Dating velox paper
When no postmark is available, the type of materials used can often aid in narrowing down the years it may have been produced in.This too is not foolproof for many publishers had large stocks of photo papers using them for decades after they stopped being manufactured.
Collotypes, which provide the finest detail of all printing methods are sometimes confused with real photo postcards.
This practice was only cost effective on cards printed in large numbers; individuals and small photo studios could rarely afford to do so. While many amateur photographers numbered their cards this was most often done by larger studios.
Sometimes a photographer might expose a logo onto the image or hand stamp a name to the back of the card. Numbering was an essential way of keeping tract of large inventory.
As time passes this silver tends to migrate to the surface of the print creating tell-tale metallic patches.
Observing this shiny crust, no mater what the color, is a quick and sure way of telling if you are looking at a real photo.