Concretion question

[ncsuwolf]

Question for those on here that have much more experience than me on cleaning things found in the ocean. Spent thanksgiving at the coast, came back with a double handful of clad and various other finds. Some cleaned easily, others not so much. Have been soaking in freshwater (changing every few days) for a month. Some items cleaned up following a week or two, others haven’t improved. Any suggestions on best ways to remove concretions without destroying items? Assuming I have found nothing of value, but would still like to know.