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Colorado Prospector - Gem and mineral prospecting and mining forums > Prospecting, Mineral Collecting and Treasure Hunting Forums > Equipment - Prospecting and Lapidary
Caren Seabeneck
Hello, Good People! Yes, these kilns really do smelt gold and silver in a microwave! Being new on the market, here are some answers for your questions:

Microwave
Q. Can I use a microwave with less than 1200 watts?
A. Yes but it will take longer to reach smelting temperature inside the crucible.
It is also possible that the magnetron may over-heat and thermally shut down before sufficient temperatures are reached if using a smaller wattage microwave.

Q. Can I use a microwave with more than 1200 watts?
A. Yes. It will take less time to heat the material inside the material to liquid state. Smelting time varies between microwaves and is approximate – you will need to experiment at first to determine the best length of time for your equipment and materials.

Q. What does microwave transparent mean?
A. Microwave transparent means that the microwaves travel through the material, rather than bouncing off or heating it. It is important that the kiln material and kiln shelf and crucible are microwave transparent, so that the heat occurs inside the crucible, rather than outside it. If the heat occurs outside of the crucible, it then becomes a radiant heat, rather than a form of plasma arc within the crucible.

Q. Why do you recommend a microwave with the magnetron on the side?
A. It just seems to work better than a microwave with a magnetron on the top. Rear is ok, too.

Q. I thought you couldn’t put metal inside a microwave. How can gold be smelted in one?
A. Some metals can reflect microwaves and some metals can absorb a portion of the microwave energy. The microwaves that are emitted from the magnetron are both a magnetic and electrical energy wave, so when the any metal becomes charged with electrical energy or reflects the electrical energy, it basically acts just as nature does during an electrical storm. The air is ionized around the metal and the energy that follows these ionized paths of conductivity, discharge, not unlike a lightning bolt. Lightning bolts are actually a form of plasma containing very high energy and heat.

Cast Iron Conical Mold
Q. Can I use a different mold?
A. Yes; anything that is made of cast iron will suffice, however the conical mold is designed to help the melted metal to form into a single button by taking advantage of gravity and the restricting design of the conical shape, similar to a funnel.

Q. What temperature does the mold need to be preheated to before pouring into it?
A. No particular temperature. Preheating the mold has two purposes, first to drive any moisture out of the mold. 2300 degree metal and glass and any form of water do not mix. Preheating the mold also helps the flux to flow into the mold more readily. I like to place the mold on a stove top burner and heat it for 10 or 15 minutes, this seems to accomplish both tasks well.

Q. Can the mold be preheated inside an oven?
A. It can be preheated inside an oven but it will not get as hot as it will on top of a gas or electric burner.

Q. How long does the slag need to cool inside the mold?
A. 10 to 15 minutes. Always wear your protective gear when inverting the mold and working near the cooling or cooled pour.

Q. How do you know when the slag inside the mold has cooled enough to handle?
A. When the orange color is gone and the flux turns black, and it doesn’t set your gloves on fire when you handle it.

Crucibles
Q: How long will these ‘one shot’ crucibles last?
A: They should last through several firings if seasoned beforehand and treated properly.

Q: Can other types of crucibles be used?
A: Not usually; only clay crucibles are microwave transparent. Graphite and Silicon Carbide crucibles have carbon in them and are not microwave transparent. When placed inside of the kiln they absorb most of the microwave energy and only allow a small amount of the microwave energy to pass through them into the interior where the conductive flux and metals are contained. As a result the crucibles will get very hot, however the plasma arcing that takes place inside of clay crucibles is greatly reduced. In other words your heat is now a radiant heat from the crucible penetrating inward, rather than the plasma arcing and heating taking place directly inside of the clay type crucible. Fused Silica crucibles can be used, however they are much more expensive than the throw away “one shot” crucibles.

