measured for thickness
before exposure to HS and throughout the experiment
for gloss and color.
. EXPOSURE TO HYDROGEN SULFIDE
While two unexposed coated controls (one silver
coupon, one glass slide) for each coating were kept in
the conservation lab at ambient conditions, three
coated silver coupons and a glass slide for each
coating set, three uncoated silver blanks, and one
uncoated glass slide were placed in random order in
the test chamber, a Plexiglas Fisher Scientific desiccator
kept in a fume hood (fig. ). A small fan provided air
circulation in the chamber. A VWR digital hygrometer
measured  ± % RH and  ± °C inside the
chamber during the experiment. HS was generated in
the chamber by pipetting  drops of water onto  g
of powdered barium sulfide (BaS) in a Petri dish. Both
the BaS and water were renewed when silver coupons
were placed back into the chamber after removal for
measurements. During the first week the silver
coupons were removed for measurement and returned
to the chamber on a daily basis, then approximately
every  or  days for a maximum of  days in the
case of the most tarnish-resistant coated silver
coupons – exclusive of the time that the coupons were
removed for measurement.
HS concentration in the chamber was estimated to
be about  ppm using Dräger diffusion tubes for HS.
During the first weeks of the experiment the Dräger
tubes failed to measure any HS during the -hour
periods designated for measurement. The manufac-
turer’s scientists suggested that HS had preferentially
tarnished uncoated backs of the silver coupons rather
than registering in the Dräger tubes. This appeared to
be borne out by Dräger tube measurements of  ppm
HS in the chamber during -hour periods at the end
of the experiment, both when the chamber was filled
with silver coupons fully tarnished on the backs and
when it was without any silver coupons.
Exposure of the silver coupons to HS continued
until at least portions of all silver coupons showed
significant tarnish. The  days of total exposure for
the most tarnish-resistant samples was extreme. Other
researchers, by comparison, exposed silver to a
maximum of  ppm HS for just six days (Reedy
et al. ). Levels of HS in museum environments
reportedly range from about . to . ppm
(Hatchfield , ).
. COATING EVALUATION
Coating evaluation methods included visual obser-
vation, image analysis of digital photographs, gloss
measurement, and colorimetry. The first two methods
evaluated whole silver coupons, exclusive of edge
regions. In contrast, gloss and color measurements
were made on the five . cm test areas on each
silver coupon described above (see Section .), since
instrument spot sizes measured . cm or less. Two
areas on each coated glass slide and coated silver
FIG. . Hydrogen-sulfide chamber containing silver
coupons in the upper half. On the bottom of the chamber
are the hygrometer (left), Petri dish containing barium
sulfide (center), and fan (right).
 CAROL A. GRISSOM ET AL.
Journal of the American Institute for Conservation , Vol.  No. , –
control samples stored at ambient conditions were
measured for color and gloss.
For visual evaluation, three to six judges rated each
silver coupon four times: just before HS exposure and
three times during the first  hours of exposure.
Judges were “blind,” i.e. judges did not know the identity
of coatings on examined silver coupons. Judges gave two
separate rating values, each with scores from  to , to
assess color and amount of tarnish (see table ). Assess-
ment of color was based on the fact that when silver
tarnishes, silver sulfide corrosion formed on the surface
at first appears iridescent as interference colors and
changes in coloration as it becomes thicker. An ASTM
standard was used as an aid to determining the percent
tarnish scores (ASTM ). A composite visual evalu-
ation rating was obtained by multiplying the two
rating components. Hence, a rating of one ( × ) indi-
cated no change, and  ( × ) was the maximum.
Ratings were averaged for each silver coupon.
At regular intervals photographic images were taken
using a Nikon D digital camera. Image analysis was
done using Adobe Photoshop to assess the percentage
of tarnished surface. The outermost . cm edge of
each silver coupon’s image was cropped off, discolored
areas were marked solidly in a new layer, and the drop-
down histogram function under “image” provided the
percentage of the discolored areas, i.e. the percent tarnish.
Average gloss measurements were obtained on each
test area using a BYK-Gardner Micro-Tri-Gloss gloss-
meter set to °; the detection spot size was  cm in
diameter. The instrument was centered over the test
area, and three readings per test area were averaged.
Average colorimetry readings were taken on each test
area at the same time as gloss measurements using a
Minolta CR- colorimeter equipped with a ° to
° light source; the spot size of the instrument was 
mm in diameter. The instrument was centered over the
test area, and three readings per test area were averaged.
. RESULTS
. THICKNESS
Thickness measurements made using the Elcometer
 before the silver coupons were exposed to HS
are shown in figure  for each coating’s  measure-
ment areas and five areas on the unexposed coated
silver control. Acrysol showed by far the greatest
average thickness at  µm, and Renaissance Wax the
least average thickness at  µm; most other coatings
averaged from  to  µm. Average thicknesses for
coatings and ranges of thicknesses are also reported in
table .
