I remember they were sponsored by "20 Mule Team Borax."
Much later I learned (with the help of the internet) that Borax was mined in California, and that the mineral from which it was made was called "tinkle." One place it was mined was named for its elemental ingredient, Boron. Boron California is still there, and is dominated by an enormous open-pit boron mine.
The early boron miners knew how to tell they had the real thing by throwing it in a campfire; it would turn the flames green.
Didn't Ronald Reagan do the intro and outtro narration for at least some episodes?
My husband’s grandfather worked on the mule trains from the boron mine.
Yes, Ronald Reagan did do that.
The story behind Death Valley Days is quite interesting. A woman named Ruth Goodman wrote the show for radio. She was an easterner and for 14 years Woodman went to Death Valley each summer to gather information that she could use in her scripts The radio show ran from 1930 to 1951. A 1962 newspaper article noted, "Mrs. Woodman has written every one of the Death Valley Days scripts (both radio and television) for 31 years -- which amounts to more than 1,000 stories." She was really amazing.
I've heard Tide detergent does the same thing (it must contain some borax).