Although that sepia tone is strongly associated with olden times for us, these pictures did not actually get that brownish tint due to the passage of time, as many people incorrectly believe. These pictures were originally developed in that Sepia tone to increase their longevity.
Remember when there were no smartphones? Pictures had to be printed in order to be viewed. The film in those older cameras was developed in a darkroom to produce hard-copy prints of your pictures. The paper used to make prints is chemically sensitized and then exposed to a photographic negative and a positive transparency (a slide). The latent image is then chemically processed. This type of chemical development of the image reduces the silver halide (a compound) in the latent image to metallic silver. The image is then removed from the developing chemicals and washed in solution to remove the excess chemicals. The undeveloped silver halide is dissolved in a process called “fixing” the image. The finished image is then thoroughly washed to remove all the developing chemicals so they don’t damage the image.