Crucibles

Home enthusiasts discuss their Foundry & Casting work.

Moderator: Harold_V

Post Reply
pattiandruss

Crucibles

Post by pattiandruss »

Will sprayable graphite coatings work on a crucible, or will they just come off when heated? Or is it the entirely wrong kind of graphite? The stuff I'm refering to is intended as a lube. Russell
Roy
Posts: 416
Joined: Sat Jan 04, 2003 12:59 pm
Location: Central lower ALabama

Re: Crucibles

Post by Roy »

Usually graphite is used to keep aluminum from adhering to a steel core etc. I have never heard of anyone using graphite on a crucible, just the norm of a refractory wash. I don't know if the graphite and assuming a steel pipe crucible will have any adverse affects or not. Other crucibles don't need any kind of coating applied. They make a graphite crucible if thats any consolation. Certainly should not hurt doing a melt and finding out.
User avatar
Harold_V
Posts: 20232
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Crucibles

Post by Harold_V »

Russ,
An interesting question. In my refining years, one of the small furnaces I used for melting pure silver to cast ingots (these were small ingots, nothing larger than 20 troy ounces) was a little electric furnace that used a pure graphite crucible. The crucible, inside, and anywhere it had little to no access to oxygen, lasted and lasted, but the top, which was exposed to the atmosphere, readily burned away. I'm thinking that for a crucible, any graphite you applied would burn away quickly.

Foundry supply houses sell a mold wash for pouring ingots. The stuff is nothing more than what they like to call lamp black, already suspended in a solution of sorts, though quite thick. It can be brushed applied as I recall. I used to use it to face my molds when pouring silver anodes and large silver bars. You might use some of this stuff with a limited degree of success, but just using it as a mold dressing, it would burn away at the line above the anode. If I cast several anodes quickly, the dressing would eventually burn away completely just from the heat induced in the mold. As I recall, carbon sublimes (or perhaps burns off) above about 1,100° F. Wouldn't want to stake my life on that, but for sure it would disappear rapidly once the mold got hot. From my experiences, I'd suggest that graphite would not serve you very well. [img]/ubb/images/graemlins/frown.gif"%20alt="[/img]

On the good side, if you want to use a steel or iron crucible, washes are available commercially. I even tried one on the little graphite crucibles I spoke of. In the case of the graphite crucibles, they burned away under the wash, so it wasn't very effective in that particular application, though I know it works fine on ferrous containers. [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]

In the case of commercial crucibles (graphite and clay) it appears the clay content protects the graphite from burning away. I've used them as well as silicon carbide crucibles, and all my failures were from fluxing, not from ignition. I'd generally destroy them from the inside. There were cases of getting no more than eight heats from a crucible. [img]/ubb/images/graemlins/mad.gif"%20alt="[/img] Fluxing was an important part of melting in refining, so I had no choice. You generally don't have the same problems in foundry work.

If you do experiment with the graphite, why don't you let us know what kind of results you achieve?

Good luck,

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
Post Reply