Variations in average thickness measurements for
each coating are reported as relative standard devi-
ations (RSDs) in table . The manufacturer reports a
% precision in measurement for the Elcometer, but
actual precision for the three measurements in each
area was somewhat worse (average RSD %), slightly
expanding the margin of uncertainty. However, pre-
cision may also be tied to limit of detection. For
example, thickness in Agateen was shown to have –
% precision for the three readings in five out of six
areas where coating thickness was greater than 
µm. Reproducibility in thickness measurement below
this limit generally was –% or even worse for
thickness below about  µm. This suggests that
coating thickness measurements for Agateen below
about  µm were uncertain or that thicknesses were
highly variable on account of thin areas.
Within coating sets, variation in thickness between test
areas was generally more consistent on individual silver
coupons (cf. data for the best coupon and average of
the three coupons in table ). Thickness measurements
on individual silver coupons most thickly coated with
Acrysol, Agateen, HMG, Butvar B-, and Aquazol aver-
aged more than twice that of coatings on other silver
coupons thinly coated with the same materials.
. VISUAL EVALUATION
Figure  shows results for the average visual evalu-
ation rating of coated silver coupons, made by multiply-
ing values assigned to color and amount of tarnish by
TABLE . VISUAL EVALUATION RATING SCALES FOR COLOR
AND AMOUNT OF TARNISH
Scale Color of silver Scale Amount of tarnish
 No change  No tarnish
 Iridescence  <% tarnish
 Orange-brown  –% tarnish
 Dark brown  –% tarnish
 Blue-black  >% tarnish
FIG. . Thickness measured at  locations for each coating;
some symbols are superimposed.
EVALUATION OF COATING PERFORMANCE ON SILVER EXPOSED TO HYDROGEN SULFIDE 
Journal of the American Institute for Conservation , Vol.  No. , –
TABLE . THICKNESS AND PERFORMANCE OF COATINGS USING VISUAL EVALUATION AND DIGITAL IMAGE ANALYSIS, GROUPED BY
EFFECTIVENESS
Coating Thickness Visual evaluation color x
amt. tarnish at  hours
(RSD)
% Tarnish from image
analysis
Avg. (RSD) Range At 
hours
At 
hours
At 
years
Most effective coatings
Acrysol Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Agateen Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
PVAc AYAT Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
HMG Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Mowiol (PVOH) Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Less effective coatings
B-N Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Butvar B- Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
B- Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Butvar B- Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Least effective coatings
Aquazol Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Continued
 CAROL A. GRISSOM ET AL.
Journal of the American Institute for Conservation , Vol.  No. , –
four to six judges. As can be seen at  hours of
exposure, some silver coupons exhibited tarnish even
before exposure to HS. This phenomenon was most
pronounced for uncoated controls and Renaissance
Wax-coated silver coupons, but small amounts of pre-
exposure tarnish were observed on at least one silver
coupon in every coating set, except for those coated
with Acrysol and HMG.
By the end of the -hour time period for visual
evaluation, sets of silver coupons ranking best on
average were those coated with PVAc AYAT,
Acrysol, HMG, Acryloid B-N, Agateen, and Butvar
B-, in that order (fig. , table ). Evaluators estimated
that silver coupons coated with the first three materials
had less than % tarnish at the end of the time period.
In contrast, Renaissance Wax- and Aquazol-coated
silver coupons had failed completely or almost comple-
tely. Remaining coated silver coupons fell in between.
For these data, ranking order of coatings did not
appear to correlate to average coating thickness, with
the exception of Acrysol.
Better performance on individual coated silver
coupons was demonstrated by two of three silver
coupons coated with PVAc AYAT and Acrysol and
one of three silver coupons coated with Agateen,
HMG, and Butvar B-. These better-performing
silver coupons, which were more thickly coated than
their cohorts, remained protective to the end of the
visual evaluation period (table ). Data for the silver
coupon most thickly coated with Agateen are included
in figure  to show how much better it performed than
the average for all Agateen-coated silver coupons.
Visual evaluation was a relatively easy technique to
implement, and results could be readily graphed and
compared. Discrepancies occurred in a small number
of evaluations and are reflected in higher RSDs for
those evaluations. This might be remedied in future
by a better evaluation set-up and improved evaluator
training.
. DIGITAL IMAGE ANALYSIS
The average percent tarnished surface on each coated
silver coupon was determined by software-assisted
image analysis of the digital photographs taken just
before placement of the silver coupons into the HS
chamber,  days after exposure to HS, and approxi-
mately every two weeks thereafter. Photography was
FIG. . Average visual evaluation ratings for three coupons
coated with each material based on color and amount of
tarnish. A rating of  indicates no change;  is the
maximum. The average for Agateen-coated coupon # is
included to illustrate how much better many more thickly
coated coupons performed.
TABLE . CONTINUED
Coating Thickness Visual evaluation color x
amt. tarnish at  hours
(RSD)
% Tarnish from image
analysis
Avg. (RSD) Range At 
hours
At 
hours
At 
years
Aquazol + wax Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Renaissance Wax Control  µm (%) – µm – – – %
Best coupon #  µm (%) – µm  (%) % %
 coupons  µm (%) – µm  (%) % %
Blanks (uncoated)
 coupons  µm (%) – µm  (%) % % %
EVALUATION OF COATING PERFORMANCE ON SILVER EXPOSED TO HYDROGEN SULFIDE 
Journal of the American Institute for Conservation , Vol.  No. , –
stopped when a silver coupon coated with a given
material became fully tarnished. During the experiment
six photographs each were taken of silver coupons that
were least tarnish resistant;  photographs were taken
of the most tarnish-resistant silver coupons. Image
analysis was also done on coated silver control
coupons in ,  years after they were coated.
