I read a book sometime in the late 70s/early 80s that was purchased from my school through a flyer or book fair. It was a chapter book that may have had an illustration or two. Best I can remember, a girl is spending her summer break maybe in a new home or at a relative’s home. It seems like the summer may have been rainy and she was exploring the house. She found something in a basement or cellar that was either magical or had to do with witchcraft. She experimented with the magic but it seemed to turn out badly and there was an explosion or climactic event in the basement. In the end, all was resolved with a pleasant ending. I don’t remember the book centering on any other characters so only one main female child. I don’t remember the title or the cover but I have a vague recollection of orange and purple on it. Here’s hoping someone remembers.
It sounds like a Carol Beach York plot. Take a look at I Will Make You Disappear. That is a girl with a younger sister, a summer rental house in the country, and a cellar with materials of witchcraft in it.
I bet this is it. Thank you!!
The Active-Enzyme Lemon-Freshened Junior High School Witch by EW Hildick.