As part of their analysis, the Center's conservation team worked to identify the pigments used for the
map's hand coloring. To do this, they used polarized light microscopy (PLM), which makes it possible to
see the distinctive structures of some pigments with a microscope, and x-ray fluorescence (XRF), which
can reveal pigments' elemental composition.
Testing revealed a number of the pigments to be standards: lead white, red lead, vermillion, yellow
ochre. These show up in art centuries older than the Blaeu and quite a bit newer, too. They do not do
much to help us pin down a date for the map's coloring.
One color, though, appeared to offer conclusive testimony. When we looked at the samples of each green
or greenish pigment under a microscope, our team noticed small circles known as spherulites. In greens,
these little orbs are often associated with a pigment known as emerald green, which brings together
copper and arsenic. And XRF results showed a response at the energy-level associated with arsenic,
seemingly supporting the visual identification. Emerald green, though, was not commercially available
until 1814. In large part because arsenic is highly toxic, the pigment was not in use much beyond the
early 20th century, either, suggesting a fairly narrow date range for the application of our map's
greens. Because the greens are integral to the overall color design, from the decoration around the
hemispheres to the maps themselves and the letterpress text at the bottom, it appeared that our map was
black and white for around two centuries before being colored.
It is not uncommon to see later color on early maps, especially those extracted from bound atlases, but
it would be odd for a wall map that had been mounted and displayed in the 17th century to remain
monochrome. Moreover, while later coloring often reflects later aesthetic preferences, what we see on
our Blaeu is what we expect from 17th-century artists. So, we revisited our initial conclusion, only to
realize that there is another pigment that includes spherulites: verditer, which comes in both blue and
green varieties. And unlike emerald green, verditer was available in the 17th century, serving as a less
expensive alternative to other pigments. When we revisited our XRF findings, we also noticed that the
response we had associated with arsenic was weaker than it should be for emerald green and could be
better explained by the presence of another element, lead, which likely migrated from another colored
area when the map got wet—or was already in the water that saturated it. With the revised conclusion
that we are looking at verditer and not emerald green, we can now say that our Blaeu was very likely
mounted, colored, and displayed not long after its publication in 1648.