Results for the average percent tarnished surface on all
three silver coupons coated with a given material are
shown in figure  and listed at  and  hours in
Table . At  hours exposure, silver coupons
coated with  out of  coatings had completely tarn-
ished on average, leaving only those coated with
Acrysol, PVAc AYAT, Agateen, and Mowiol with
average tarnished surfaces ranging from  to %.
Among these four, Acrysol-coated silver coupons
delayed tarnish formation best. Two of the Acrysol-
coated silver coupons had only  and % tarnish,
respectively, after more than  hours of exposure to
HS, and none of the Acrysol-coated silver coupons
had completely tarnished after nearly  hours of
test exposure. The superior performance of Acrysol in
FIG. . Average percent tarnished area for all coupons determined from digital photographs. The average for Agateen-coated
coupon # is included to illustrate how much better many more thickly coated coupons performed.
FIG. . Change in glossmeasurements for Agateen-coated areas showwide spread of data. Solid geometric symbols reflect gloss
measurements on the most thickly coated areas with least loss in gloss, outlined geometric symbols reflect less thickly coated areas,
and other symbols reflect most thinly coated areas. The key at right lists the average coating thickness and its RSD for each area.
 CAROL A. GRISSOM ET AL.
Journal of the American Institute for Conservation , Vol.  No. , –
terms of delayed surface tarnish developmentmay in part
be ascribed to its being much thicker than others tested.
More thickly and evenly coated silver coupons per-
formed markedly better according to percent tarnish
based on digital image analysis when the best-
performing silver coupons coated with Acrysol, PVAc
AYAT, HMG, Butvar B-, and Agateen were com-
pared to their cohorts (table ); these were exactly the
same thickly coated silver coupons that performed
better according to visual evaluation. For example,
the silver coupon most thickly coated with Agateen
(ranging from  to  µm in thickness) delayed
tarnish formation much better than the average for its
set (ranging from  to  µm in thickness). The most
thickly Agateen-coated silver coupon was estimated to
have only % surface tarnish after more than 
hours, while the average percent tarnish for all Agateen-
coated silver coupons was already almost % by 
hours.
Data are also included in table  for coated silver
control coupons after  years exposure to ambient con-
ditions. Silver control coupons coated with eight
materials were still without tarnish, while the Renais-
sance Wax-coated control coupon was fully tarnished.
The Agateen-coated silver control was still protective
except where it was thinly coated and had turned
black, while the Mowiol and Aquazol controls had
begun to color slightly in some areas.
In general, once even lighting was achieved on the
shiny silver surfaces, this photographic- and computer-
based surface tarnish evaluation technique proved to be
a useful and relatively easy method for ranking coat-
ings. Within experimental error, results at  hours
of exposure were generally consistent with those at
the endpoint for visual evaluations at  hours
(table ). Regular digital photography throughout the
experiment also provided invaluable documentation
that assisted with interpreting and checking results of
instrumental analyses. Less evaluator-dependent
methods of assessing the photographs could be used,
such as selecting particular color ranges in Adobe
Photoshop before using the histogram function to
determine the percent surface tarnish. If available,
digital image analysis software could also be used as
an alternative method.
. GLOSS MEASUREMENT
Gloss measured on coatings applied to glass slides
was constant throughout the HS test exposure; thus,
change in gloss measured on silver coupons could be
attributed to the change in reflectance caused by
tarnish on the silver surface. At the beginning of the
experiment the majority of coated silver coupons
showed gloss measurements in a range between 
and  gloss units (at the ° setting, the instrument
produces values of approximately  gloss units for
% reflectance). Lower average gloss was measured
FIG. . Average change in (a) gloss and (b) color measured on individual coupons coated with different average thicknesses of
Acrysol listed at right. The most thinly coated coupon (#) began to lose gloss and color after – hours. Gloss, measured
through  hours, did not clearly show changes on the more thickly coated coupons (#, ), but color, measured through
 hours, showed definite changes after  hours.
FIG. . Average gloss measured on five top-performing
areas for seven representative coatings. Coupons coated
with Acryloid B-N and Renaissance Wax were removed
from the chamber after  hours because they had comple-
tely tarnished.
EVALUATION OF COATING PERFORMANCE ON SILVER EXPOSED TO HYDROGEN SULFIDE 
Journal of the American Institute for Conservation , Vol.  No. , –
for duller coatings, notably Aquazol, PVAc AYAT, and
Renaissance Wax in decreasing order of gloss.
When gloss measurements for the  test areas on the
three silver coupons in each coating set were plotted,
the spread of data was almost always large and
increased over the exposure time as areas tarnished. A
graph showing change in gloss for the  areas on
Agateen-coated silver coupons, for example, showed
that the average gloss at any exposure time would not
adequately represent the relatively good performance
of the best-performing areas, which have thicker coat-
ings within a relatively narrow thickness range
(fig. ). As was found by visual evaluation and digital
image analysis, gloss measurements were often more
consistent and showed later onset of loss in gloss on
individual silver coupons that were more thickly and
evenly coated than their cohorts. This is illustrated by
graphs of the three silver coupons coated with
Acrysol (fig. a).
