I found ruby bohemian ambrotype backing glasses have a very long history and that tint even used at text editors as a text font color today. I think toning with selenium is a problem that could be answered with studying ruby glass color. Paper discusses glass composition and finds manganese is the main colorant for these glasses. I dont know whether it would be possible to mimic that chemistry at toning but as I said before selenium could be used at same chroma to create better toning. Please find Part 1 HERE and Part 2 AT COMING POST PDF files. There are good reproductions in paper which would light an idea in your minds.
A Technical Study of Five Ruby Ambrotypes
Lisa M. Duncan
Winterthur/ University of Delaware Program in Art Conservation
August 9, 2009
ABSTRACT- Ruby ambrotypes are images made photographically upon a
colored glass support. A descriptive study of ruby ambrotypes was carried out
on five samples. Goals of analysis included identifying the colorants in the
glass support, classifying coatings on the surface and characterizing tarnish.
Elemental analysis performed with x-ray fluorescence (XRF) spectroscopy and
scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS)
revealed the presence of manganese as the colorant in the glass. Surface
coatings were found to be complex mixtures of natural resins and fatty acids
using Fourier transform infrared (FT-IR) spectroscopy in tandem with gas
chromatography-mass spectrometry (GC-MS). Tarnish of the silver image
was identified with Raman spectroscopy and SEM-EDS with backscattered
electron (BSE) imaging to be a combination of predominantly silver sulfide
and silver chloride salts.