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@pdon-pinkham, @egardner, @rnarducci, @rleder Noooo, do not use acid of any kind! The matrix and the fossils are are the same – calcium carbonate – so any removal of matrix will also dissolve the fossil surfaces. Typically with Brachs that are bonded into limestone matrix, the weakest plane between them is actually within the brach shell so mechanical removal will split the upper surface of the brach away from the rest of the brach. If you like this specimen I would suggest doing nothing. An experienced person properly using an airabrasive unit could clear out material but it would take a lot of time and money to do that. If these were rare echinoderms or trilobites it might be worth it. Keep collecting. You may just find these same brachs weathered out and free of all matrix at another location.
Having said all of that, you can test the brachs with vinegar (acetic acid) to verify they are indeed carbonate. Some Kentucky brachs could be silicified. If they are not carbonate then soak them in a container of household vinegar for as long as it takes to remove all matrix. Muriatic acid works quicker but is more dangerous to handle. Before disposal of acids you will need to neutralize them. And guess what! You can do that by soaking more limestone in it until all chemical activity stops (no more bubbles).