In order to narrow the spread of data for comparison
of coatings, the five best-performing test areas for each
set of silver coupons were selected from photographs
taken during the experiment based on optimal delay
in tarnish over time. Plots of gloss for the average of
the five areas separate with test exposure, illustrated
by seven representative coatings (fig. ). In order to
rank all coatings based on such gloss plots, the “onset
of tarnish” was then manually derived at the intersec-
tion of a pair of trendlines on each plot (fig. ). The
first trendline consisted of an initial slope near zero
describing relatively stable unchanged gloss, and the
second trendline consisted of a negative slope describ-
ing the declining gloss that followed. For most coatings,
the trendlines, which roughly correspond to an induc-
tion period followed by relatively steady loss of protec-
tion, qualitatively fit data reasonably well. Results for
Mowiol-coated areas were an exception in showing a
slightly anomalous trend: the gloss of Mowiol-coated
areas dropped around  gloss units in the first 
hours before becoming stable. Tarnish onset points
were generally corroborated by visual examination of
photographs taken during the experiment.
FIG. . Average gloss (outlined diamonds) and change in
color (solid squares) measured on five best-performing areas
coated with Acryloid B-. Flat and sloping trendlines for
both color and gloss intersect at about  hours.
FIG. . Estimated onset of tarnishing for five top-performing areas for each coating, derived from measurements of color
change and gloss. Average thicknesses of coatings are noted in the legend.
 CAROL A. GRISSOM ET AL.
Journal of the American Institute for Conservation , Vol.  No. , –
Results for estimated onset of tarnish derived from
average gloss for the five best-performing areas on
coated silver coupons are shown in figure . Acrysol-
and Mowiol-coated areas performed very well by this
measure, with gloss measurements remaining constant
through  hours of test exposure (after the initial
drop for Mowiol-coated areas). Results also appeared
to separate out performance of Agateen and HMG
from remaining coatings. Rankings for all coatings
were similar to those on best-performing silver
coupons evaluated visually and by image analysis
with the exception of Mowiol, which performed
better according to gloss results.
In general, this method of coating evaluation was
found to be more labor intensive than visual evaluation
or digital image analysis, and it did not allow direct
ranking of coatings as the other two methods did.
Rankings based on the gloss measurements were
derived by determining the onset of tarnish for best-
performing areas, but gradual changes in gloss as
silver coupons tarnished, variability of measurements,
and the anomalous behavior trend for Mowiol made
interpretation difficult. Coupled with thickness
measurements taken in exactly the same areas,
however, gloss measurements provided precise data
about the success of coatings of given thicknesses.
. COLOR MEASUREMENT
Color change (E*) and the total color difference from
initial pre-exposure readings (ΔE*) were calculated for
all measurements in each area based on the  CIE
L*a*b* formula. The ΔE*s measured on coatings
applied to glass slides were essentially nil throughout
HS test exposure; thus, change in color measured on
the silver coupons could be attributed to tarnish of
the silver rather than the coatings themselves. Color
difference generally increased as samples tarnished,
first becoming iridescent, then turning orange-brown,
dark brown, and finally blue black.
Spread of data for the  test areas was as broad as for
gloss. Aswas found by the other three measurement tech-
niques, ΔE*s were often more consistent and showed a
later onset of color increase on individual silver
coupons that were more thickly and evenly coated than
their cohorts. This is illustrated by graphs of the three
silver coupons coated with Acrysol (fig. b). Color
measurement continued for more than  hours of
exposure for Acrysol-coated silver coupons, and results
showed changes for the two more thickly coated silver
coupons at about  hours, as was also the case for
areas more thickly coatedwithMowiol. Note that colori-
metry plots for the three Acrysol-coated silver coupons
were similar to those for gloss (fig. a).
In order to narrow the spread of data for comparison
of coatings, the same method of plotting data for the
average of the five best-performing areas was employed.
A plot for Acryloid B- shown in figure  is a near-
mirror image of the coating’s gloss plot, as occurred
for most coatings. Using the same trendline-method
for determining the onset of tarnish as was employed
for gloss, colorimetry showed approximately the same
onsets of tarnish on the five best-performing areas as
gloss measurements, within experimental error (fig. ).
Rankings were also similar to those of better-
performing silver coupons evaluated visually and by
image analysis, with the exception of Mowiol. Color
change was somewhat anomalous for Mowiol, as was
the case for its change in gloss: ΔE* increased to
nearly two units over the first  hours, then main-
tained stability up to around  hours.
As was gloss measurement, colorimetry was more
time-consuming than visual evaluation or digital
image analysis, and ranking of coatings also had to be
determined indirectly by deriving the onset of tarnish
for best-performing areas. Interpretation of data also
proved to be difficult because of variability in measure-
ments, gradual change in color, and anomalous pat-
terns such as that exhibited by Mowiol. Agreement in
the onsets of tarnish determined from colorimetry or
gloss, however, allowed higher confidence in the
results for both techniques. They also tended to
confirm that the most important evaluation factor for
coating performance was the initial development of
tarnish, a key moment commonly identified by visual
evaluation in studies of coatings on silver (Reedy
et al. ; Thickett and Hockey ; Luxford and
Thickett ).
