It's hard to see what's going on in this photo, so I will clarify: I tacked a string with a small weight (a "plummet") tied to the end from the rafter of the roof over the kiln. The purpose is to make sure that the stack goes up straight, as over 30-plus courses of brick, a slight list can turn into a giant, unstable one. As you can see, the 2 courses above the standard come in slightly; fortunately we hung the plumb line before it got any taller. Such simple technology; but nothing better has yet been invented.
Though the exact function it was made for is unknown, the little stone being used as the plummet has a small groove carved around the narrow end, making it perfect to tie a string around. That groove was carved over 3000 years ago. The rock was found on the Sebastacook River, a remnant of the people who lived there before Europeans came to this contintent. We've already brought this item to the attention of the archaologist at the Maine State Museum. who deemed it authentic but not particularly unique or interesting, which is why we still have it; but it pleases me to use it to plumb my stack, and imagine somebody millenia ago used it to make straight lines as well.