Surviving Daguerreotypes exhibit several kinds of surface
deterioration. The appearance of mechanical scratches and large
area abrasions can effectively be eliminated by retouching a
copy (not the original). Stains may be cancelled by copying
with colored filters, and weak images can be improved by
high-contrast copying. However, large area silver tarnishing
that obscures image detail on many Daguerreotypes cannot be
compensated by optical copying methods. For this reason,
chemical removal of tarnish on the original plates was a common
practise for many years.
A Scanning Electron Microscope Looks at a Daguerreotype
Potassium cyanide was first used to remove tarnish and,
inevitably, some of the image information, since it dissolves
silver, but in the early 1970's a "new and improved" formula
was published that utilized acidified thiourea. It became
widely used because, besides being less toxic than cyanide, it
produced bright clean surfaces that appeared not to have
sustained noticeable damage or loss of image.
Of course it was realized that tarnish returns quickly to clean
silver unless the storage environment is completely free of
sulfur. But some cleaned Daguerreotypes soon developed
unsightly blemish spots that were dubbed "measles", rather than
the expected hazy film of tarnish.
In March 1973 the author, at the request of colleague Leon
Jacobson, examined corrosion spots on a sixth plate
Daguerreotype of an unknown subject using a scanning electron
microscope (SEM). The results were published in a short article
 in 1974. Following is a more complete discussion of the
technique and results, including previously unpublished SEM
micrographs from that work.
Figure 9 shows the appearance of the test picture chosen for
analysis, after it had been cleaned in the thiourea solution.
The "measles" spots are hardly visible in this specimen, but
they were sufficient for analysis. They were of much greater
concern on other historically valuable Daguerreotypes.
Because of the vacuum environment in electron microscopes, it
was necessary to remove the Daguerreotype plate from its case
and from its binding tape and cover glass. Thus prepared, the
bare Daguerreotype plate is better able to withstand a vacuum
environment than any other photographic image. The plate was
larger than our available SEM specimen stages so a holder was
improvised that, unfortunately, did not allow optimum tilt
angles, but the resolution was not seriously degraded at
magnifications less than about 10,000.
In the years since this work, many other SEM analyses have been
reported, notably by M. Susan Barger and coworkers, and by Swan
et al. But the earlier work still usefully illustrates the
nature of a corrosion problem and one of the pitfalls of
restoration. It also reveals details of the Daguerreotype
microstructure that a light microscope cannot achieve.
Microstructure of a Daguerreotype Image
Fig. 10 is a low magnification (about 15x) SEM micrograph of a
portion of the white shirt chosen for its sharp edge contrast.
Various kinds of blemishes are visible, some of which are
nearly invisible by light microscopy. The SEM image depends on
the secondary electron emission properties of surfaces rather
than on light reflection. This fundamental difference between
the two imaging processes often reveals organic and inorganic
thin film contaminants not visible in light, even though the
concept of color is inapplicable to electron images as it is to
Figure 11 shows the light/dark boundary at about 200
magnification; the particulates in the white region are
becoming visible. Figure 12 clearly shows the amalgam particles
in the white area, as well as buffing scratches in the
silver-plated base metal. Figure 13 shows details of the
amalgam particles at about 5000 magnification. This sequence of
pictures shows that the "white" expanse of the shirt contains
many more amalgam particles than the dark regions. The
particles are silver-white in visible light, and their shape
scatters incident light so that the viewer's eye has an
appreciable acceptance angle for this reflected light. Light
that is reflected from the highly polished areas where there
are no particles is efficiently reflected, but in a narrow
angle that depends precisely on the angle of incidence. This
has the effect of sharply reducing the eye's acceptance angle.
Thus a viewing angle can be found where the contrast is at a
maximum, within perhaps twenty degrees on either side of the
perpendicular. The actual dependence of contrast on viewing
angle depends on several factors; it has been studied by Barger
et al . If the eye is far off to the side, contrast is
nearly zero, and the image vanishes.
Daguerreotypes have been described as "grainless", but from
these pictures this is obviously in error. The grain of the
particles is apparent in a light microscope at 300x. They
appear textureless in comparison with salt prints, their
contemporary competitors, which had a visible paper texture.
The mechanism of particle nucleation and growth which accounts
for the range of particle sizes is discussed more fully by
Barger [8, 12] and by Pobboravsky . Since we examined only
one specimen, we have no information on the original effects of
process and materials variations.
The fact that the particles are bright by reflected light and
also bright in secondary electron images does not have an
intuitively obvious explanation. It has been said that the
earliest secondary electron images surprised the pioneering
workers because of their unpredicted resemblance to light
microscope images. SEM images, besides being capable of more
than fifty times greater magnification, have some five hundred
times greater depth of field than light micrographs. It is
convenient that the two imaging technologies complement each
other so well.
The width of the black band at the bottom of some of the
pictures is a micrometer marker (not all the pictures have a
marker because one of the SEMs we used lacked a marker
mechanism). The band marked '100 microns' thus represents about
0.004"; '4 microns' represents 0.00016". One micrometer, or its
formerly-used synonym "micron", equals one thousandth of a
millimeter or about 4 one-hundred thousandth of an inch; the
wavelength of green light is half a micrometer. It is more
accurate to refer to these internal markers, because apparent
magnification may change during subsequent reproduction. The
maximum magnification of which most light microscopes are
capable is less than 2000x.
Figure 14 shows one of the "measle" spots near the left side at
about 850x; it consists of a dark center surrounded first by a
narrow white ring, then a broader dark ring. This specimen had
been cleaned in the acidified thiourea solution. The corrosion
site is approximately twenty-six times larger than a typical
amalgam particle, making it visible to the unaided eye.