. DISCUSSION
Results provided comparisons of performance between
coatings on silver in an HS environment. Despite wide
variation in data for individual coatings, mainly ascribed
to variations in coating thickness, the rankings of coat-
ings were similar using the four different evaluation
methods and three selections of data: all data, data for
best-performing individual silver coupons, and data for
five best-performing areas. The main discrepancies in
rankings were for the performances of Mowiol and
Agateen. According to gloss and color measurements,
Mowiol performed best of all the coatings when the
five best-performing areas were compared (fig. ), but
least well among the five most effective coatings when
data from other evaluation methods were considered
(figs.  and , table ). Agateen’s excellent results on
the most thickly coated silver coupon and five best-
performing areas but mediocre performance as an
average of all data clearly could be attributed to
uneven, thin coatings on some coupons.
Experimental results were not correlated to expected
tarnish levels or hours of exposure in normal museum
EVALUATION OF COATING PERFORMANCE ON SILVER EXPOSED TO HYDROGEN SULFIDE 
Journal of the American Institute for Conservation , Vol.  No. , –
environments, as others have attempted (Luxford and
Thickett ). Results for the five best-performing
areas, however, suggested a potential for good perform-
ance when coating application was optimal. Times for
the onset of tarnish derived from color and gloss
measurements showed that the best-performing
Mowiol- and Acrysol-coated areas lasted about twice
as long as the best-performing Agateen-coated areas
(fig. ). In turn, Agateen-coated areas protected the
silver almost twice as long as HMG- and
PVAc-AYAT-coated areas.
Use of a relatively inexpensive thickness-measuring
device based on eddy current technology (around US
$) proved to be essential for the evaluation of coat-
ings in our experimental study. In many cases it showed
that coating thickness was not even over the surface of
the silver coupons and suggested that some coatings
may not have achieved sufficient thickness or suffi-
ciently even thickness for adequate protection of the
metal. Based on this experience, it is essential to deter-
mine thickness of coatings and variability in coating
thickness in any study that compares coatings.
A critical issue was whether the experimental
coating thicknesses in the study were comparable to
those used by conservators. As far as can be deter-
mined, nothing has been published in this regard.
Thicknesses of two types of coatings on objects in
the collections of Winterthur Museum were sub-
sequently measured using a BYK-Gardner (Byko-test
) thickness gauge, and those results are incorpor-
ated into discussions of individual coatings below.
Unfortunately, measurements on artifact surfaces are
limited when using thickness gauges based on eddy
current technology: results are only accurate when
measurements are made on relatively flat uncorroded
surfaces, which can be difficult to find on artifacts
and may not be representative. In practice, thickness
measurements proved to be unreliable in recesses
where coatings typically pool, at edges where they
were thin, and on curved (especially concave), deco-
rated, or tarnished surfaces. Attempts to measure
thickness on artifacts using an inverted metallographic
microscope proved unreliable. Successful thickness
measurement using an FTIR microscope has been
reported at the British Museum without including
any results (Thickett and Hockey ).
The most promising result of our study was that
Acrysol appears to have potential as a coating for
silver. It has been used for dip coating brass (Dow
) and as a consolidant for damp plaster, bone,
and ceramics, but it has not been used for coating
silver as far as is known. Acrysol has a significant
safety advantage during the coating process over coat-
ings that rely on organic solvents for application,
since it is delivered in an aqueous dispersion. The coat-
ing’s success in our experiment, however, must be
attributed to some extent to its thickness, which aver-
aged  µm in the five thickest areas. By all four evalu-
ation methods Acrysol’s performance on the most
thinly coated silver coupon # (see fig. ), which
also had all five of the most thinly coated areas, was
not as good as the average for Agateen. This was in
spite of the fact that the average coating thickness on
silver coupon # was  µm, higher than the average
thickness for all other materials. It remains to be seen
if Acrysol would be acceptable to conservators and
curators if it were as thickly applied as on better-
performing areas in this experiment. The coating
might also be less desirable on account of a slight
haze, but its appearance might be improved by appli-
cation technique. Dilution with water-miscible alcohols
such as isopropyl alcohol and ethanol improve its flow
but increase viscosity. Additional study and experimen-
tation are required to develop an optimal application
method and evaluate Acrysol’s aging properties in
this use.
In this study, Mowiol, also unknown as a coating for
silver, exhibited excellent performance on the five best-
performing areas (also relatively thick at  ±  µm)
according to gloss and color measurements. It, too,
has the advantage of being applied in an aqueous sol-
ution, and its appearance was satisfactory. Its relatively
mediocre rankings for visual evaluation and digital
image analysis, however, raised questions. Additional
work would be required before it could be considered
for coating silver.
