Sciencemadness Discussion Board

Porcelain is cracking annoying

MeshPL - 26-10-2016 at 11:09

Or porcelain cracking is annoying.

At school we have a relatively huge stockpile of laboratory porcelain and I am free to use it for my own experiments. The problem is, during last few months I cracked 10 crucibles and evaporating dishes, and by the time I leave school, the stockpile may get a little smaller. Also, I haven't spilled any dangerous and hot chemicals due to cracking, but this is a matter of time.

Has anyone had simmilar issues with porcelain cracking?

The way I use this porcelain is I put it above burner (mecker type) on a kaolin triangle on an iron tripod ring, than fire the burner. When I put the flame off I usually wait till the porcalain cools before I use metal clamps to take it off (or I do it by hand since it is already cool), but sudden cooling cannot be a reason for all the cracks I have as some porcelain cracked during heating. Things I heat are somewhat corrosive, by example I had a crack when I was heating potassium hydroxide with urea to make potassium cyanate. However I never heated pure hydroxide or acids and their anhydrides and had cracks from less corrosive materials by example sulfur mixed with dichromate.

Can anyone explain what I may be doing wrong? May the flame heat or tripod size be an issue? Or is it just crappy porcelain (likely possible)?

aga - 26-10-2016 at 12:20

If it's all Brand New porcelain, without a glaze over every bit of it, then it may have absorbed some moisture depending on how it's been stored.

Apply a lot of heat all at once, water expands, material cracks.

Get a batch of say 5 or so and cook them at 80 C in an oven for a an hour then crank it up to 150 for another hour.

Mark them somehow then see if they fare better than the un-dried ones.

Edit:

i had a similar experience with an un-dried graphite 'A3 Salamander' cruicible. Large chunks blew off at high speed !

[Edited on 26-10-2016 by aga]

wg48 - 26-10-2016 at 15:21

All of the porcelain crucibles I have used have failed in as little as the second use.

It seemed they where very prone to failure when heating a solid compound that had previously solidified in the crucible. Possibly due to the differential temperature between the parts in contact with the solid and those not in contact or the expansion of the solid relative to the crucible.

aga: some graphite clay crucibles are sold unfired. They should be fired according to the manufactures instruction before use or they may burst.

crystal grower - 26-10-2016 at 20:42

I also have a history of destroying porcelain crucibles and dishes...

MeshPL - 26-10-2016 at 21:05

Quote: Originally posted by aga  
If it's all Brand New porcelain, without a glaze over every bit of it, then it may have absorbed some moisture depending on how it's been stored.

Apply a lot of heat all at once, water expands, material cracks.

Get a batch of say 5 or so and cook them at 80 C in an oven for a an hour then crank it up to 150 for another hour.

Mark them somehow then see if they fare better than the un-dried ones.
[Edited on 26-10-2016 by aga]


Well, the porcelain has glaze over it. But it is rather old and the quality may be questionable, so maybe it did absorb water over a few years? I will try your oven method. Maybe it will help. Or maybe it will break them. :P But it is worth trying.

@wg48 The porcelain cracks whether the compound is fresh (powdered or in chunks) or has been melted before (example: burning out all the sulfur from the crucible, as it is easier than other methids of removal).

Apparently I'm not the only one with such problems...

Thank you for all responses!

[Edited on 27-10-2016 by MeshPL]

Deathunter88 - 26-10-2016 at 21:27

Porcelain crucibles are some of the most fragile pieces of lab equipment. They must be treated like a delicate flower if you don't want them to break. Here is a video that demonstrates the proper technique for heating them for least chance of breakage.

https://www.youtube.com/watch?v=YwudANt37UM

aga - 26-10-2016 at 21:58

Ignorance of the relative contraction rates of the vessel & what was just made, plus it's 'stickiness', fractured a 500ml RBF here once.
Quote: Originally posted by wg48  
.. some graphite clay crucibles are sold unfired.

That one was just badly mal-treated, then somebody turned up wanting other stuff whilst it was being dried out.

Lessons:-

Treat your equiment kindly.
Let people get their own stuff.
Always buy at least two.

pneumatician - 12-11-2016 at 20:55

all other students also broke crucibles like crazy or only you? :)

can be a bad production serie. Is a good brand made in Berlin, Germany?

what say the professor?

MeshPL - 20-11-2016 at 05:10

Other students are not allowed to do their own experiments. :cool:

The porcelain is not so good grade, Polish, rather old (may be older than me).

The professor says: "This happens all the time, we have more, throw this cracked one away".

Dr.Bob - 21-11-2016 at 18:04

Just get some Vycor beakers. They will last better with heat. You can get some for half decent prices on Ebay occasionally:

http://www.ebay.com/itm/10-Corning-Vycor-Glass-Evaporating-D...

[Edited on 22-11-2016 by Dr.Bob]

Liamatpm - 9-12-2016 at 09:34

You probably are not doing anything wrong, I bet they are just old and are in bad condition. At my school about 30 test tubes break a day (we reuse test tubes). I would recommend just getting your own on eBay or something then they will not break (or at least have a less likely chance of breaking.

MeshPL - 10-12-2016 at 00:52

That stupid porcelain... heat it gently... breaks. Melt some zinc in a crucible and accidentally drop it into water... survives! Not even a crack! I guess smaller crucibles are less likely to break, at least by what I've seen so far.