Gold Processing Kilns
On this page we are going to pass on any tips or suggestions from GPK users and us on how to make your smelting experience successful, easier and help keep it fun rather than frustrating.
MICROWAVE SMELTING TIPS
FLUX: Understanding how the GPK Flux works can help everyone with their smelting experience. You cannot smelt without flux; the ore will not get hot enough on its own. The objective of the flux is to conduct the microwave energy, suspend and trap the impurities contained in the concentrates during the smelting process - thus allowing the precious metals to separate from them – whereby they can then be poured into a button in your mold. We designed a non-lead based flux for home use due to the health and environmental concerns raised when using the conventional lead based fluxes. The lead based fluxes with litharge can be successfully used with the GPK Kiln, however, litharge is only necessary when doing a fire assay.
We are constantly experimenting with different formulas to improve our flux performance and will pass on any information or additives to GPK users as we perfect them.
THE PROCESS: When your black sand concentrate is thoroughly mixed with the flux and put into the microwave, the flux conducts the electrical energy contained in the microwaves emitted by the magnetron. Most of us understand what happens when a fork is placed into a microwave and it’s turned on; plasma type arcing occurs and sparks fly. This same type of arcing occurs within the crucible, as the kiln and crucible are microwave transparent and the flux is electrically conductive of the microwave energy. The crucible then acts as an electrical insulator so that the electrical arcing cannot escape the inside of the crucible and fry the microwave itself. So what you have is electrical plasma arcing occurring within the crucible, which generates very high temperatures. As the flux and precious metals melt, the impurities are taken up by the flux and suspended so that we can pour the smelted precious metals from the mixture after sufficient temperatures are reached inside of the crucible. Note: The flux must be absolutely dry, or it can cause boiling over in the crucible. If it gets moisture in it, try warming it to 300F or 400F it in a pan on the stove – look for steam – until it is thoroughly dry and there is no steam.
BLACK SAND CONCENTRATES TO FLUX RATIOS: The formulas given in the instructions are guidelines for everyone to work with, not absolutes. An easy mistake to make is to start out with an excessive amount of black sand concentrate to flux. Now, if we overload the flux’s ability to suspend and take up the impurities contained within in the crucible by adding too much black sand concentrates or jewelry, we won’t get a good recovery of our precious metals. Rather, some of the precious metal will be bound up within the remaining sands or concentrates in the crucible. So as a remedy, add less of the concentrates or more flux to the crucible to get the correct ratios. If you start with small amounts of your concentrate and provide an excess amount of flux, the process is much easier to get a handle on. You can then ramp up the amount of concentrate to flux as your successful experiences allow and, within a few smelts, you will have arrived at a ratio that provides both good recovery and a small amount of excess flux in the smelt to deal with the impurities. The good thing about this smelting process is that you can always crush your flux after it’s cooled and do it over when things go bad. So here’s a simple rule of thumb; Start small and ramp up as your smelting experience grows. Once you get a handle on things, the 1 part concentrate to 3 or 4 parts flux is going to be pretty close and works well with the black sand concentrates I’ve been using. Note: the fewer impurities contained in the concentrates to begin with, the more concentrates can be added to a given amount of flux and still get a good precious metal recovery.
GRINDING BLACK SANDS: Often the very fine gold contained in black sand can be literally locked up inside the sand particles, called “micro-encapsulation”, or simply stuck to the sand particle itself. A GPK user suggested this tip to use prior to roasting and it seems to work really well: Take a trip to the thrift store and pick up a used blender. Put your black sand into the blender and grind the heck out of it. This can help liberate the “micro-encapsulated” particles and free up the stuck gold particles so they can be smelted more readily. Use small amounts, so you don’t overload the blender with your black sand. You want it to flow around and contact the blender blades as much as possible. The finer your black sand is, the better it is for smelting.
