DISCLAIMER: Bronze casting can be dangerous. This tutorial is provided for information only. The author assumes no responsibility for any injuries that the reader might incur should they injure themselves casting bronze. The reader does it at their own risk.
Casting Bronze with the Delft Clay Method of Sand Casting
Casting bronze in a home workshop is not as daunting as it might seem at first. It does require a few specialized pieces of equipment but the metal itself is easily melted with a MAPP gas torch which is available from a home center. The molding process is a type of sand casting called Delft Clay which is easy to do without access to a sink and running water.
Safety - You should wear closed-toed leather shoes and clothing made of 100% natural fabrics (cotton, wool, linen). You should wear a respirator (available from a home center), leather gloves (you will probably want the type made for TIG welding), and shade 3 glasses (available from a welding supplier). I generally wear Carhartt cotton overalls and a long sleeved cotton shirt as my base work clothing.
Patterning – Sand casting requires the use of a model to impress the design into the sand. They can be made of metal, wood, plastic, wax or any other hard, durable material. For the projects detailed on this website, I first created paper patterns from archaeological drawings reproduced at the actual size of the originals, and pasted them onto card (manila folders and old business cards work well). I then used these to transfer my patterns to sheet brass which I cut out and filed to the exact size of the original plates. For more dimensional items, I made a paper pattern to the correct size and marked it out on carving wax. Then I carved them out in preparation for casting.
Creation of the Casting Flask – This method of casting requires a two part casting flask. Using modern production methods, I made this in a metal shop from 4” square tubular stock which I cut to size, finished the edges, welded a piece of ½” square tubular stock on the side, and a piece of steel sheet on the bottom. I then welded a piece of ½” square stock on the side of the top piece to create a register with the bottom half of the flask.The photo on the left shows the flask prepared for casting with the impressions of the strap end. Note the card above it with the pattern and the sheet brass model.
Moldmaking - In the Roman period, belt fittings and other small bronze items could have been cast in sand, cuttlebone, clay, or stone molds. I prepared my mold by packing both parts of the mold with the Delft Clay, making sure the surfaces were even and dusting the surface with mica powder. I then gently pressed the metal or wax model, face down, into the clay of the bottom mold and placed a steel plate on top to press it down evenly so that it was almost completely pressed in. Then I placed the top part of the mold on and gently pressed it in contact with the model. I removed the top part again and in the area compressed by the model I made a vent hole(s) with a skewer and a pouring hole with a larger rod. Then I cut a pouring funnel in the top part. After removing the model and a small amount of additional clean-up of the mold, I put the two halves back together and placed them by the hearth in preparation for the pour.
Bronze Melting and Pouring – I use a rented workshop where 1 cannot have a forge or furnace so I must heat the metal for casting with a MAPP gas torch. I heated the bronze in a ceramic melting dish in small hearth made of refractory bricks. The melting dish had been seasoned with borax prior to its initial use and as the metal melted I add some additional borax as flux to help the metal flow more easily. When it had melted and formed a distinct ‘button’ in the melting dish it was ready to pour. Any additional metal was poured into an iron dish on the side. After a few minutes, the mold was opened to see if the casting was successful.
Clean Up - The raw castings had flashing lines, vents and sprues which were cut off and coarsely filed smooth. After I had rough filed the pieces, I went over them with a very fine file to remove any deep scratches. Note that the filing is labor-intensive and it took many hours of work to prepare the pieces. Then I coarsely polished them on the bobbing wheel to remove any final scratches and finely polished them to a shine with jeweler’s rouge. Here is the buckle from the previous photo attached to its belt plate.