Author Topic: Are crucibles fragile?  (Read 6088 times)

Offline ironman

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Are crucibles fragile?
« on: July 24, 2012, 12:31:14 AM »


The below video's are not the accepted industry or home shop practice. This disclaimer is from the forum staff, don't remove

Hi everyone

                    I have been told by a few people that crucibles are sensitive to heat shock, so I did two experiments to see if  they  were right. I also did two experiments with molten metal and water. I must stress that when water is completely surrounded  by molten metal and the steam has no where to escape then it is dangerous.  Have a look at the two videos  <youtube>HWUF6wuYDt0<youtube>                <youtube>49gX1Nlzy1A</youtube> 
       
« Last Edit: July 27, 2012, 11:42:42 AM by PTsideshow »

Offline trevoratxtal

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Re: Are crucibles fragile?
« Reply #1 on: July 24, 2012, 02:12:15 AM »
Many thanks Iron Man.
You have taken the time and trouble to dispel a misconception.
I am so pleased that you have done this this will allay the fears of many folk thinking of using melted metals in poor surroundings, like back yards when it could rain.
I have for many years run a outside forge, and often melt metals, with never a problem even if it rained.
But so many times I have been told it is dangerous by folk that knew better  :D  :D
The filming was perfect. :beer:
Thanks again
Trev 

Offline ironman

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Re: Are crucibles fragile?
« Reply #2 on: July 27, 2012, 03:02:18 AM »
Thank you for at my videos. 

I have another video about a lead casting myth click onto the link 

Offline Anzaniste

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Re: Are crucibles fragile?
« Reply #3 on: July 27, 2012, 07:43:36 AM »
Steam explosions occur when you pour molten metal into a damp or wet mould. In the examples shown on utube  the water floats on the surface of the molten metal or in the case of pouring the metal into the water the mass of water compared to the metal is very large.
The wet mold scenario is completely different. The seam is produced very rapidly below the surface of the molten metal and produces an "explosion" The mass of the water is very small compared to the molten metal but the volume of the steam which is produced almost instantaneously is huge.
I know I did it as a kid many years ago and was lucky not to have suffered injury. I was using plaster of paris molds which I had not dried thoroughly.
Scrooby, 1 mile south of Gods own County.

Offline PTsideshow

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Re: Are crucibles fragile?
« Reply #4 on: July 27, 2012, 10:07:55 AM »
Warning before following the above advice in his video's Consider the following information!
Water and hot metal do not mix, it is a more of a BLEVE than an explosion But since water will flash into steam and expand almost instantly to between 1,100 to 1,200 times its volume depending on the books or sources cited.

The warnings about it in a small scale home shop backyard casting,and steam expansion, flinging molten metal. Are very real. In a school shop they had a student that washed the pop cans out and crushed them before they were completely dry insides trapping water inside.

When the water flashed into steam,flinging droplets of molten aluminum everywhere with in about a 15 foot radius of the furnace. The good luck that the students were all down at the other end of the shop 30 feet away, and the emergency gas valve was near the door of the classroom rather than by the furnace. Which at the time was continuing to spit steam and molten metal out as the upper cans melted.

Pouring molten metal in to water, is an old jewelery technique and it is also used to make bead or shot for casting. Most bronze, pewter, lead shot for casting and other work is made this way. Silver shot for a number of granulation techniques are made this way. This a well known and widely covered in most jewelry casting work books.

The problem is your are mixing a number of different statements, Liquids in the material you are melting cans, or oils that are clinging to or inside the material are what will cause the problem. Your results are not the correct ones for the things you are trying to disprove.
That is why the statements about clean, dry scrap or materials for melting are in all the books, and forums dealing with casting.

Your videos are disingenuous in the premise and the results! They are dangerous, because people aren't smart enough to think things through with basic science or physics.
As been said when you pour molten metal into a larger container/mass of water the cooling effect of the water can handle the heat transfer.

If you have ever seen a steel/iron foundry and forming mills in action, you will notice that they use very large amounts of water to keep the equipment and process running. 

Also I hope you have continued good luck with that crucible, and it doesn't let go one day as your are pour with a full charge. Since your attire will cause your injuries to be compounded.
You did use a brand new crucible, The point of warming up the crucible and materials in it is to slowly drive off any liquids that may be in the mix and burn off other trash since it expanding at differing rates.
Also the rain part is aimed at not adding wet materials into a crucible, not the heating up to drive off moisture.
The point of the instruction with new crucibles to warm them up again is to drive off moisture that may be in the materials of the crucibles. Since they are compressed from graphite and or carbon materials and a binder, unless they are home made and then are clay based which means they will have moisture in them ( inside the walls of the crucible). Which can cause the water to expand and crack the walls thus leading to failure of the crucible.

Your video's and your statements will be a danger to people that don't better. And I hope no one or your self get injured by your folly and misunderstanding of the process.

The only thing your videos prove is the old adage that" a little knowledge is a dangerous thing!" :zap:
« Last Edit: July 27, 2012, 03:02:21 PM by PTsideshow »
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Offline yorkie_chris

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Re: Are crucibles fragile?
« Reply #5 on: July 28, 2012, 09:17:48 AM »
 :Doh:

Have you seen the videos where ally is poured into a damp steel ingot mould? The flash boiling is that rapid it sounds like somebody hitting a girder with a hammer!

Be bloody careful. If in doubt preheat everything over the furnace exhaust.