Mark Twain claims in the preface of Tom Sawyer that the superstitions he describes were real in the American ‘West’ among children and slaves circa 1836-1846. Is this true, or did Twain really make them up?
Also, as a follow-up question: the boys acknowledge slaves as experts in these supernatural beliefs. What kind of religious / supernatural beliefs did slaves have in that era, and how did this compare to what the free population believed?
This is by no means comprehensive, but here are some superstitions which I recall from the books:
Spunk water and a verbal charm will remove warts
Throwing a dead cat at the devil at midnight in a graveyard will remove warts
A loaf of bread filled with quicksilver will float to a drowned body
Firing cannons will cause a drowned body to surface
If a dog looks at you and howls you are going to die
Touching a snakeskin is the worst luck
If a murdered body bleeds it means the killer is near
A worm crawling on you means you will get new clothes
If you have a hairy chest you will be rich
Friday is an unlucky day
If you lose a treasure (the text says a marble), bury a like treasure and come back in two weeks to find your lost treasure
Killing a spider is bad luck
Turning around three times and crossing your breast will ward off bad luck
Counting the things you cook for dinner is bad luck
Shaking out a tablecloth after sundown is bad luck
If a bee-keeper dies you must tell the bees before dawn or else the bees will die
Seeing the moon over your left shoulder is bad luck
Young birds flying a yard at a time and landing means it will rain
An unburied corpse is more likely to result in a ghost than a properly buried one
This is a difficult question to answer since Twain may be the only authority for some of these, so they may not all be able to be verified by a real folklorist who has collected the beliefs. Some of these I recognize as occurring elsewhere: murdered body bleeding on the approach of the killer, cannon fire to raise a sunken body, Friday is unlucky, retrieving lost treasure, killing a spider, the bee-keeper dying, looking at the moon over the left shoulder, unburied corpses resulting in a ghost. Others more familiar with Southern or specifically Missouri folklore may be able to verify more. But a lack of verification for all on this list may not mean that Twain made them up.
Twain is problematic because he made up a great deal, but my sense of his work is that this is an area where he is likely to exhibit more fidelity to what he experienced. His use of dialect and his observations of folk culture were not areas where he was inclined to manufacture things. He grew up surrounded by African-Americans and he consistently praised their storytelling skills, so I suspect that even those elements you identify here that I do not recognize were accurate descriptions of folk traditions the way Samuel Clemens knew them growing up at that time and place.
African Americans slaves had a rich body of folklore, some of which was shared with the white population and some of which was their own unique blend of African survivals and American folk culture. An example of something that they brought with them from African (and was unique to their culture) was the practice of burying altered coins beneath a structure for good luck. Archaeologists found these examples from beneath the 1866 floorboards of the Boston Saloon in Virginia City, Nevada. Similar altered coins have been found elsewhere in the South associated with slave quarters. For context, the Boston Saloon was operated by William A.G. Brown, a freeborn African American from Massachusetts. There is good evidence that Twain knew Brown and patronized his saloon (at another location) during his Nevada sojourn, 1861-1864.