Cards previous to that had to have the Private Mailing Card Statement. There was often a statement that said "this side for address only." The postal service started allowing the use of divided back postcards in March of 1907.
So, if your card is marked "Private Mailing Card," is dates from 1893 - 1901. A divided back postcard (example 3) has a line down the middle, or some other indication that one half of the back is for the address and the other half is for a message. If your postcard has an undivided back, it is from 1907 or earlier. during World War I in order to save ink and estimated to have saved 20% more ink.
Research has already been done by earlier historians and guidelines have been put into place.
Mashburn (Colonial House) The dating of the postcard for years or eras of issue can be accurately determined if the card is studied for identity points.
There were seven eras for the postcard industry and each one has distinguishing points to help establish its respective identity.
The following helps determine the era of the card in question.
But even here the effect is more of a softening of detail than a observable texture.
But even collotypes will exhibit a discernible grain when magnified.
Does the postcard back indicated that it was printed in Germany? The German cards were of exceptional quality and are some of the best examples of old postcards available. Does the postcard have a shiny finish on the front, with a color photographic image?
However, World War I shut down the German printing presses and cards started being printed in the United States or imported from Great Britain. Photochrome postcards have been produced from 1945 to present.
It is easy to identify these because of the inscription.
It may be noted that many of the early Pioneer views were reprints as Private Mailing Cards.