Firm silicone lasts longer than soft silicone. However, it can be more difficult to remove a delicate casting from a firmer mold. If your casting are breaking when you try to remove them from the mold, stress relief cuts should be made. Properly placed cuts will help relieve pressure points without affecting the shape or detail of the casting. Always use a ruler and with multiple strokes cut. Note DO NOT CUT all the way through, or you will have to band your mold together every time you use it. If you have a mold with thicker walls, a mold knife may be a better option for cleaner registration.