The coal industry of Maverick County is located along a section of the Olmos Coal Formation immediately north of Eagle Pass. Mining operations developed by Dolch at Dolchburg and by the Olmos Coal, Coke, and Oil Company at Olmos were the largest coal producers in Texas around the turn of the 20th century.
Those photos still fade over time, plastic doesn't prevent the materials from fading or deteriorating. Pencil that's the old lead pencils, or the new graphite pencils will not fade. Graphite is the most stable form of carbon. It's color is grey rather than black, so the line will be lighter than a black color pencil, but they are not susceptible to fading, they are extremely stable.
The most common type of pencil casing is of thin wood, usually hexagonal in section but sometimes cylindrical, permanently bonded to the core. Similar permanent casings may be constructed of other materials such as plastic or paper.