Used as optional or replacement insert. Simply installs into your Halder hammer head housing allowing you to fine-tune your mallet, with single or even dual component inserts materials in one mallet for optimum versatility. Insert is made from TPE (Thermoplastic Elastomers) that offers excellent stiffness and abrasion resistance. Hardness 2 on scale of 5. Medium hard, non marring. Color: Grey. Diameter: 30mm. Weight: 22 gram.