If you aren't familiar with lost wax casting, you will probably be surprised at how involved it is. For me, being a woodturner, the process always begins by turning a bowl on a lathe. I intentionally do not repair cracks or remove natural imperfections in the wood so the final bronze piece looks more like real wood. After I complete the shape and hollow out the inside, I sandblast the exterior to accentuate the grain. Turning the bowl and getting it ready for molding takes a couple of days.
The next step is to make a mold of the wooden bowl using several coats of silicon resin (rubber) and plaster. Silicon coats the wood and picks up the finest details including the grain, cracks and imperfections in the wood. A thick layer of plaster (+/- 1") is applied over the rubber to provide stability and to fortify the flimsy rubber mold. This whole thing is called the "mother mold." The mother mold is reusable, allowing multiple editions of a single wooden bowl to be made. Making the mother mold takes several days.
The next step is to make a wax replica of the wooden bowl. This is done by melting a couple of pounds of victory wax in a turkey cooker. Melted wax is poured into and sloshed around in the mother mold. It takes several layers of wax to get the right thickness for the new wax bowl. This makes an exact replica of the outside of the wooden bowl. The inside is uneven and lumpy.
The "wax bowl" has a perfect outside. The inside looks like five hundred brown crayons that were melted and congealed inside the bowl. So, its lumpy, uneven, and loaded with ridges. Its a lot easier to fix the lumpy wax than lumpy bronze so I put the wax bowl on the lathe to fix the interior. I use a vacuum chuck to suck the bowl onto the lathe. I make a new vacuum chuck for each project so the wax bowl is precisely and securely held in place while I'm working on the inside with a round nose scraper. If the vacuum is too strong it will either leave damaging marks on the outside of the wax bowl or crush it (I've done both).
The next step is to attach wax sprues (gates, vents and sprue cup). Together, these form a system allowing me to pour molten bronze into the piece and provides a relief vent for super-heated, expanding gases. I afix the wax sprues to the inside rim of the bowl so the wood grain on the outside is not altered or damaged. These last two steps take several hours to complete.
After the sprues are attached, yet another mold (the final one) is made by dipping the whole wax assembly into a silica-ceramic material. This step is called the investment (although, the whole process seems like a great big investment to me). Like the silicon mold made earlier, the ceramic, investment material picks up the most minute details from the wax bowl. This mold has to be strong enough to handle the high temperature of the molten bronze (around 1,920 degrees) and the pressure created by super-heated, expanding gases. There are often 8 or more ceramic layers built up during this stage. Each layer must dry completely before the next dip is performed. If moisture is trapped inside the mold, it will turn to steam and explode when the mold is heated in the kiln during burn-out. The investment step can take a week or longer.
At this point, the wax is burned out of the ceramic mold leaving an empty cavity. This is how the term "lost wax casting" came about. Before the bronze is poured into the mold, the ceramic shell must be cured by heating it until it is white hot. If the shell is allowed to cool, it must be heated up again before pouring in the molten bronze. For pieces the size I make, the bronze should be very close to 1,920 degrees.
It takes several hours for the bronze to cool. After which, the piece is "divested." Divesting involves breaking away the ceramic shell, lightly sandblasting the surface to remove any remaining ceramic material and a thin surface layer of carbon. The sprue cup, gates and vents must be cut off (they are no longer wax but, now, solid bronze). I use a high-speed metal cutter on my air compressor to get rid of the big stuff and a Foredom hand tool with a tungsten burr to get as close to the surface as I can.
Then, it is back to the lathe again. The bronze bowl is sucked back onto the lathe by the vacuum system. Usually, I polish the inside of the bowls to a mirror finish. This involves a lot of sanding and polishing. Depending on how smooth or rough the inside of the wax was, I usually start "polishing" with 60 grit sandpaper. Once the rough patches have been smoothed out, I can progress through the various grits (P80, P100, P120, P150, P180, P220, P320, P400, and P600) before I switch over to Micro-Mesh for the final mirror polishing. Micro-Mesh is a polishing system developed to restore aircraft windshields to optical clarity. I use the entire spectrum starting with 1,500 grit and culminating with 12,000 grit. I've dry sanded and I have sanded wet. Dry works well. Wet works too if you use mineral spirits as the lubricant. Water tarnishes the bronze as it polishes it (I've done that). I track my time pretty carefully at this stage since it is so labor intensive. I average about 40 hours polishing the inside of a high-gloss bowl.
Like wood, I've discovered bronze can have its imperfections. I had a batch of bowls that had an unusually high degree of porosity (voids in the metal). When this occurs, I usually polish the inside of the bowl only to 320 grit.
After polishing the interior of the bowl, a patina is applied to the exterior. Metal patinas are basically chemicals that react with the metal. Different colors (mostly in the green, brown and black spectrums) are created by applying different chemicals; some on a cool surface, others on a hot one. I use a weed burner type of torch to heat the bowl and then spritz or dab on chemicals. After the bowl has cooled, I apply then buff a metal wax on the exterior surface.
If you made it this far, good for you! If you didn't know before, now you can see that making bronze bowls out of wooden ones is pretty time consuming and fraught with more than a few opportunities to destroy them along the way.