Q: What will happen if I use the wrong type of crucibles?
A: The microwaves will strike the crucible and heat it up from the outside instead of the inside and the flux and ore concentrates will take much longer to heat and defeats the plasma type arcing that is so effective in the process. Please refrain from using anything other than clay crucibles and always make sure there is 1” of space between the top of the crucible and the kiln lid.

Q: Can I fill the crucible full of flux and ore concentrates?
A: No. Leave at least 1” of space from the top of the crucible, so the flux can expand and not overflow the crucible.

Flux
Q: What does the flux do and why must this particular flux be used?
A: The flux is custom mixed and designed for the microwave process being used here. The flux protects the crucible, allows the plasma arcing to occur and refines the gold processed to a higher degree of purity.

Kiln
Q: What is so special about this kiln?
A: The kiln is comprised of the highest quality high density no smoke high temperature refractory ceramic fiber kiln lining insulation available that is microwave transparent (allows microwaves to go through the material) and is suitable for applications up to 1260°C (2300°F). It has low thermal conductivity and high heat storage and is thermal shock resistant for fast thermal cycling without cracking or spalling. It is factory cut and rigidized to provide for long life and excellent handling strength and material integrity at high temperatures.

Kiln Shelf
Q: What caused my kiln shelf to crack? Is it still useable?
A: Moisture absorbed from the atmosphere will affect it, so keep your kiln shelf dry and protected from moisture at all times. Also: due to the high temperatures achieved in this microwave process, all kiln shelves will eventually crack due to thermal shock and you may notice a small hairline crack after using it. This is cosmetic only and does not affect the structural integrity of the shelf, as it is thicker than necessary and is well supported by the layer of kiln material beneath it. Even if the shelf were to break completely in two, it would still be safe to use, with caution.

Q: Why is it important to use a certain type of kiln shelve and not just any one?
A: All kiln shelves are not alike. They are composed of different materials targeted for different types of kilns. The kiln shelf that is provided with our microwave kilns is made of high alumina, which is more microwave transparent than other kiln shelf materials. It is also denser and therefore less susceptible to warping and has a higher cone rating of 11, which means it is safe to use up to 2400F degrees.
Shelf materials not to use in your microwave:
Silicon carbide shelves have carbon in them. Although rated up to 2730F, carbon is conductive. Cordierite kiln shelves contain around 60% silica, with some alumina and small amounts of other minerals (mullite and corundum) and are rated for up to 2320F degrees.
CP
Hi Caren and welcome to the Colorado Prospector forums.
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Great Q & A's you've provided about the Microwave gold kilns and kits, thank you.

For anyone who's interested in trying one of these kits or kilns, you can view details and pictures in the Miners Market where the kilns, kits, and supplies are now available.

I'll be looking forward to the results reports and pics of the processes from some of the folks who uses one of these too. char098.gif

CP
Caren Seabeneck
Here is our new, homemade 6 minute YouTube video that shows the microwave smelting process in action.

http://www.youtube.com/watch?v=tB3oQeTG-Mc
CP
Cool video Caren, thanks for posting that up. I can see why you'd want to be patient with the cooling process.

With so many jewelers using the precious metal clays these days, Denise and I were wondering if the Microwave Gold Kilns could be used for the clay cure or settng process also?
We haven't used any pmc's yet but there are quite a few folks who do use them.

CP
Caren Seabeneck
Good question, CP, and a new one for us! We had to actually research what pcms are and, upon finding out what silver clays, for instance, are used for, here is Patrick's answer:
"We don't have the temperature control to deal with pcms. You would need a pyrometer and control to keep it a specific temperature inside the microwave 300 degrees less than the melting point of silver, or the material is just going to melt."
Well, that makes sense. After all, our objective with microwave smelting is to melt everything, especially the precious metals. However; it is a valid concept and something Patrick (the kit designer) is interested in looking into eventually. It would be a nice addition to the microwave gold smelting kit, perhaps even with a specially designed microwave.
Caren
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