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Author: Subject: Crucibles and Metalcasting
12AX7
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[*] posted on 24-3-2005 at 19:13


Ah, partially crystalline glasses are fun bits of stuff.. didja know that Macor machinable ceramic is basically glass with mica plates grown in it?

Anyways, if that's what it is, it'll basically act like glass, and probably melt at usual temperatures, be sticky and not very shock resistant (although that depends).

Tim
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tumadre
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[*] posted on 18-10-2006 at 20:55


I have found that my local natural clay does hold up to iron at a temperature upwards of 1700C but the strength is to be laughed at.
although it will absorb 20% its weight in water, melting steel in it via an arc under sand was a quick and dirty success.

and on the production of graphite:

"two carbon electrodes in a horisontal furnace and a mix of carbon and carborundum, heated through the application of 9000 amperes at 80 volts alternating current....for at least 30 hours"

"Electrical engineer's Pocket-Book" seventh edition copyright 1913
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[*] posted on 22-3-2008 at 14:39


I have read over the thread, and decided to resurrect it. I have been working on a very small crucible for melting precious metals, and need ideas. I have made a Titanium oxide and sodium silicate crucible, and it is rock hard, but I still want to be sure it is good before melting any of my expensive stuff. Any thoughts?

Also, it might make a decent normal crucible if sized up.
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microcosmicus
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[*] posted on 22-3-2008 at 14:46


Try it out with cheap scrap metal before melting the expensive stuff.
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StevenRS
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[*] posted on 22-3-2008 at 15:03


I just did, it worked fine. Zero weight change.
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microcosmicus
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[*] posted on 22-3-2008 at 15:37


Zero cracks too, hopefully --- I doubt you want the
expensive stuff spilling all over the place should
the crucible break :o

Good luck with our casting project!

[Edited on 22-3-2008 by microcosmicus]
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[*] posted on 20-9-2008 at 15:34


I have recently been using my microwave as a heat source to melt metals, up to brass. I am using a kaowool insulated brick of silicon carbide bonded with sodium silicate, and a ceramic crucible sits on top of it.

While this works, the problem is that the crucible is slow to heat as it only sits on the SiC. 30 sec in the microwave, the SiC is orange hot, but the crucible is still cold. What I need is a small SiC crucible, so the container for the metal is directly heated by the microwaves.

I have tried to make a SiC crucible bound with waterglass, but this does not work, it is to soft at high temp and bubbles form in it; I also have tried decomposition of Pitch in the SiC to bind it with carbon, and the result was a soft powdery clay. No good.

Does anyone have any idea how to make a SiC crucible without pressure? Heating is no problem, I have achieved temperatures hot enough to melt the kaowool into a puddle.
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[*] posted on 20-9-2008 at 17:32


Have you tried sintering with clay? Certainly more refractory than Na2SiO4.

Tim




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[*] posted on 20-9-2008 at 17:38


I have not, as I assumed most sintering processes used high pressure, but on research, I realize this is not so. What type of would be best? Regular pottery clay? I have some of that so i will try it.
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[*] posted on 20-9-2008 at 17:50


Ball clay would be reasonable... high fire and sticky. Not as sticky as bentonite, but doesn't melt as easily; stickier than kaolin. Random pottery clay may be a mixture, check to be sure.

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[*] posted on 20-9-2008 at 18:28


Alternatively consider making a sandwich from two sheets of rolled clay with a filling of SiC with a couple percent of clay as the filling. Cut out a circle for the crucible bottom, trim the remaining part and roll it into the crucible wall, attach the bottom, then seal exposed SiC with more clay. Obviously pick a high fire clay. End result is a clay crucible that heats well in a microwave.

If you want stickyness and are going to try a fully SiC crucible, use a plastic resin that goes to char without much bubbling, or a thick tar softened with a drying oil, as the binder; fire the stuff as hot as you can so as to get some bonding of the Sic to itself. The SiC should be at least 1/3 very fine grained material.
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[*] posted on 20-9-2008 at 18:39


I tried charring various organics, and none of them were very effective. The resulting structure (or lack of) heated very well though, red heat in under 30 seconds. Tar worked for a while, I might try that again.

Also, I ball mill my SiC into dust, like 350 mesh. Is this to fine, or should there be coarser material mixed in? I could get it even finer if that is needed.
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[*] posted on 20-9-2008 at 19:07


That's probably fine, it should make a lovely consistency with clay.

I'd beware of differential expansion building a layered stucture like that. Better to have it evenly distributed I think.

The problem with pitch is, it needs to be fired ca. 2000C to sinter and recrystallize the graphite.

Tim

[Edited on 9-20-2008 by 12AX7]




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[*] posted on 20-9-2008 at 19:25


Yea, I guess I did not get it hot enough. (the tar attempt). Will definitely try again. How much tar or clay should be used? I was thinking about 12% by weight tar, but I have no idea how much clay to use.

EDIT: I tried with tar, and had poor results. The resulting amphorous carbon burned off before it sintered. I thought I had kept O2 away, but I guess not.

[Edited on 20-9-2008 by StevenRS]
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[*] posted on 17-10-2008 at 19:10


I had an idea for making aluminum oxide by heating the crap out of aluminum phosphate. It was also a possible way to make phosphoric oxide too.
Could this be doable? just make a paste of AlPO4 and form it into a shape, then cook it?
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[*] posted on 17-10-2008 at 19:51


Hmm, and what temperature does AlPO4 decompose at? I bet its really high.
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[*] posted on 18-10-2008 at 09:35


Phosphate refractories are quite resilient and used for specialty purposes (tundish etc.). The active phase usually consists of calcium, magnesium or aluminum phosphate, forming a matrix around dead burnt magnesia, silica, alumina, mullite or whatever. Phosphates withstand considerable temperatures, ultimately forming glassy melts as silicate does, but with somewhat different properties (IIRC, the two are immiscible, so a dispersion of calcium phosphate in a glassy silica matrix creates a translucent material, known as bone china.)

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