Each painting is an individual work of art. Lovely, crystal snuff bottles are even more challenging. Bottles are filled with acid, which roughs-up the inside walls so paint adheres. This results in a pretty, frosted look. Then, using a long, thin brush with just small, bottle top openings.
Artists usually lie on their backs, holding bottles up to the light.
Many people collect reverse-painted snuff bottles simply for their beauty. Others collect them as investment pieces, since many of these labor-intensive arts are disappearing. The same technique applies to framed-glass paintings. Imagine painting eyes, shadows, expressions and clothing details before painting the face. Layer upon layer of oil paint is applied before the image is finished.