[drcy]

20th century black and white photos are called gelatin silver prints. The 1800s albums also have silver in them. Silvering can appear in both, but it's less common in albumens. So the silvering happens in photos that use silver salts. Most 1800s paper photos are albumen and the vast majority of 20th century b & w photos are gelatin silver.

Albumen (egg whites) was the clear binder or glue that held the photochemicals to the photopaper. It replaced by gelatin.

Silver salts turn dark when exposed to light, which is how images develop and why silver was used in photos.

Other, usually much rarer processes don't use silver and won't have silvering even if old. Platinum prints (uses platinum), carbon prints (uses carbon), cyanotypes and others don't use silver in the photo paper.

Silver prints isn't really a good term for 20th century black and white photos, because 1800s albumens are just as much of 'silver prints.' Gelatin silver is the best term. Though some people say silver gelatin-- same thing.