Many homes built in the early 1950s often featured little in the way of wall insulation. However, when insulation was used, it usually consisted of a product called rock wool or stone (or slag) wool. Still in use today, it's made by melting down rock and sand and then spinning it together to make an insulating fiber. Because it's made mostly of rock, it is fairly resistant to fire as well as to the buildup of mold and fungus.
Spun fibers of rock wool combined together often resemble cotton candy. Known originally as slag wool, it made its first commercial appearance in Germany in 1871. Developers at the time noted that it worked well as a thermal insulating material for structures and also around pipe. Rock is melted to 1600 degrees Fahrenheit, making it molten. It is then put on fast-rotating spinning wheels and bound together using food starch and a little mineral oil.
Finished rock wool fibers pressed into sheets and rolls have a strong ability to take the air passing through them and partition it. It's this partitioning action that helped to make it a useful thermal barrier in '50s-era wall insulation. It also possessed a notable degree of fire resistivity due to the material (rock) it was composed of. For early 1950's home building, it normally came in rolls or sheets, which were then placed between open wall studs.
Besides its thermal insulating and fire resistivity characteristics, rock wool also strongly resists moisture buildup. Additionally, because of its material composition, it doesn't support mold or fungus growth within it. There's no moisture, which means mold or fungus has little chance to begin growing. In the early 1950s, the material was appreciated more for its insulating properties than anything else. Rock wool was also attractive because it's good at absorbing sound, making for a quieter home.
Like almost any fiber product, if you find you have rock wool in your walls, some precautions should be taken if handling it. Though it's made from naturally occurring materials, dust and small fibers emanating from it can be an eye, skin and respiratory tract irritant. They're not carcinogenic, though, unlike other fibers used to insulate pipe or around furnaces, such as asbestos. Just use work gloves and goggles as well as a standard dust mask if you'd like.