ROASTING YOUR CONCENTRATES: Here’s a suggestion from us and a couple of GPK users that can help with some types of black sand concentrate and is absolutely necessary for sulfide and partially oxidized ore concentrates. Remember: the less junk in your crucible, the easier it is to smelt out your precious metals from the concentrates and the better your flux will work during the smelting process. You need a metal gold pan and a propane fired type stove burner to accomplish this roasting step: Put your metal gold pan on your propane burner and turn the burner up on high. Spread a thin layer of your black sand concentrates in the metal gold pan and heat it up. Stir it occasionally and keep roasting it until it stops smoking and smelling. Obviously, this needs to be accomplished outside in good ventilation and you need to stay the heck out of the way of the smoke and fumes coming off the roasting concentrates. Don’t worry; your gold is not going anywhere - unless the wind is blowing about 40 miles an hour - then everything is going away. Once the concentrates are roasted, let the gold pan cool off and put your roasted concentrates in a suitable container. When you have enough of the roasted concentrates, you can then smelt them as outlined in the instructions. These steps can really enhance the effectiveness of the smelting process. Even if you don’t “grind” your black sand, the roasting step will definitely help your recovery. Another benefit of the “grinding” process is that it will help increase the homogenous mixture of your black sand with the flux that is necessary prior to microwaving it. If your mixture is not homogenous and there is an excess of black sand contacting your crucible during microwaving, a phenomenon I call a “thermite type” reaction can take place and burn a hole in the side of your crucible. Not a good situation to say the least; the finer your black sand is the better.
ROASTING SULFIDES: Roasting your sulfide and partially oxidized ores prior to smelting is pretty much the same as roasting black sands, with some exceptions. Obviously, your concentrates need to be concentrated. Panning your crushed ore, or using some other method of removing as much of the gangue material prior to smelting or roasting, is absolutely necessary. The next step is to take your propane burner and metal gold pan out somewhere where there are no neighbors or your wife to call the police or 911 on you, thinking you’re creating some kind of a bomb. Roasting a sulfide ore and driving the sulfur off to oxidize the ore smells really bad - ten times worse than the morning after any card playing, beer drinking, pickle egg eating party we used to attend (or still do). As with the black sands, spread them out in your metal gold pan, turn the burner on high and stir them occasionally until they reach 800F to 1000F and stop smoking and smelling like brimstone and hell itself. Again: stay the heck out of the fumes and smoke - not good. Roasting sulfide ore prior to smelting is absolutely necessary. Sulfides not thoroughly roasted will steam and spit and mess everything up when you go to smelt them; the driving off of the sulfur causes acidic reactions in the smelting process, raising heck with the crucible and the ph of the flux itself. Separate the magnetics out and use the non-magnetics and start off with ½ shot glass instead of a full shot. Glazing the crucible before smelting helps a lot, too. Note: This process works with free milling gold only – complex (chemically bound) sulfides cannot be roasted off.
STIRRING YOUR CRUCIBLE CONTENTS: I like to stir the contents of my crucible about two thirds of the way through a smelt to homogenize the contents and observe the status of the contents of the crucible. I’ll put the kiln in the microwave for about 45 minutes and then take the kiln out of the microwave. Remove the lid and carefully, by looking from above and to the side of the kiln, look into the crucible and see how the smelt is doing. If the contents are liquid yet still bubbling or “rolling”, take a stir rod and carefully stir the contents of the crucible to help homogenize the smelt, then replace the lid and place the kiln back into the oven and continue the microwaving for another 10 to 15 minutes. Then take the kiln out of the microwave and carefully observe the smelt again. If the smelt is calm or still, it’s about ready to pour. Stir the crucible contents a final time to help homogenize the precious metal beads and allow them to collect into a button and place the kiln back into the microwave for about another 10 minutes. After the time is up, remove the kiln, remove the top cover and first layer together and grasp the crucible with the tongs and pour it into the mold. Obviously, always wear all of your protective gear when performing any of these procedures.
