# Using a Kiln



## MrCrusher (Mar 19, 2017)

I am trying to figure out if a kiln would be a reasonable way to make PM bars. I am only considering Silver and some Gold but mostly Silver.

I am not a good search person and have been looking and not found anything that relates. If there Is another thread on this can someone point me to it?

I now use a Oxy/Acyl Torch to melt but I want to make 10toz bars in a mold. I can get 5toz bars with the torch but to get it a nice flowing molten mass to pour takes some effort. At least to get the quality of pour I am looking for.

Does anyone have any experience trying to use a kiln? I was also wondering if I would need to do it in an inert gas? And what would be the best mold material. Could I use a Stainless Steel mold? It has a much higher melting point and my thought would be to soot the mold well before placing it into the kiln to stop it from sticking. 

Any feedback would be welcomed. I am totally in the thought process at this point.

Thanx
Mr_Crusher


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## 4metals (Mar 19, 2017)

This thread in the library may get you started. 
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=85&t=22204

As far as molds are concerned most use graphite but a graphite mold will not last long without an atmosphere to protect it in the kiln. The expensive bar making equipment today is a belt furnace with an atmosphere where the mold (usually graphite) is loaded up with an exact quantity of shot and it progresses through the heat zone and cooling zone, emerging on the other side as cool to the touch bars.

You could do the same with an electric kiln with an atmosphere to protect the graphite and just unplugging it to cool off before removing the atmosphere and you should get a smooth topped bar with the sides and bottom mirroring the mold. 




This is what kilo bars look like coming out of one of the belt furnaces.


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## MrCrusher (Mar 19, 2017)

4metals,

Thank you for that link I have been looking for a post like that.

What atmosphere would you recommend, for using graphite molds? I did not see any recommendations in that post on the best gas to use to purge the chamber. If it is sealed fairly well I don't think it would use much gas to keep it purged and to have a shot at bars that look that good would be well worth it to me.

While I will admit making electrical stuff is one of my weak suites it could be a good learning experience for me if I decide to try and build one. I just don't like electricity.

As a side note: I am a machinist by trade and have access to CNC Mills So I can actually make a mold out of anything I want. The reason I was thinking Stainless was because I could polish the bottom and sides which I can not do in graphite the grain structure is just too coarse to get a mirror finish.

Thanx
Mr_Crusher


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## 4metals (Mar 19, 2017)

Larger operations use disassociated ammonia to produce a forming gas but I believe it is available in cylinders as well. I don't know how you could seal a furnace to limit use of excessive gas. That would be the real trick.


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## snoman701 (Mar 19, 2017)

In an electric kiln with standard coil electrodes, an oxidizing atmosphere is a requirement of operation. When atmosphere is reducing, it will allow the oxidized surface of the electrode wires to flake off the oxidized outer skin. Then upon change of atmosphere, it will again oxidize. This will cause a very premature element failure.

There was a group of ceramic potters about ten years ago that figured out that coating the elements with a ceramic coating called ITC 213 allows one to use the electric kiln in a reducing atmosphere. It protects the elements with a refractory coating that keeps the surface of the electrode stable. Cost last time I looked was around $200. 

Now, I have no clue on the PM side if the using of the "forming gas" is required (to prevent the deterioration of graphite), I have not read up on it, or if merely a reducing atmosphere is required. If it's merely a reducing atmosphere, this is pretty easy to achieve using the above, and just a gas line leading in to the kiln once above autoignition point. Some, who wanted to maintain reducing atmosphere from initial exposure would use a bunsen burner and a small hole drilled in to the base of the kiln.


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## 4metals (Mar 19, 2017)

Excellent suggestion, it is common to use a reducing flame over crucibles and over molds when melting and pouring to burn off any atmospheric oxygen and produce a reducing environment. This is doubly important when melting silver. for this application it seems like a good way to go, inexpensive and simple.

I believe the larger units rely on disassociated ammonia because it has a controllable effect on the temperature which is closely monitored in the preheat, melt and cooling zones. 

