First, you make a form with clay, then you use plaster to create a mold with the same shape. Cover the mold with casting material, make a copy its form to create an outer mold, and then create a slightly smaller inner mold. Pour molten metal into the interstice between the outer mold and the inner mold, and remove the metal from the mold after it cools.
How to make
"What Are Traditional Crafts? -A Guidebook to Seeing, Learning, and Enjoying-" Edited by the Japan Kogei Association Eastern Branch. Published by Unsodo ／ List of works English translation: Kazuko Todate (Art critic / Art historian)