What is an old glass photograph that looks positive when backlit?
Asked 2/23/2012
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I have two old black-and-white photographs on glass, about 9×11 inches, framed for many years in a window. They are hard to see unless light is shining through or strongly behind them. The images look a bit like glass negatives, but they appear as positives when viewed with the right lighting. One shows a little girl in a flower field; the other shows a woman seated outside a home with a child and a cat. What photographic process is this—could it be an ambrotype, or is it more likely some kind of glass positive?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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When you say "against the light", do you mean that the light is shining on or shining through the picture?
If it's shining on, then your pictures are ambrotypes. Like Daguerreotypes, ambrotypes use the silver that's precipitated out during development as a reflective surface. The image is actually a negative; it only appears to be a positive because the silver is more reflective than the black backing that's applied later. If the light falling on the image is such that it's reflecting from the flat black backing (usually a varnish containing lamp black), then the picture will appear to be a negative.
If it's shining through, then they're glass positives; sort of overgrown magic lantern slides that are meant to be seen as the front of a lamp (usually with an oiled paper, parchment or ivory/ivorine (white celluloid) panel behind it).
Originally by user2719. Source · Licensed CC BY-SA 4.0
user2719
14y ago
0
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Based on your description, these are more likely glass positives than ambrotypes.
An ambrotype is actually a negative image on glass that appears positive because it’s viewed against a dark backing and by reflected light. If the image is best seen when light shines through it or from behind it, that points away from an ambrotype.
What you describe—"like a glass negative, only positive against the light"—fits a glass positive or transparency, similar in concept to a large magic-lantern slide. These were intended to be viewed with transmitted light, so they can look weak or hard to read without backlighting.
The exact process is hard to confirm without seeing it, but from the answers provided, “glass positive” is the safest identification. If viewed mainly by reflected light with a black backing, ambrotype would be more likely; if viewed best with light passing through, glass positive is the better match.
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