Preparing ceramic shells for a bronze pour
On Saturday morning when I was loitering in the yard looking at lots of bronzes that had just been sand blasted, one of the foundry guys beckoned me over to asked me if I wanted to see them loading the ceramic shells into the kiln. “Si Grazie…”
They have a large trolley on rails that was loaded with about 4 large shells and lots of smaller ones packed around them. I was surprised that the large shells were the right way up, as I’d assumed they would be fired upside down to let the wax out. However, they explained that they had made holes in the ceramic shell to let the wax out. The smaller ones were all upside down.
They then raised vertically the large door of the kiln and I felt a blast of heat from the inside of the kiln which was already at around 650°C. The trolley was moved into the kiln and the big door came back down with a large metal drain pipe poking out stil. Within a couple of minutes the wax started pouring out of this and was caught in a very large saucepan which had to be emptied regularly.
They explained that the kiln would be kept at 650 till all the wax had melted, then it would be taken up to 900 for about 6 hours to fire the ceramic shells and then cooled slowly over the weekend. I should come back on monday morning and they would show me the rest of the process.
So about 8 am on Monday I returned, this time with camera in tow to watch the next stages. They were going to cast a large piece of the fountain they are currently working on. As it was so large they wouldn’t be doing anything else with it. First of Enzo was having to fill each of the holes that had been made to let the wax escape – this was done with a dab of ceramic liquid and a little chuck of ceramic powder – followed by a quick flame throw to ‘fire’ it. For the large fountain – this took around 1 1/2 hours!
Meanwhile, Maurizio was getting the bronze weighed out and ready for the furnace, as well as the large container that the shell will sit in. This is lowered into a hole in the ground to make the pour easier. They waited until Enzo had nearly finished patching all the holes in the shell before lighting the furnace – they wouldn’t want to have to keep the bronze at full melt temperature for any length of time. The furnace was loaded up with off cuts of bronze from previous cast’s runners and risers, and eventually some ingots too.
When the shell was finally patched, it was hoisted up, and lowered carefully into the large container, which had disks of felt positioned under where the steel legs of the framework came. These legs also had rectangular pieces of rubber attached to them.
The container was then filled with sand from 2 large drums. I was then hoist out of the hole, and had a machine attached to it to vibrate it so that all the sand was well compacted.
The container was returned to its hole in the ground, and the top was sealed with a sheet of plastic before the vacuum pump was attached to put the whole container under negative pressure. More sand was then placed over the sheet, presumably to protect it from melting/burning from bronze splashes.
The shell was finally ready for the pour. All that was needed was for the bronze to get to the right temperature and for all the required people to arrive.