PVAc AYAT’s generally good performance in this
experiment confirmed De Witte’s earlier study that
showed the coating’s low permeability to HS (De
Witte ). The low gloss of the PVAc AYAT
coating, however, makes it unacceptable for use on
silver (fig. ). Moreover, the low Tg (°C) of PVAC
AYAT would make it a poor choice in dusty environ-
ments, since dust would adhere to the surface.
Agateen performed relatively well in the experiment
on the most thickly coated silver coupon and five best-
performing areas. Thin application in one area was
clearly responsible for the otherwise excellent appear-
ance of the coated silver control coupon after  years
at ambient conditions (table ). Untarnished Agateen-
coated objects in the collection of Winterthur
Museum measured somewhat more than the  ±
-μm thickness of the best-performing silver coupon
and five best-performing areas measured in the exper-
iment. A still-protective Agateen coating applied by
Heller to a silver tankard (Acc. No. .) more
than  years ago, for example, averaged  µm
based on  measurements (% RSD). Equally pro-
tective were slightly younger Agateen coatings applied
to a silver bowl (Acc. No. .) and a silver sauce
boat (Acc. No. .) in the museum’s collection.
The former measured  µm (RSD %) and the
 CAROL A. GRISSOM ET AL.
Journal of the American Institute for Conservation , Vol.  No. , –
latter  µm (RSD %), based on  measurements
each. At the outer limit of acceptable thickness would
be an Agateen coating applied to the Mace of the U.S.
House of Representatives. Since the Mace is moved
to the House chamber each time the body convenes, it
has been intentionally coated with a thick layer of
Agateen measuring – µm in thickness (RSD
unknown) in order to protect the object from the
regular handling (Gleason et al. ; Williams ).
In this experiment the best HMG-coated areas did
not perform quite as well as the best Agateen-coated
areas, despite similar coating thickness. To some
extent this may attributed to the fact that Agateen is for-
mulated with solvents to promote leveling for its use as
a coating, whereas HMG is formulated as an adhesive.
Like Agateen, HMG’s performance correlated with
thickness according to all evaluation techniques.
Visual evaluation and image analysis, for example,
showed no tarnishing at  and  hours, respect-
ively, for the most thickly coated silver coupon 
(average thickness  µm and RSD %), while the
ranking was below the middle for the most thinly
coated silver coupon (average thickness  µm and
RSD %). Thicknesses of Frigilene, the commercial
cellulose nitrate coating similar to HMG, have been
successfully measured on silver objects at the British
Museum but have not been reported (Thickett and
Hockey ).
Butvar B- generally ranked somewhat lower than
the cellulose nitrate lacquers. According to all four
evaluation techniques, its performance was best when
coatings averaged  ±  µm and poor when coatings
averaged  ±  µm.
Despite the high quality and excellent aging proper-
ties of the Acryloid resins B- and B-N, these
acrylics ranked in the middle of coatings in our exper-
iment. Difficulties in measuring thicknesses on
B--coated silver objects prevented acquisition of rel-
evant data about a range of objects despite a number
of attempts. Measurement of the thickness of a B-
coating was successful on only one object, the untarn-
ished bottom of a Gorham coffee pot (Acc. No.
..) in the collection of Winterthur
Museum; it showed a coating thickness of about 
µm based on  measurements (RSD %). This is
thicker than B- coatings in our study, which averaged
 µm on the five best-performing areas, and may have
contributed to the relatively poor performance of B-
in this study. Thicknesses of  and  µm on test
samples coated with B- and B-N are reported in
the literature; not surprisingly these thicknesses corre-
lated with “good” and “bad” results, respectively
(Mourey and Czerwinski ). In any case, inferior
test results for the acrylic coatings compared to
Agateen in this experiment were consistent with those
reported in another study in which B- was applied
according to normal treatment practice by an experi-
enced conservator (Reedy et al. ).
All four evaluation techniques showed poorer per-
formance for silver coupons coated with Butvar B-,
Aquazol (with or without wax overcoat), and Renais-
sance Wax exposed to HS. All Aquazol-coated silver
coupons were blotchy and uneven in appearance and
would be unacceptable to conservators for coating
silver. However, Butvar B- and Aquazol still
appeared at least somewhat protective after  years’
exposure of coated silver control coupons stored at
ambient conditions (table ). The thinness of Renais-
sance Wax coatings ( µm with RSD % on best-
performing areas) may have contributed to its particu-
larly poor performance, only slightly better than the
performance of uncoated silver blanks exposed to
HS. The Renaissance-Wax-coated silver control
coupon was also thinly coated ( µm with RSD %)
and had completely tarnished after  years exposure
at ambient conditions. In the study mentioned in the
previous paragraph, by comparison, silver test
samples coated with microcrystalline wax measured
 µm in thickness, and they performed in the middle
between good and bad results for acrylic resins
(Mourey and Czerwinski ).
Considerable information was gained about assess-
ment methods during the experiment. Results were
found to be easily compared for data acquired by
visual observation and digital image analysis, while
gloss and color measurements were relatively time con-
suming and more difficult to interpret and compare.
Measurement of coating thickness in the same area as
that used for quantitative measurement of gloss and
color, however, provided data that allowed limited con-
clusions to be drawn regarding correlations between
coating thickness and performance (protection from
tarnish).
