I’m choosing to interpret the complete lack of response to yesterday’s helpful guide to making clasps as an urgent demand for more of the same. Today’s guide describes what to do with a cobalt-chromium denture framework once you’ve got it out of the casting machine.

We start by leaving the mould for half an hour. This is partly because the metal inside has a temperature of about 900 Celsius, and is therefore impossible to work on unless you have asbestos hands; but mainly because if you cool cobalt-chromium alloy too quickly it plays merry hell with the metal’s crystalline structure and the denture will break.

After about half an hour the metal has completely solidified, and can be immersed in cold water to cool it down. Doing this also damps down the investment, the dust from which contains silicone and other nasties that you don’t want to breathe in. When it comes out of the water, the muffle looks like this.

Ready to remove the investment. The delicate technical instrument we use to do this is in the top-right corner

Ready to remove the investment. The delicate technical instrument we use to do this is in the top-right corner

A few scientific taps with a hammer and most of the mould falls off. Once you can get your fingers around the cone at the top of the sprues, do so and smack it with the hammer until most of the investment falls off. If you put your hand underneath the framework to support it, it will get bent and you’ll have to start all over again. Presuming that this didn’t happen and that you haven’t broken a finger or something equally daft, you should be left with something like this.

Free of the investment, the metal clearly needs a lot of work.

Free of the investment, the metal clearly needs a lot of work.

One of the reasons I like this process is that I get to hit things with a hammer, then get out some power tools. In this case, it’s a high-speed grinding unit, with a carborundum disc rotating at 25,000 rpm. The sprues are cut off as close to the framework as is possible without cutting into it (that gets you belted around the head with the hammer). This leaves us with something like this.

The sprues have been cut off, so now we can get on with finishing

The sprues have been cut off, so now we can get on with finishing

Now we need to get rid of the investment and oxides that are all over the job. We do this with a sandblaster full of 25 micron aluminium oxide blasting compound. Two minutes of that, and you have sand everywhere and a chrome that looks like this.

Nice and clean. Time to start the real work

Nice and clean. Time to start the real work

Now we need to clean up the casting. There are the stumps of the sprues to remove and pieces of flash metal all around the edges, and the rests and other delicate shapes need to be defined and shaped. Once it fits on the model and we’re happy with the result, it goes back into the sandblaster to homogenise the surface. By now it’s looking more like something you’d put in someone’s mouth.

It fits. Time to make it shiny

It fits. Time to make it shiny

The sandblasted job goes into an electrolytic polishing bath for ten minutes at just under five milliamps. Then it’s smoothed off using hard rubber wheels and rubber points on the high-speed grinder. Once that’s been done it looks a bit shinier, but not shiny enough.

Better, but it needs more polish

Better, but it needs more polish

Next we get out the hand-held motor, fit a mounted steel wire wheel to it and set it to 9,000 rpm. The whole chrome gets a good going over with a fine metal polish and the wire wheel until any lines or marks have been removed. Since cobalt-chromium is quite a lot harder than steel, the wire does no harm to the surface. Actually the polish is doing all of the cutting, and the brush is only made from steel because anything else will wear down to nothing in seconds. After the first polishing our casting looks like this.

After polishing with the wire wheel, it's nearly finished

After polishing with the wire wheel, it's nearly finished

Now we need to get that extra bit of lustre, for which we use felt wheels and points and an even finer metal polish. Once this is done and the whole thing has been scrubbed in near boiling water and laundry detergent (that combination will get rid of anything – try it on that tannin-stained coffee mug), it’s ready to have the fit checked and go out to the client.

Polished and cleaned off, the chrome shines forth in all its glory. My work here is done.

Polished and cleaned off, the chrome shines forth in all its glory. My work here is done.

We do about five or six of these a day, so several other castings were going through the same stages alongside this one. That takes us from about eight in the morning, when the casting happens, to about three in the afternoon, when it’s time to start making the wax patterns that will be cast into frameworks the next day. It’s a busy life, but you do get to cut things up with power tools; and is that not what man has dreamed of since first he looked upon the stars? Probably not, but it’s not bad.

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