Search
  • David

PLAYING WITH FIRE 6

BRONZE CASTING PROCESS: POST 6


KEEPING MY EARS


When the wax trees have been assembled, their branches heavily fruited with most exquisite art chunks, they are ready for slurry, which will form the ceramic shell into which the metal will be poured. The slurry is a mixture of silica flour and colloidal silica water. Slurry is applied in layers, and allowed to dry between coats. The wax tree is first immersed into the slurry mixture, and then before it has a chance to dry, a coating of stuccoing sand is broadcast onto the wet surface, where it sticks like feathers to tar,

creating a good textured surface for the next coat to stick to. In my process it will take a build-up of about nine coats of slurry to make a good casting mold. I typically put two coats on, each day, for 4 1/2 days. After nine coats, the shell will have a thickness of

between 3/16 and 3/8ths of an inch, which is a good thickness for a casting mold that will hopefully withstand thirty pounds of molten metal being poured into it without breaking. A casting mold is what we are after. We’ve been after it since the very beginning, but of course there has been all that damned work we’ve had to do to get here.


The fundamentals of building a good shell on a wax pattern are complicated by properties of the slurry. The fact that the solids begin to fall out of suspension the very minute that the solution is allowed to rest, means that the mixer can never rest. From the time the slurry is first mixed, in most foundries, the slurry will be mixing away, day and night, month in, month out, until the equipment fails. In this entire process, my foundry differs from any other I know of.


My slurry mixer only runs a few weeks a year, and the rest of the time, it sits quietly in a corner behaving itself. I knew that in order to do ceramic shell castings, I would need a new type of mixer. There was simply no way to constantly tend to a slurry tank without employees. Slurry tanks are quite miserable things. They end up quitting when down-time can be least afforded (which is most any time).

Things break, so when a mixer quits working, the foundry is out of business until the thing is replaced or repaired. Sometimes the tank wears a hole in itself and the contents go onto the floor. Sometimes a propeller stops working and the slurry solidifies into a 750 pound vat of silica clay, with a few gallons of water on top. Sometimes bacteria invades the tank and compromises the slurry's ability to hold up to the pressure of molten bronze. Sometimes the contents of the tank can start to cycle in a "dead eddy" where the slurry begins to solidify around the edges of the mixer, anywhere the materials are not circulating well enough to remain fluid. When this happens, the tank, in effect, grows smaller by the day, eventually resembling something you might see in a hot springs at Yellowstone, with dried deposits of stone-like material intruding into an ever smaller pool of liquid. Sometimes the slurry man just goes completely bat-shit crazy and pitches a fit of rage and breaks stuff.


There are reasons I know these things about a slurry tank. So it occurred to me that before I went completely berserk, in perhaps the fashion of Vincent Van Gogh, and started cutting off portions of my ear, I needed a new way to deal with slurry. I knew that the solids that precipitate out of the solution were not actually damaged by their settling. They were just settled. The near sandstone-like hardness of the precipitate was not because of any chemical change in the material, as would be the case with cement, but rather, because the fine particle size of the silica flour allowed it to settle in a very dense form. What I wanted was a way to allow the slurry to solidify when I am not using it, but not allow it to dehydrate. So, obviously I would need an airtight cover. That was something I could manage. Next, I needed some sort of

machine that was capable of “drilling” the precipitated solids back into suspension. In order for my idea to work, the entire tank and its mixing bar, had to function as a large drill press, peeling off a thin layer of solids each revolution of the tank against a stationery boring bar. Making the tank spin was the easy part, as that is a fairly common method to avoid propeller problems in slurry systems. Making the mixing bar cut a peel of solids off, during every revolution, was less easy. The solution to making my tank work was creating a speed-controlled boring bar, that, over the course of several hours, could automatically lathe all the solids from the block of precipitate until it is in suspension again. My miniature breakthrough was the idea of turning the leading edge of my mixing bar into a row of what are essentially potato peelers. As each strand of solids peels off with the peelers, the bar comes along behind and shaves off the ridges left by the peelers. This effectively took away the inherent chatter of a very wide cutting edge as it peeled a layer of solids with each rotation of the tank.


Of course, after successfully solving my slurry tank conundrum, my bride wanted me to patent this machine and get rich, so we could go to exotic places, like other “successful” people do. I haven’t patented my tank, but we did go to exotic Denio, Nevada, and stared longingly at the now defunct Diamond Bar, imagining what it would be like to go in and have a cold beer, as we had done so many years ago, just before the fights started at the Denio Barbecue. But I mostly wanted to be able to do my work and keep my ears. (for appearance, of course, because I don’t really listen that much.)


In the long run, it is altogether possible that most people don't have a use for such a slurry tank, let alone their own foundry, so any exotic trips afforded by the patenting of this tank concept might be whatever trip it takes to get to wherever one has to go to apply for a patent. I imagine that there is a place to leave large quantities of money for the patent people, and then there would be a ten year waiting period (I guess we could hang out in Denio) to find out whether the Chinese have absconded with the idea, and have brought the product to market for $29.95 plus shipping and handling.

All Rights Reserved, David Crawford; 2019