. CONCLUSIONS
Results indicated that the relative effectiveness of 
coatings tested on silver in a high-sulfide environment
was linked to achieving even coatings of sufficient
thickness, along with inherent differences in the coat-
ings themselves. Correlation of thickness and perform-
ance on many test silver coupons underscored the
importance of even and adequate coating application.
Results of performance rankings were generally consist-
ent, within experimental error, whether determined by
visual evaluation, digital image analysis, gloss measure-
ment, or colorimetry. Slight inconsistency in rankings
determined by these methods was interpreted to relate
primarily to variations in coatings thickness on individ-
ual silver coupons within coating sets. All four evalu-
ation methods ranked Acrysol, Agateen, HMG, and
PVAc AYAT among the five best coatings when
EVALUATION OF COATING PERFORMANCE ON SILVER EXPOSED TO HYDROGEN SULFIDE 
Journal of the American Institute for Conservation , Vol.  No. , –
limited to consideration of best-performing (optimally
coated) test areas on the silver coupons, and Acrysol
appeared to be a promising coating for future study
and development. The fifth coating, Mowiol, per-
formed best of all in the five best-performing areas
with respect to color and was equal to Acrysol in
terms of gloss. It showed mediocre performance com-
pared to the other four coatings, however, by visual
evaluation and image analysis. The Acryloid and
Butvar coatings showed varying ability to delay
tarnish for a limited time but did not perform as well
overall as the previous coatings. Aquazol and Renais-
sanceWax were the least effective of the twelve coatings
tested by all methods on all areas. Renaissance Wax in
particular performed only marginally better than
uncoated controls.
ACKNOWLEDGMENTS
This research was carried out at the Smithsonian’s Museum
Conservation Institute, supported in part by the Kress Foun-
dation. The authors thank Lynn Brostoff for guidance
during the experiment; Rene de la Rie and Christopher
Maines at the National Gallery of Art for the loan of equip-
ment; Margaret Little, formerly at Winterthur Museum near
Wilmington, DE, who assembled coated objects from the
museum’s collection along with their coating histories and
assisted with measurements of thickness; and Ellen Chase,
Blythe McCarthy, and Paul Jett at the Freer Gallery of Art/
Arthur M. Sackler Gallery for assistance with providing
objects for measurements of coating thickness there.
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SOURCE OF MATERIALS
Acryloid B-N, Acryloid B -, and Acrysol WS-
Rohm and Haas Company
 Independence Mall West
Philadelphia, PA -, USA
 CAROL A. GRISSOM ET AL.
Journal of the American Institute for Conservation , Vol.  No. , –
(now a subsidiary of the Dow Chemical Company,
Midland, MI)
Agateen Lacquer No.  and Thinner No. 
Agate Lacquer
Tri-Nat, LLC
 South Avenue
Middlesex, NJ , USA
Aquazol 
Talas
 Broadway
New York, NY , USA
Butvar B- and B-
Solutia Inc.
PO Box 
St. Louis, MO , USA
HMG Heat and Waterproof Adhesive
H. Marcel Guest LTD
Riverside Works
Collyhurst Road
Manchester M RU, UK
Mowiol -
Sigma-Aldrich
 N. Teutonia Avenue
Milwaukee, WI -, USA
PVAc AYAT
Union Carbide Corporation, a subsidiary of The Dow
Chemical Company
PO Box 
Houston, TX , USA
Renaissance Wax
Picreator Enterprises LTD
 Park View Gardens
Hendon, London NW PN, UK
Sterling silver sheet
Armstrong Tool and Supply Company
 West Eight Mile Road
Livonia, MI , USA
BYK-Gardner micro-TRI-gloss and Byko-test 
BYK-Gardner, USA
Rivers Park II
 Guilford Road
Columbia, MD , USA
Dräger diffusion tube (/a-D for HS)
Fisher Scientific
 Park Lane
Pittsburgh, PA , USA
Elcometer  digital coating thickness gauge
Elcometer Inc.
 Rochester Industrial Drive
Rochester Hills, MI , USA
Minolta CR- Colorimeter
Konica Minolta
 Williams Drive
Ramsey, NJ , USA
AUTHOR BIOGRAPHIES
CAROL A. GRISSOM has been Senior Objects Conservator at the Smithsonian Institution since , specializing in treatment of
plaster, stone, and metal sculpture. An authority on zinc sculpture, she published Zinc Sculpture in America:  to  in
. She received her Master’s degree in art conservation from Oberlin College, took advanced training at the national conser-
vation institutes of Belgium and Italy, and worked as a sculpture conservator at the Center for Archaeometry, Washington Uni-
versity, St. Louis, and exhibitions conservator at the National Gallery of Art. Address: Museum Conservation Institute,
Smithsonian Institution,  Silver Hill Road, Suitland, MD , USA. Email: grissomc@si.edu.
NICOLE GRABOW is Objects Conservator at the Midwest Art Conservation Center, a non-profit regional center dedicated to the
preservation and conservation of art and artifacts. She received a Master’s of Science degree from the Winterthur/University
of Delaware Program in Art Conservation and completed postgraduate work at the Smithsonian’s Museum Conservation Insti-
tute and NationalMuseum of the American Indian. Address: Midwest Art Conservation Center,  Third Avenue South,Min-
neapolis, MN , USA. Email: ngrabow@preserveart.org.
