@sadie-mills@carl-lewis A lot depends on the preservation material for the trilobite. Most will be calcitic and will be adversely affected by any chemical cleaner that will dissolve limestone. To remove loose or soft shale or mud a toothbrush with water and an optional mild detergent will help. If the limestone matrix is covering part of the trilobite there is no free lunch . . . air scribes and air abrasive cleaning by an expert preparator are required. Inexperienced use of these tools can ruin a fossil in the blink of an eye.