Ok, why did I do all of that instead of just waiting for the timer and then pouring the smelted metal into the mold? First of all, it gives me a chance to see how well the smelt is progressing. By stirring the crucibles contents, you allow the precious metal beads to contact one another, which encourages them to coalesce into a single button. Also, it lets me judge the thickness of my flux and the status of the black sand concentrates contained in the crucible. Sometimes the black sand concentrates have a tendency to stick to the bottom of the crucible and you can dislodge them if they’re not fused into the bottom of the crucible. The final stirring allows me to judge the completed viscosity of the flux as it reaches its final temperature and the state of the impurities in the smelt at its end stage. Your crucibles contents and the crucible itself should be a yellow orange color at the end stage of the smelt, which is approximately 2000 to 2200`F. Note: smelting times vary, according to the wattage of your microwave. Ours is 1200 watts. Higher wattages heat faster and therefore decrease smelting times significantly.
AMALGAM: The microwave smelting kiln is a tool for smelting only. It does not separate out the precious metals that are in your concentrates. You still need to have your buttons assayed to determine their purity and content.
CARBON: Can gold be smelted out of carbon from a leach pad? No; microwave smelting cannot break the carbon bond with any loaded precious metals. Burning the carbon off can take temperatures up to 4000 degrees Fahrenheit.
PC BOARDS: Yes, you can smelt precious metals from computer parts. Get rid of as much base metals as possible first. Cut the pins off the plated parts, then, do a nitric acid or hydrochloric acid digest to dissolve and get rid of as much of the base metals as possible before smelting.
PLATINUM: The microwave smelting kiln is not designed to melt platinum, as it requires temperatures in excess of 3000 degrees Fahrenheit. However; you can use silver to ‘catch’ platinum. Silver, which melts at a lower temperature (1700 Fahrenheit), acts as a collector metal and will amalgamate with platinum during microwaving. You can then do a nitric acid digest, since platinum is not susceptible to the acid. The silver can be precipitated from the nitric acid after recovering the platinum by adding salt to the nitric solution. Add salt and stir the nitric solution vigorously, then allow the solution to sit overnight. The silver will precipitate as silver chloride and will grow crystals in the bottom of your vessel.
SCRAP GOLD & SILVER JEWELRY: Ok guys, if you want to pour a big button and your concentrates just aren’t doing it for you, go out and scrounge up some scrap gold or silver and pour a really big button. The GPK kiln and flux work great for that. The biggest gold buttons I’ve poured have been with scrap jewelry. I take a pair of wire cutters and cut the jewelry up into pieces to about ¼ of an inch, add 3 shot glass measures of the flux and smelt it. I’ve poured buttons over 5 ounces using this method with great success. There’s not nearly the amount of impurities contained in the jewelry as when smelting black sand concentrates, so the flux doesn’t get loaded up with them. The gold buttons come out with a mirror finish on them and they’re just amazing.
STERLING SILVER: So you want to gather up and smelt silver out of old silverware? You can do that. Just be aware that sterling silver is only .925 pure. To go to .999 pure is an involved refining process beyond microwave smelting.
STERLING SILVER PLATED: Don't even do there. Until silver reaches the price of gold, there is not enough silver on a silver plated fork to even begin to be worth messing with - trust me on this.
PROBLEMATIC THICK AND VICOUS FLUX: Occasionally, you may get to experience having good precious metal recovery from your concentrates; however the flux has become overly thick and viscous and won’t pour readily into the mold, trapping the smelted precious metal within the flux in the mold and along the side of the crucible poured from. You can thin the flux slightly by adding a small amount of sodium tetraborate (Borax) to the flux prior to charging the crucible. Twenty Mule Team Borax found at your local grocery store works fine, however most of the store borax has been exposed to moisture and is clumpy when the box is opened. Take a portion of the borax and sift it through a wire mesh strainer, breaking up the clumps and then dry it in a Pyrex type glass dish in your oven at 300 degrees for 10 or 15 minutes to remove any moisture from it prior to using it for thinning the flux. Thinning the flux is a pretty much a case by case basis and the ratios for thinning the flux are dependent upon your situation, Be cautious; over thinning can reduce the electrical conductivity of the flux, increase smelting times and potentially reduce final achievable temperatures during your smelting. Start slowly, dilute no more than 25% by volume of the flux with Borax and do so prior to charging the crucible. A better solution is to reduce slightly the amount of concentrates used when charging the crucible. Don’t try to add Borax to the melt when it’s hot. Some people will prefer to use silica sand rather than Borax to thin the flux. If so, you will need pure silica sand sized to 70 mesh. I’ve used both and still prefer to use the Borax with this microwave process.
