I've recently started work on casting each of my daughters hands in bronze. http://athimblefullofhell.blogspot.com/
Welcome Dave. Nice job. Cracked your shell did ya? This is the Achilles heel to ceramic shell. Things that help is more layers, vent holes and altering your dewax procedure. What kind of wax are you using?
Yes, but a Dremel and a carbide Burr helps a lot, it takes a gentle touch to match textures. I'll add more layers to the next one. It's green casting wax, I was burning out the wax from the bottom with a propane torch. I kept the hand in a small enclosure. I might skip the enclosure next time until most of the wax is gone
Welcome to my world. I run a low expanding microcrystalline wax. It still cracks. Allow me to entertain you. I have had pretty good results with this method. I cracked my recent one, but that's because I hit it with a torch to remove my hook before boiling. Flame dewax must be done as rapidly as possible. The better solution I've found it to take the shelled wax and place it into a furnace or kiln that's preheated already to 1000 or so. I have a build in mind for a chamber with a lifting floor so the piece can be loaded onto the floor and raised up into the awaiting hot chamber. A little busy to pursue it at the moment, but some day.
I came across a variation of this where you team out the wax, and as the heat only really hits the surface of the wax, it's suppose to avoid the problem with the wax expanding and cracking your shell. I may try that with my next hand. I have not vitrified the shell, but I sit it in dry sand for the pour. That gives is some extra support.
Crash and burn, it turns out that the flashing on the thumb hid the fact that the shell had actually broken away, the sand held it mostly in place, but half the thumb is about 1/8" too thick. That hand is destined for the crucible again. Wish I'd seen that before i'd done the rest of the chasing.
Placing an empty shell in sand is a good precaution.. However, the cracking is taking place during dewax. Understand there is actually 2 phases of this process. Dewax just rids the shell of most of the wax. Burnout removes the remaining wax and "vitrifies" the shell making it hard. Vitrification I think is technically the wrong name for it. Whatever it is, after the shell is fired to white, it's hard, glasslike and very strong. Steaming also cracks shell and in most cases, is worse than the flame. The million dollar trick is bathing the entire shell at one time in a consistent and even heat all over. The wax is going to expand no way around it. BUT if you can get the entire layer of wax where it meets the shell liquid as fast as possible, it will have some place to go. Uneven heating is your enemy if it comes from a big propane torch or wall paper steamer.
If your evacuated fired shell cannot be filled with water without pissing all over the ground, it's not ready for bronze! You can salvage cracked shell by applying slurry and more silica. I ran a boring tutorial years ago where I drill holes into the shelled wax object, this does help relieve pressure. I mix up a paste of slurry and silica and patch the holes with a few layers. Fired shells can also be filled if necessary.
