Ruby Ambrotypes
Some people have asked me what a ‘ruby’ ambrotype is. A typical ambrotype is basically a glass negative, often intensified to make the silvered image area (negative, hence light areas) brighter. The back is then painted black, or else a black cloth or piece of cardboard is placed in the frame behind the negative. In the better quality ambrotypes there is often also a cover-glass, so the negative may be placed emulsion-side up in the case, correcting the lateral inversion inherent in all direct negatives.
In the ruby ambrotype, the negative is on red-tinted (hence ‘ruby’) glass, so the black background or paint is not required. Occasionally, dark green glass was used instead of red. For some reason these ruby ambrotypes are slightly higher valued among collectors, perhaps because the warmth of appearance is appealing.
To detect a ruby ambrotype you need to take the ambrotype out of its case, and hold it up to a light source, so you are looking through it, instead of light reflected off it. This will not only show the red tint of the glass, but will also show the image in negative, since it is only in reflected light that the image appears positive.