COREY SMITH RILEY is a conservator in private practice at Caryatid Conservation Services inMiami. She was formerly conservator
at the Ah-Tah-Thi-Ki Museum belonging to the Seminole Tribe of Florida and located on the Big Cypress Reservation in the
Florida Everglades. She received a Master’s degree in Art History and Certificate of Art Conservation from New York University
in , following a year-long internship at the Smithsonian’s National Museum of the American Indian. She also completed a
postgraduate fellowship at the Smithsonian’s Museum Conservation Institute. Address: Hollywood Blvd., Hollywood, FL
, USA. Email: jcoreys@gmail.com.
A. ELENA CHAROLA is Research Scientist at the Smithsonian’s Museum Conservation Institute since , after several years as
Research Associate. She has been a Lecturer in Advanced Conservation Science in the Graduate Program in Historic Preservation
at the University of Pennsylvania since  and a Visiting Professor at the Raymond Lemaire International Centre for Conser-
vation at the Catholic University in Leuven, Belgium, since . For more than  years she has consulted extensively in various
EVALUATION OF COATING PERFORMANCE ON SILVER EXPOSED TO HYDROGEN SULFIDE 
Journal of the American Institute for Conservation , Vol.  No. , –
projects for World Monuments Fund, in particular for World Monuments Fund-Portugal. Address: As for Grissom. Email:
Charola_ae@yahoo.com.
Résumé – Douze revêtements appliqués à des échantillons d’argent ont été exposés à des concentrations élevées de
sulfure d’hydrogène pour évaluer leur potentiel à prévenir la ternissure dans des environnements intérieurs. Lors des
tests, les échantillons d’argent ont été évalués par observation visuelle, par analyse d’images numériques, par la
mesure du lustre et par colorimétrie. Les meilleurs résultats ont été obtenus pour l’Acrysol WS-, l’Agateen Air
Dry Lacquer No.  (lacque Agateen n.  pour séchage à l’air), l’adhésif HMGHeat andWaterproof (HMG résist-
ant à la chaleur et à l’eau) et le PVAc AYAT. Le Mowiol – a permis de bons résultats à quelques endroits loca-
lisés, mais s’est avéré généralement médiocre. Le Butvar B- et l’Acryloid B-N se sont démontrés moins efficaces
pour retarder le ternissement, quoique leurs résultats étaient un peu supérieurs à ceux pour l’Acryloid B- et le
Butvar B-. L’Aquazol , seul ou avec un revêtement de cire Renaissance, ainsi que la cire Renaissance seule,
ont donné les moins bons résultats. L’épaisseur du revêtement s’est avéré un facteur déterminant dans sa
performance.
Resumen – Doce barnices aplicados a cupones de plata esterlina fueron sometidos a altos niveles de sulfuro de
hidrógeno (ácido sulfhídrico) para probar su efectividad al prevenir el oscurecimiento en ambientes interiores.
Los cupones de plata fueron evaluados durante el transcurso del experimento utilizando observación visual, análisis
de imágenes en fotografías digitales, medición de brillo, y colorimetría. En general fue mejor el desempeño de: el
Acrysol WS-, el Agateen Air Dry Lacquer No. , el HMG Heat and Waterproof Adhesive, y el PVAc AYAT.
El Mowiol – se desempeñó muy bien en algunas áreas, pero en las demás resultó mediocre. El Butvar B-
y el Acryloid B-N fueron generalmente menos efectivos en retardar el oscurecimiento; y el Acryloid B- y el
Butvar B- se desempeñaron menos bien. El Aquazol  y el Aquazol  cubierto con una capa de Renaissance
Wax (cera Renaissance), y el Renaissance Wax resultaron ser los menos efectivos. El grosor de la capa demostró ser
un factor significativo en el desempeño del recubrimiento.
Resumo –Doze revestimentos feitos em coupons (pílula) de prata de lei foram submetidos a níveis altos de sulfeto de
hidrogênio, a fim de testar sua eficácia na prevenção de manchas quando em ambientes internos. Coupons (pílula)
de prata foram avaliados no decorrer da experiência por meio de observação visual, análise de imagens de foto-
grafias digitais, medição de brilho e colorimetria. Acrysol WS-, Agateen Air Dry Lacquer No.  (Verniz de
secagem ao ar livre Agateen nº), Adesivo HMG Heat and Waterproof (HMG resistente ao calor e à água) e
PVAc AYAT obtiveram, de modo geral, melhor desempenho. Mowiol – teve atuação muito boa em
algumas áreas, mas foi medíocre em outras. Butvar B- e Paraloid B-N foram geralmente menos eficazes no
retardamento de manchas. Paraloid B- e Butvar B- tiveram desempenho inferior. Aquazol , Aquazol
 revestido com Renaissance Wax (cera Renaissance) e a Renaissance Wax foram os menos eficazes. A espessura
do revestimento provou ser um fator importante no seu desempenho.
 CAROL A. GRISSOM ET AL.
Journal of the American Institute for Conservation , Vol.  No. , –