||[Jan. 31st, 2003|10:18 pm]
Did the casting today. Started off by following my usual procedure: put in some cardboard between the base block and the crucible, which keeps the crucible from sticking when it's time to do the pour; fired up the melting furnace, ramping the gas pressure and air flow until it hit optimal burn rate (flame front just at the nozzle, just a bit of flame coming out the vent, that errie noise, some pure tone loud enough to be felt in the chest, and heard even through industrial hearing protection); adding in bronze as the heel (the liquid portion of the melt) formed and grew.
I use a commercial covering flux, added in when the heel first forms -- it gives off quite a flood of smoke, which as it contains flourine compounds I avoid breathing in. I used to use just plain glass, but this stuff works much better at removing oxides and keeping the metal clean. I also use a degassing flux, which is used just before the pour, using a special tool to plunge the flux to the bottom of the melt.
Once the first charge of bronze was melted, I poured the sickle blade I'm working on, then had enough to do another casting. I recharged the crucible and did another melt, this time for a couple of mace heads I decided to do. Using the last of the pre-mixed bronze, I got two mace heads cast, with none left over. Before I do bronze casting again, I'll have to make another batch of bronze.
Having had problems in the past with removing castings before they had fully solidified, I took a couple of hours to let them cool off, then broke them out of their molds. The thinner castings came out fine, and even the mace heads, which were still smoking, were hard enough to hold together.
The odors of casting are some of the most noxious I've encountered. The covering flux must contain some acid, as it has a very piercing, sharp odor that can still be tasted, despite avoidance of breathing in the vapors produce, more than 3 hours after the casting is done. The smoke of the petrobond is almost as bad, smelling like the burnt remnants of engine sludge mixed with epoxy, so it sticks around for a while. I think that if I do casting more regularly, I'm going to have to use a respirator to keep my lungs from rotting out from these no-doubt hazardous fumes.
At any rate, I think these were all successful, though of course I'll have to wait until the grinding and polishing phase is underway to ensure I do not have objectionable gas bubbles or slag inclusions. Tomorrow, if all goes as planned, I'll cut off the sprews and start the grinding and shaping. I'll also be running the material though a tempering oven, as I've found that the relatively rapid cooling of the casting process leaves the bronze too brittle for normal use. On the other hand, too soft isn't good either. A couple of hours at 450 deg. F. though will help the bronze to soften enough to avoid brittleness, while leaving the material hard enough to do the job. I'll also do some work hardening on the edges to ensure that those are as hard as possible.
Overall, I think today went well. I'm still on track for hitting my deadline for the commisioned work, and have a couple of other pieces underway as well.