Fluorescence

Maria Brunskog writes:

>On microscopic examination of cross-sections sampled from aged,
>japanned surfaces on furniture, a bright and saturated red
>fluorescence colour has been observed. The light was in the lower
>UV-range, 330-390nm. At a higher UV-range 430-450nm the fluorescence
>was not visible. The red colour (like good quality wine) has been
>interpreted as silver. I would appreciate any comment from anyone
>with experience of metal/silver fluorescence, either from metal
>foil or powder incorporated in lacquer media.

I have not observed visible fluorescence of metal foil, leaf, flakes
or powder in cross-section samples illuminated with wide-band
ultraviolet excitation (ca. 330-390nm) or narrow-band blue-violet
excitation (ca. 430-450nm).

The presence of metal foils and metal flakes can sometimes be
discerned as dark lines or particles within the fluorescent lacquer
or size used to adhere them to a substrate.  The presence of metal
leaf and metal powder layers (much thinner or smaller than foils or
flakes) can be difficult to discern in fluorescence illumination,
unless they lay between fluorescent coatings.

The presence of a metal layer in a cross-section samples can be
determined or confirmed by specular reflection when the sample is
examined in brightfield visible illumination (darkfield visible
illumination often is adequate, too).  This is one of the reasons I
advocate that conservators examine cross-sections using a single
microscope equipped for both fluorescence and visible illumination.

I have observed anomalous visible fluorescence colors associated
with metal foils, leafs, and flakes that cover, or are covered by, a
transparent coating that fluoresces.  The phenomenon appears to
involve internal reflection of visible fluorescence by the
reflective metal layer.  That is, white light emitted by the coating
when excited by ultraviolet wavelengths is reflected by the metal
layer within the transparent coating.  If the opposite side of the
coating covers, or is covered by, a colored layer, then the coating
can appear colored.  For example, the coating layer in the following
structure might appear colored (e.g., pale red or orange) if viewed
in ultraviolet excitation:

    regilding

    red bole
    (reflect visible wavelengths back into the coating, causing it
    to appear pale red or orange)

    coating
    (emits visible fluorescence)

    metal leaf
    (reflects visible fluorescence from coating) yellow bole

A small amount of finely divided colored pigment within the coating
layer could give a similar appearance.  Some colored organic
pigments and dyes emit intense visible fluorescence, and should not
be ruled out too quickly.

In Maria's case, I would re-examine the samples in brightfield
illumination (or darkfield illumination) to confirm the presence of
a metal layer.  If her examination confirms that the deep red color
is in fact associated with an actual metal layer and not a coating,
I would be willing to collaborate with her and post results to the
group.

James Martin
Orion Analytical, LLC


                                  ***
                  Conservation DistList Instance 15:3
                  Distributed: Thursday, June 14, 2001
                        Message Id: cdl-15-3-002
                                  ***