You could experiment with the flame size (gas flow) to get the smallest flame possible so the bar will cool enough to solidify even with the flame burning when the coils are powered down. I would think it would be best to direct the flame at the top of the mold but far enough away that the gas flow doesn't ripple the pool of metal in the mold. 

If the gold or silver is pure enough the bars should look shiny, the graphite just has issues when the temperature gets over 500ºC and there is oxygen to react in contact with the exposed graphite. Actually oxygen is reacting with graphite at room temperature as well but very slowly, once things heat up it is noticeable quickly. The graphite will go from a smooth machined surface with sharp square corners to a grainy looking rounded corner mold which will not produce the shine and surface finish on the bar's side walls and bottom. The top will cool slowly and can be quite attractive and smooth.


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## snoman701 (Mar 20, 2017)

If someone chooses to go the ITC 213 route, www.potters.org should have the archives to see how to coat the elements. Prep is really important. One pinhole in the coating defeats the entire purpose of it. 

This coating has been mentioned on the forum before, but given it's cost, people went back to the "lets reverse engineer" the coating. Don't waste your time. These companies have deep pockets...even with the patent, you'd only have a range of values for the composition. Given the cost & time involved with changing kiln elements, it just hardly seems worthwhile.


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## g_axelsson (Mar 20, 2017)

You don't need a reducing atmosphere to protect the graphite mold, it only needs to be inert. Any protective gas for welding should work well, and I think nitrogen would also work.
Vacuum would also work but could be problematic to create for an amateur. Actually, that is something I have plans to test. I got a carbon evaporator, a vacuum pump, bell jar and a high current transformer. Using a graphite mold I should be able to use it as both heating element and mold by passing electricity through it in a perfect vacuum. If that doesn't work I can make a heating element from a molybdenum plate.
I'll write it up when I have tested it, so far it's only a plan and the machine is behind a pallet in my storage.

Göran


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## MrCrusher (Mar 20, 2017)

Thank you everyone for sharing so much information.

To be honest some of that went over my head. I will need to study some of those terms.

I was going to use an electric kiln, with no plan on having a flame involved. After looking at all the responses my thought process leads me to this plan of attack for my first effort. If anyone sees a flaw in the plan please share good or bad.

I was going to use a small electric kiln with a programmable controller. My thought there is after I determine the time needed for the melt it can auto shut down. This should give me some repeatability in the process. I was going to use some thin steel to aid in sealing the top / door ( not sure which yet ) the rest of the kiln is closed. Then insert a feed tube for an inert gas. Not sure if the tube should be in the top or bottom. Will it matter if the gas is heavier or lighter then air as to where to place the gas input. I would think it would.

Since no one said anything about other mold materials then graphite I guess I will start with that. One question about the premature coil failure. Will an inert gas have the same deteriorating effect?

Thanx
Mr_Crusher


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## snoman701 (Mar 20, 2017)

Moly is where it's at, but the elements aren't easy to power. If I recall correctly, they were using phase angle fired SCR's and pretty high voltage (otherwise the moly just sucks the amperage). Not nearly as easy as plug and play of nichroome, but also much much nicer to play with....as long as you have an off switch on your door 

I enjoy making things that make things hot.

I honestly don't know about the inert gas. I've personally always wanted to try it. The need to go to a full reducing atmosphere for ceramicists is really to utilize the colors of reduction in the glazes. I am a pottery collector, and there's still something entrancing about a good copper red that draws me to it....just because it's such a difficult feat to pull off. So please, report back.

If you are looking for a well constructed but affordable kiln, I was always a fan of the AIM 8x8x9 test kiln. It appears they now make this with a built in controller. If I recall correctly, an extra element will cost you around $30, and you could theoretically use this kiln as a model for your own homemade version...It's either less than or equal to a single box of insulating fire brick. If you are going this route, please make sure you wear a good respirator when cutting the firebrick.


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