We performed X-ray fluorescence analysis in the SEM by focusing
a stationary electron beam with an energy of 9 kilovolts on the
center of the corrosion site and on other selected sites for
comparison. The energy spectrum of the X-rays emitted at the
site of electron bombardment was analyzed by a solid state
energy dispersive detector. X-rays are emitted from a
pear-shaped volume substantially smaller than the overall
corrosion site but large enough to include several of the
Analytical results were as follows:
1. Center of corrosion site: strong silver and sulfur, trace
chlorine and mercury.
2. Small white particles surrounding corrosion "petals": strong
silver and sulfur.
3. Dark zone - Fig. 10: strong silver and sulfur.
4. Amalgam particle: silver, mercury, trace chlorine.
5. Clean base metal between particles: silver, mercury. Gold
was not detected; not all Daguerreotypes were toned.
Towler  listed the following materials used to make
Jeweler's rouge (iron sesquioxide).
However, this list is oversimplified: there were many
variations. Other polishing compounds such as pumice were used,
and combinations of sensitizers were used, including chlorine,
as discussed by Swan et al .
Iodine, and sometimes bromine, sensitizer.
Sodium hyposulfite ("hypo").
Gold chloride toner, not always used.
Both the narrow light ring and the broad dark ring showed
strong silver and sulfur. The results indicate that these
collars are spreading contaminants that hide the normal
composition of the clean surface, and that they are largely
responsible for the expanded visibility of the corrosion sites.
Neither the amalgam particles nor the base metal between
particles contained detectable sulfur.
The plate was then cleaned again in thiourea solution, followed
immediately by several distilled water washes and an ultrasonic
wash in distilled water. A second SEM analysis showed
essentially no change in the appearance of the crystalline
corrosion, but the sulfur and chlorine peaks in the X-ray
spectrum were almost undetectable. The measles were much less
apparent to the eye, and did not change over a storage period
of six months.
Conclusions and discussion:
The crystalline corrosion spots act like tiny sponges that
retain traces of the thiourea cleaning solution. This thiourea,
which contains sulfur, effused outward over a period of days,
forming a collar of increasing visibility around each corrosion
site. It was the thiourea residue that was largely responsible
for the visibility of the measle spots: the original
crystalline centers were much smaller and relatively obscure.
The ultrasonic wash was vigorous enough to remove the residual
traces from the interstices of the microcrystalline "sponges".
Thiourea is the active ingredient in most commercial silver
cleaners. It is an organic compound containing sulfur,
nitrogen, carbon, and hydrogen (H2NCSNH2). It had been
recommended by reputable restorers in the 1970's for removing
tarnish on Daguerreotypes (eg Weinstein & Booth , and
the 1979 edition of Eastman Kodak Publication F-30.) But there
is no cleaning process that removes chemically bound corrosion
without also losing some picture information. Chemical cleaners
cannot convert silver corrosion compounds back to metallic
silver and redeposit it precisely in its original sites.
Cleaners convert the corrosion products (usually sulfides) to a
soluble organo-metallic complex that can be washed away. This
selectivity is useful: the silver in the corrosion is lost but
not the uncorroded silver. Dirt and inactive foreign
substances, if they are not chemically bound to silver, may be
removed by solvents or detergents.
The cause of the crystalline form of corrosion is unknown. The
fact that no copper was detected was interpreted to mean that
there was no pinhole in the silver plating to expose the base
copper. This is not conclusive: the crystalline structure may
have grown in several phases, effectively concealing the
original defect. We believe that the most effective means of
analysis would be to remove the corrosion by argon or krypton
focused ion bombardment in the SEM; Barger et al  discusses
this technique. This would permit SEM inspection during the
dissection process and eliminate exposure to other chemical
reagents that would confuse interpretation. At the time of our
original work (1973) this technique was being explored but was
not then operational. It has become a recognized tool in recent
Neither bromine nor iodine (the usual sensitizers) were
detected in our analyses. Pobboravsky  has measured
typical silver iodide film thicknesses of the order of 30
nanometers, or about 300 atomic diameters. Because of the
unfavorable placement of the specimen plate in our SEM, it is
likely that this was below our detection limit. The presence of
these materials was not of particular interest unless they were
concentrated in the corrosion sites, which was not the case.
The origin of the chlorine traces is not certain. It may have
been added as an accelerator during sensitizing. It may also
have been a trace impurity in the original process (before the
days of 'Chemically Pure' reagents), or simply have come from
recent handling or during more than a century of storage.
Particles of the original polishing compound may have been left
on the surface, which could have served as corrosion nucleation
sites. Our SEM had a substantial iron background X-ray peak
caused by wall scattering and aggravated by the unfavorable
specimen position. Therefore no conclusion was justified on
Other limitations of the SEM analysis:
The X-ray spectrum at the time of this analysis detected
chemical elements but did not yield information on the chemical
compounds or on the quantitative amounts. In a heterogeneous
surface such as this specimen, quantitative information would
be meaningless unless the analyzed microvolume could be
There are detection problems with elements whose atomic numbers
are below about fluorine, which includes carbon, oxygen, and
nitrogen. The X-ray yield is small at low atomic numbers, and
the escape probability of the low energy X-rays also decreases,
especially in heavy matrices such as silver. Instrumentation is
continually improving, and many new analytical techniques are
emerging that are capable of identifying organic compounds in
Our results, like those of many other investigations, leave
unresolved a number of questions. They did lead to a conclusion
regarding a cleaning process that was experimentally verified,
which is a useful outcome for a small volunteer effort. The
study has been discussed in detail to show the power of the
scanning electron microscope, a modern analytical tool in
common use in many fields. Hopefully this experience may
encourage other workers to make similar efforts.