NO BUTTON: Why didn’t I get a metal button when I poured the smelted material into the mold? First of all, everyone, you must have gold or silver in your concentrates to begin with and the more of it the better. For example: In a standard gravimetric fire assay, the lab will place 29.17 grams of the ore in a crucible along with their assay flux. After the fire assay has been completed by their method, they will recover the metallic bead and weigh the bead. Once the lab has determined the weight of the bead, they use a basic calculation that 1 milligram is representative to approximately 1 oz. of precious metal to a short ton of 2000 lbs. We all have a pretty good idea of what the size of a 1 gram gold nugget is - a nice picker in our gold pan - so we would need 1000 of the 1 milligram beads to make up that 1 gram nugget in our gold pan. A single 1 milligram bead is pretty small, especially when we take into the consideration the specific gravity of gold. If you’re looking at your black sand concentrates with a 10 power hand lens or a 30 power pocket microscope and you believe that you can see fine gold in there, just keep in perspective how small those gold particles actually are and how many it takes to add up to a gram or even just a milligram.
The photo below on the left is from an extremely rich sample of silver concentrates that fire assayed at 9000 ounces per ton. The button is approximately 9 grams and reference to its size is shown by the dime beside it. If that assay button was gold rather than silver and the button came from a 29.17 gram sample of our black sands, a single ton of our black sand would be worth approximately $11,000,000 at today’s prices. The photo below on the right is of an assay button that weighs 375 milligrams, representative of 375 ounces per ton. If that button came from a 29.17 gram sample of our black sands, a single ton of our black sand would be worth approximately $450,000 dollars at today’s prices.
The cupels in the two photos above are approximately 1 inches across. If I could pour a button the size of the one in the above photo to the right, out of approximately 30 grams of my panned concentrates, 50 lbs of that concentrate is worth about $12,000 at todays prices. Look for those small hard to find beads in your pours - you may have to crush the flux to find them. If there is gold in your black sand concentrate, the beads are there somewhere. You need to experiment with the process and see what works best for you. If you are finding beads at all, you are looking at potentially rich concentrates. I only keep the nuggets and large pickers out of my concentrates and smelt the rest of the fine gold and black sands together.
Remember to keep your expectations in proper perspective when smelting your concentrates. We receive many calls from people with large amounts of black sand on hand tested to be extremely rich in gold that is microscopic. We have not had any success with smelting gold out of black sand that does not contain a large amount of visible gold, so please refrain from purchasing a microwave smelting kit if you expect to create large gold buttons from a few microscopic grains of gold. Now, when someone out there does manage to break that molecular bond of gold bound up in rich black sands, please let us know so that we can pass on the information!
We hope these tips help explain the process when using the GPK Kit; we will be updating this page as we progress with the microwave smelting process.We are getting tips and suggestions from GPK users all the time and will pass them along to everyone. If you have a tip that you would like to share or any questions please email us at: email@example.com. Happy smelting!
And finally - remember the old prospector's saying: "There's always black sand where there's gold but there's not always gold where there's black sand!"
Color Chart Approximate Temperatures
Approximate Melting Points of Gold and Silver
900 - 940
1650 - 1720
463 - 671
865 - 1240
600 - 655
1190 - 1215
865 - 955
1587 - 1750
Cast Iron, gray
1175 - 1290
2150 - 2360
1170 - 1240
2140 - 2260
Gold, 24K Pure
1320 - 1350
2410 - 2460
1390 - 1425
2540 - 2600
1390 - 1425
2540 - 2600
1482 - 1593
2700 - 2900
Iron, Gray Cast
1127 - 1204
2060 - 2200
349 - 649
660 - 1200
865 - 890
1590 - 1630
1300 - 1350
2370 - 2460
990 - 1025
1810 - 1880
1425 - 1540
2600 - 2800
905 - 932
1660 - 1710