I don't know what a tetra bath is, but from your description, I'm going to guess that the diamonds are set without the metal being drilled through beneath the stones. Unless some other open path, such as graver cuts that extend below the girdle of the diamonds, is provided, cleaning solutions may have a difficult time. Even though dirt, boric acid, grease, etc. will somehow find a way to squeeze behind the diamonds, cleaning solutions may not be able to circulate enough behind the diamonds to be able to remove things. You can try long periods in a heated ultrasonic with a strong cleaning solutions, perhaps followed by another fairly long time in pure isopropyl alcohol. If the cleaning solution can fully dissolve the junk, then hopefully the alcohol will be able to rinse it off, and if so, on drying it should leave the stones clean. If this doesn't work, you may not have much choice but to remove stones. Another approach would be to try and avoid the problem. Don't coat the diamonds with boric acid, but also don't get them hot. If your goldsmith uses a torch, have him or her hold the ring with a third hand so that the stones are under water in something like a small tin can. This should avoid most of the problem. Another would be to do the sizing with a laser welder. Then the stones need no special handling at all.