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Tim Cook

Crucibles

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Question for anyone familiar with casting brass/bronze or whatnot.  I am going to try my hand at lost foam sand casting.  Problem is my forge has an opening too small to use a cup style crucible large enough to hold the amount of metal I need to pour.  I need one that is in the shape of a bread loaf pan to fit.  Haven't found one yet.  I did find some cast iron bread pans.  I was thinking of buying one and welding the handle I need onto it.  Has anyone seen this?  Would it work?  Or does someone know of a supplier that sells the graphite version?

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2 hours ago, Tim Cook said:

Question for anyone familiar with casting brass/bronze or whatnot.  I am going to try my hand at lost foam sand casting.  Problem is my forge has an opening too small to use a cup style crucible large enough to hold the amount of metal I need to pour.  I need one that is in the shape of a bread loaf pan to fit.  Haven't found one yet.  I did find some cast iron bread pans.  I was thinking of buying one and welding the handle I need onto it.  Has anyone seen this?  Would it work?  Or does someone know of a supplier that sells the graphite version?

That won't work. Cast iron will melt along with the brass/bronze. Even regular steel won''t last well, as it dissolves into copper alloys well below the melting point of the steel. An alternative just getting a much larger crucible, and cutting it down. I know there are bowl shaped ceramic crucibles available, which are used f.e. for gold and silver casting. I don't know if they exist in the size you are looking for, such as: https://sunnymountain.net/products/alumina-ceramic-crucible-3-inch

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Here's Jake Powning casting bronze with a more shallow crucible, probably such a ceramic one: 

 

I'd advice though to make tongs that enclose the crucible, rather then grab the edge, as it make it less likely that the crucible will break and fall. 

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Really appreciate your reply Jeroen.  I wonder if u could explain further on the cast iron.  According to online metals, the melting point of cast iron is about 2200 degrees and bronze is 1675.  Why would a propane forge with pyrometer holding a temp of about 1800 or so not work?  Thanks!

Edited by Tim Cook

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A friend who has had years casting bronze said to me once that you never have the burner blowing against the crucible wall when melting bronze as it is detrimental to the metal, it causes bubbles in the metal if I recall correctly.  But using a propane forge should be just fine so long as you keep the temperature right and use a neutral flame.  The reason that cast iron or steel won't work is that they melt at high temperatures on their own but adding some other metals in contact to them begins to alloy with them and causes them to melt.  As said above, copper is one of those metals. The rule of thumb is that an alloy of a metal will always melt at a lower temperature than the pure metal itself.  The second you expose molten metal to another metal you begin to have an alloy form at the contact surface, some metals are more corrosive than others and that is why you cannot use a cast iron crucible for this purpose.  Ceramic is the way to go. I have some nagging memory that carbon is taken up by copper alloys and it makes them more brittle... something to check on.

 

Edited by Tim Mitchell

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Ok I get it.  Thanks for the clarification.  Now just need to find a ceramic crucible that holds about 3 or 4 kg in the shape of a bread pan.  Hope they make such an animal or I might have to build a melting furnace.

Edited by Tim Cook

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1 hour ago, Tim Cook said:

Ok I get t.  Thanks for the clarification.  Now just need to find a ceramic crucible that holds about 3 or 4 kg in the shape of a bread pan.  Hope they make such an animal or I might have to build a melting furnace.

@Dave Stephens did a nice thread about casting bronze.

 

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On 4/6/2019 at 11:14 AM, Tim Mitchell said:

 I have some nagging memory that carbon is taken up by copper alloys and it makes them more brittle... something to check on.

 

That's not the case. Copper alloys can take up hydrogen and oxygen. Hydrogen causes brittleness and oxygen causes porosity. So you need to ensure the metal is in a reducing environment (preferably have some charcoal floating in it if you use a gass forge), and don't overheat it. The hotter and longer the metal is heated, the more gasses it absorbs. 

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Thanks for the clarification Jeroen, I do remember something about them industrially using coke over the molten copper in the old days to avoid picking up Oxygen and Hydrogen.  Perhaps it wasn't picking up carbon in the copper readily I was remembering, but just the effects of carbon when introduced in the melt of an Iron Carbon Copper alloy. 

The original research that was done on Iron copper alloys back in the late 1800s by several different researchers showed that the higher the carbon content of an iron copper alloy ingot, the more brittle it is and the less able to be forged. The copper acts as a hardener for the Iron and the carbon compounds the hardening and embrittlement effect as it is also a hardening element.

I haven't gone over my copper iron alloy notes for some time, I will have to dig them out ;)

Edited by Tim Mitchell

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