I don't think this was the research I'd come across before (I thought that was more recent but could be wrong), but this piece of research appears to date to 1980 and is very interesting. It's worth reading the whole report however -
See page 28 of the report for a description of the red coloured vehicle on the back of the gilt or silver decoration of the medallions
https://www.cmog.org/sites/default/files/collections/85/8595D55B-378A-4400-91C2-B7D20E08F616.pdfpage 28
'...The red pigment is dispersed throughout its
vehicle as fine particles having a strong, pink,
transparent color. They can be seen at magnifications
of -40 X . The color of the particles at a
microscopic level is better described as "rose"
than "deep-red," although the latter is an accurate
description of the color of the film in bulk....'
and
'... Two possibilities considered for identification
of the red pigment were dragon's blood and rose
madder. Dragon's blood seemed a likely choice,
because it is traditionally thought to have been
used as a red varnish in association with gold
decoration. While it has not been strictly eliminated,
it now seems unlikely that the red pigment
is dragon's blood. Samples of modern dragon's
blood, in the form of an orangish-brown resin,
transparent in small pieces, were found to be
soluble in ethanol and unaffected by dilute aqueous
sodium hydroxide solution. In both these
features it is unlike the red pigment under investigation.
In addition, the orange-brown color ofthe
modern dragon's blood under the microscope is
distinctly different from the characteristic rose
color of "the unknown."
In contrast, however, the microscopic appearance
of the pigment from the Zwischengoldglas is
virtually identical to that of a pink powder found
in a small Egyptian glass bottle of Roman date.
While that seems an unlikely place to look for a
parallel, that powder had previously been identified
as rose madder by one of the authors (Robert
Brill). Particles of the two pigments have exactly
the same color, transparency, appearance under
ultraviolet illumination,14 lack of birefringence,
and more-or-less indefinite shapes. Having not
come across evidence to the contrary, we proceeded
on the assumption that the red pigment is
rose madder, although there are in fact other
candidates. Red gum accroides, which seems to
match at least some of the properties of the Zwischengoldglas
pigment, is one. The yellow form of
gum accroides has been used most frequently as a
yellow dye, along with gamboge and saffron, to
produce "gold" varnishes for bright metal surfaces
such as tin foil and silver. Much less known is a
similar use for the red gum. This resin is thermosetting,
obtaining greater hardness and becoming
virtually insoluble upon hardening. Furthermore,
it is totally soluble in aqueous sodium hydroxide
solution. Mixed with shellac and spirit alcohol, it
frequently saw use as a red varnish stain for metal.
The lac dyes are another possibility. '
Under the section headed 'Summary of results' on page 33 it is stated:
'5. The red pigment is probably rose madder. '
However, this relates to investigations on a variety of zwischengold glasses and they provide a list of the objects at the beginning of the research. There is no mention of small glass bowls with applied metal and coloured sandwiched decoration (i.e. what I have suggested are zwischengold salts).
m