Sciencemadness Discussion Board

terracotta instead of crucibles

angelhair - 11-2-2011 at 18:18

Can terracotta take temps up to 950 + deg C? I would like to turn some Ca CO3 and CaOH to CaO and also make some sodium metaborate.

I don't have any crucibles that hold any more than 20 or so grams. Also I need something that will fit in my muffle furnace. What about castable ceramics?

[Edited on 12-2-2011 by angelhair]

Sedit - 11-2-2011 at 21:06

I dont knowq about terracotta right off hand but im sure castable ceramics will take over double that temperature.

blogfast25 - 12-2-2011 at 14:36

Terracotta is fine for very high temperatures but non-labware crucibles tend to be very sensitive to thermal shock. Heat and cool very gradually to avoid cracking/shattering. Garden variety terracotta will withstand (not melt or catch fire) the blast of a hot Thermite ® reaction but breaks into many pieces (usually still held together by the solidified reaction melt) due to the extremely fast rise in temperature…

For ‘burning lime’ they should work just fine.

m1tanker78 - 12-2-2011 at 19:16

I routinely use terracotta pots as expendable crucibles. They can take the heat but crack after 1 or 2 uses. Luckily they almost always crack after or as they're cooling. I've always been afraid to try ceramics due to their high cost, mostly.

Tom

Sedit - 12-2-2011 at 19:56

Quote: Originally posted by blogfast25  
Terracotta is fine for very high temperatures but non-labware crucibles tend to be very sensitive to thermal shock. Heat and cool very gradually to avoid cracking/shattering. Garden variety terracotta will withstand (not melt or catch fire) the blast of a hot Thermite ® reaction but breaks into many pieces (usually still held together by the solidified reaction melt) due to the extremely fast rise in temperature…

For ‘burning lime’ they should work just fine.


Honestly almost all extreme temperature fluxuations will crack ceramics. With something like thermite it would be hard to make a ceramic that would not crack under the stress of raising from relatively 0 degrees to 3000 or whatever it maybe.

blogfast25 - 13-2-2011 at 08:34

Sedit:

I’ve never tried ceramic lab crucibles for thermites because terracotta/cheap china does the job and a cracked crucible is in fact an advantage at the metal recovery stage. But lab crucibles are designed to withstand larger thermal shocks than terracotta/kitchenware china. Tough ceramics with fairly thin walls (less temperature differential) make crucibles with good thermal shock resistance…

See also kitchen ware pyrex v. lab ware pyrex...

Sedit - 13-2-2011 at 09:44

There is little difference from things I have been seeing between china and lab ceramics to be honest. I think they add extra alumina but China is already high in alumina anyway. I have been looking around as much as possible so that I would be able to offer the best thermal shock resistant ceramic crucibles I can.

Even though they are all succeptible to shock like you already said the thinner the better. Its a balancing act between mechanical strength and thermal resistance. I have found some materials with almost zero thermal expansion coefficient but im not sure the temperatures they can reach and most of the ones I found such as astrolite I believe the name was is more concerned with shrinking since there use it in space telescopes and what not. Im getting all my materials sometime this week so im going to attempt to design some experiments to test the expansion coeficient of various materials.

Fleaker - 13-2-2011 at 09:50

I've melted aluminum in terracotta in a charcoal-fired furnace. The terracotta held up for the melt, but was very brittle afterward.

blogfast25 - 13-2-2011 at 10:29

Quote: Originally posted by Fleaker  
I've melted aluminum in terracotta in a charcoal-fired furnace. The terracotta held up for the melt, but was very brittle afterward.


I see from the latest sodium thread that you built a charcoal fired furnace using a Perlite based cheap refractory material. I’m sure that’s the same design I used for mine, which has a furnace cavity about the size of a large pint glass in a 5 l empty (mild steel) paint can (the refractory thickness was definitely overkill in my design: nearly 2 inches). I use an inverted old Hoover as blower, works very well...

It’s still going after all these years, even though the ‘refractory’ is now extremely crumbly and bits of Perlite keep flying off during operation. Not a huge problem because you can’t use it indoors anyway. Amazing just how much lump charcoal I get through when set to ‘high heat’… At those temperatures terracotta will suffer but so will steel which oxidises away like mad and is also softened (forging temperatures). Graphite-clay is probably best value for money.

I’ve vowed myself to build Mark II from a 20 l steel bucket and the same refractory (but thinner). One fine day…


[Edited on 13-2-2011 by blogfast25]

Fleaker - 14-2-2011 at 06:54

I've built about half a dozen furnaces. My first furnace actually was made of hard fire brick and fired with charcoal and/or wood with a leaf blower. I melted iron in that thing and vitrified the brick in the bottom! That's the furnace I used terracotta flower pots as an impromptu crucible. They only last about one heat, any more than that and they're liable to crack in a catastrophic way. The second furnace I built was with a very large stock pot. I plasma cut the hole for the tuyere and a hole in the lid for the exhaust. I then silver-brazed on a steel band and welded little screws into the circumference. I used a greased blocks of wood that I turned down on the lathe for the forms for the burner plug and exhaust hole and drain hole in the bottom of the pot (in the event of crucible failure). My insulation was perlite + commercial stuff in a 3:1 volume (5-6 cm) on the base and sides overlaid with 2 cm of the real deal castable high alumina refractory. A 22 kg bag cost me $40, perlite was probably less than $10 for all I needed. This furnace was the one that I attempted the Downs cell (and my P distillation disaster) in and it fit a #4 graphite crucible quite well.

I have a design for that stock pot furnace. Anyone here could build one with fairly rudimentary tools and have a furnace that'll get to about 1400 C/ 2600 F in c. 25 min (based on my own experience w/ a pyrometer).

Magpie - 14-2-2011 at 08:57

I'm not sure how big your crucibles need to be or what you consider excessive cost, but here is a rugged crucible that can take the heat:

http://www.minerox.com/index.cfm?fuseaction=product.display&...

blogfast25 - 14-2-2011 at 09:40

Quote: Originally posted by Magpie  
I'm not sure how big your crucibles need to be or what you consider excessive cost, but here is a rugged crucible that can take the heat:

http://www.minerox.com/index.cfm?fuseaction=product.display&...


That's a good price but they are relatively small...

Sedit - 14-2-2011 at 11:45

Honestly compaired to what I have been seeing from science outlets thats a great price. And for the large size thats an awsome price. Im seeing 50ml crucibles going for roughly 20-30$ which is just unreal and one of the reasons I started looking into making my own by the mass.

Fleaker - 14-2-2011 at 20:05

Go to graphitestore.com, or even better, legendminingsupply (google this, it's off the top of my head).

watson.fawkes - 15-2-2011 at 05:57

Quote: Originally posted by Fleaker  
Go to [...] legendminingsupply (google this, it's off the top of my head).
http://lmine.com/

The assaying crucibles posted upthread are from ANH Refractories, if I recognize the shape correctly. The come in boxes of 24 (I'm pretty sure; might be 12). In the USA, there are distributors in most major metro areas. The one near me takes walk-ins. It's where I buy my castable refractories.