Melting Silver

maybe you need MAPP gas rather than propane? It burns hotter.
Del

OK.... but I don't know if my local filling station would know what to do if I walked in with my cylinder and send fill it up with MAPP please. Where does one find it?

Make sure the crucible is heated as well as the silver. The material of the crucible may not conduct well, but there's quite a lot of it - so it has a relatively large ability to soak up the heat applied to the silver (it's a function of actual mass and heat capacity of the crucible).

Yep, done that. The semi fused lump of silver has formed an irregular cup shape which I can turn over in the crucible. I am then able to apply the flame to the metal, and the crucible surface containing it. I get the flame coming back up around the metal so the entire thing is getting heated. After a good amount of time - like minutes - it still isn't even like bad rubber. I can make the crucible surface glow red too - takes a bit of time, but doesn't change anything. Right now, I'm thinking Oxy-Acetylene is the only option...

I think you need a flux

The flux is in there... (Sorry, forgot to mention it). Sprinkled on top when it's hot and red.

I gave you the link to read and contemplate the principals of smelting.

It's my nice way of avoiding saying "you are doing it all wrong"

It should be spelled principles.

Well I see by your subsequent posts - you at least read the link. And might have realized he's oxidizing faster than melting because of his methodology.

Just a guess on my part, do you have the flame blowing around the crucible or is the the tip of the flame just under the crucible?

Think the torch is above the crucible, and directed into it (OP states "The (fireclay) crucible is placed on top of a fireclay block.").

Flame is blowing into the crucible. The lump of non-melting silver is wedged in the crucible, making space for the flame to pass all round.

125 g is 4 ozt that is a lot of silver, plus your fire clay absorbs a lot of heat, and heat is being lost all around if you do not have a setup to reflect the heat back, as in, arrange your bricks around the crucible being heated. You may also get more heat from the setup if you can raise the crucible off the firebrick by an inch or two on a tripod. The brick below it is adding all its weight to the amount of heat required for the operation.

Besides raising up off the brick (which will make a huge difference) you might also consider whether the casting can be made by melting smaller amounts at a time (I don't know how this would affect the casting).

I wouldn't worry about it not being silver - it sounds like silver behavior with not enough heat, to me. Forget about firescale it will not prevent melting.

My sister-in-law does jewelry, and use an air-acetylene torch. You can do oxy-acetylene with a soft flame, too, but it might still be overkill.

What kind of burner exactly are you using? If your gas flow is limited, the flame may not be sufficient to melt the amount of Ag that you have. Know also that the many tries at doing this, as I surmise from your description of the problem, has made a significant amount of Ag₂O - silver oxide. This will interfere with the melting process as well but, if you can get enough heat to the crucible, your silver will indeed melt and the Ag₂O will rise to the top and can be separated by decantation - the unwanted oxide is called dross.

Flame temperatures of a few fuels:

• 2200°C = 3992°F, for Propane/Oxygen
• 2927°C = 5300°F, for MAPP Gas/Oxygen
• 2700°C = 4892°F, for Acetylene/Oxygen
• 3200°C = 5792°F, for Hydrogen/Oxygen

Since the melting point of Ag is 962ºC, well below the flame temperature of even Propane/Oxygen, you just need a bigger flame.

My burner is a LPG bottle (Propane / Butane I presume) connected to a torch (without a regulator). The brand name is ORCA (see here: http://www.regal.co.nz/category.php?sub_id=189). It is atmospheric oxygen - not pure, so my temperatures are not quite that hot. I'm going to purchase a torch that will give me (bottled) oxygen to add to my LPG gas, and then my temperature should be much higher.

Let us know how you get on then. Always good to see success!

Well, I've got my dual gas torch now. Just waiting for the paperwork to go though with the gas company so they'll sell me the oxygen. The guy I bought the torch from says that he does it all the time with LPG/Oxygen mix, and trying LPG alone is a waste of time. So it looks like I'm on track. I'll let you know how it goes. Thanks. Michael

Success! Took a while for the gas company to process my account application. Apparently, the accounts department is in Christchurch, which is a very traumatised place at present. I also had to increase the size of my torch flame, with a different tip. The metal heated to red fairly quickly, and then it took a fair bit of time to heat the crucible and metal to a point where it actually began "dripping" into the bottom of the crucible. I'm amazed at the amount of energy one has to "invest" in the metal and crucible for it liquefy. The other interesting thing about this is that an open mould casting doesn't work, because like any liquid I suppose, surface tension comes into play, and the liquid silver also forms puddles with gaps in between. Thanks for the conversation everyone. Cheers Michael

Glad to hear you've got it sorted out.

BTW - why not register? You can get e-mails to let you know when someone posts on at thread you've subscribed to if you want, and keep track of such things using the "updated discussions" link (looks like this: ).

Also sure lots of folk here would love to see examples of your work - you can post pics if you register.

Jus' sayin'.

PS Sympathies if you have any friends or rellies in Christchurch. Bad.

Agreed on all points. Particularly the invitation for Michael to join our merry band of pranksters and for anyone having to deal with the tough times in Christchurch.