The Cloud Chamber Bowls are made from large pyrex containers, called carboys. The original instruments came from the University of California radiation laboratory, where they would cut out the middles of the carboys for use in cloud-chamber experiments. Partch used the tops and bottoms, which he hung from ropes from a wooden frame (the bottoms were inverted to facilitate their installation)
The bowls may be struck in two different ways, creating very different tonal qualities. The best, most resonant sound is made by striking the bowl close to its bottom edge with a very soft mallet. Another shorter, sharper sound is produced when hitting the tops of the bowl. A rigorous attention to how the bowls are struck is necessary to avoid breakage due to their delicate material. This material also restricted Partch’s ability to exactly tune them. If a bowl broke, it would be impossible to find another that matched its exact intonation. Partch’s answer was to simply rewrite the Cloud Chamber Bowl parts in his pieces to reflect the new tuning.
– From Sound American