Dickinson 26-H Electric Kiln

[Chumbawamba]

I picked up an old electric kiln the other day for what I think is a pretty good price. It's a Dickinson model 26-H. It has a ~15" cubic interior and goes up to something like 2300F. Certainly enough to melt gold. I paid $160.

Here is one that looks like it:

http://losangeles.craigslist.org/wst/zip/1475235915.html

Being a craigslist ad, that will go away very soon, but is a link to a free one in San Pedro, California, that needs repair. Good for the firebricks, at the very least.

Upon reading up on the use of kilns through various forum posts I now understand that it would have somewhat limited utility in terms of overall gold recovery or refining, but I think I can put it to use in several creative ways. I imagine it would be good for incineration of small amounts of things like connector ends or mylar with gold plate. Not sure if the resulting gasses would be a huge problem or something I can mitigate with fume hoods or afterburners? Would also be good for melting a decent amount of lead or aluminum, probably in a covered vessel. Probably will work for ore smelting on a limited scale.

I have yet to fire it up but before I do so I need to replace two missing firebricks.

I still plan to build a gas-fired furnace. I imagine electricity costs on something like this kiln would not be trivial if used a lot.

 

[4metals]

One use for an assay furnace of that size would be to refine using cupellation, Action Mining has large cupels (large enough to absorb 30 pounds of lead). By cupelling with lead you can produce dore beads which will be all precious metals and the base metals are adsorbed into the cupel with the lead.

If your alloy has silver in it you will have to add 3 times the weight in silver (inquart) and place the resultant dore bead in 50% nitric acid to dissolve the silver and you will be left with 99.5 fine gold. Melting that gold with selective fluxes will purify it even further. The silver can be easily cemented out of the nitric acid and re-used.

I would caution against using a kin as a burnout oven as the smoke may foul up the coils, plus there is usually no stack on a kiln, at most a small diameter hole in the top.

 

[markqf1]

My furnace (vcella kiln) is one of the best investments I've made for assaying and refining.
Keep in mind, lead is not the only collecter you can use.
Some pgm's like tin, copper and even gold.

All of course with the proper ventilation.

Mark

 

[4metals]

Bismuth is also used for cupellations, more costly but less toxic than lead.

 

[Chumbawamba]

Thanks for the feedback, peoples. I now feel more confident about my purchase!

 

[butcher]

now you need a pot full of gold to pay for the electric bill.

 

[4metals]

Not necessarily, I have a vcella kiln model 30, 16" x 16" x 16" it consumes 5.76 kw to run. I pay 10 cents/kw in Pennsylvania so running it for 8 hours costs me $4.60 to run it for 8 hours. (25% cheaper if I do it at night) If refining by cupellation, it is cheaper than buying acids and treating the waste. Plus you can refine and assay with the same unit.

 

[goldsilverpro]

4metals,

I just bought a #16 Vcella kiln with an auto-controller, for running assays - 16"X16"X11" - 4.8 KW. With the 4"-5" insulation, it takes about 2 hours to get it fully hot but, when it's hot, it stays hot. After several minutes with the door open, it returns to full heat in a few minutes after the door is closed. The only thing I don't like about it is that the door swings open to the side rather than sliding up and down like most assay furnaces I've had in the past. This makes it quite hot to work in front of it when the door is open.

In the past, with other furnaces, I drilled 1 or 2, 1/2" holes to introduce air for cupellation. Just cracking the door open is too slow and it works mainly for only the cupels in front. I'm thinking of a regular 1/2" bit to first go through the SS skin and then a carbide masonry bit to go (very carefully) through the firebrick. I'm thinking of a 1/2" hole, centered left-to-right in the roof, about 2/3 of the way back. Also, a 1/2" hole in the door, with a rotating disc to cover it when fusing. I can cover the hole on top with a firebrick when needed. I don't think I should chance drilling a hole in the back because of the element.

Do you think 1/2" holes are large enough? How do you introduce air? Any other tips on using this furnace? Do you swing the door all the way back to keep the heat off the left side of your face? Your #30 is only a little larger (about 30% - my chamber is 11" high) than mine. How do you dress to cope with the heat? I hate wearing those aluminized jackets and hoods, but it may be necessary.

Any advice would be appreciated.

Chris

 

[4metals]

Vcella kilns are excellent because of their price and ease and cost of rebuild. They are designed for the ceramics guys so I guess that's why the door swing is what it is. I have to wear the tin man suit if I'm doing a lot of cupellations, and the tin man sleeves if it's a short visit.

Getting air into the furnace evenly was an issue because it wasn't easy to provide a good airflow for driving the lead since getting a good airflow on the hinge side provided too much air on the opening side. I drilled a series of 5 half inch holes along the bottom of the door. Then I made up plugs out of refractory cement to fill the holes. The plugs fit easily into the holes and the end has a larger diameter to stop the plug when fully inserted and make removing a plug easier with heat resistant gloves on. As it turns out I only ever need 3 holes open for assays. I open all 5 when scorifying or cupelling with the giant cupels to make dore.

I always keep a line of dummy cupels just inside the door across the width of the furnace to act as a temperature buffer.

I just drilled a 1 inch hole in the top center which I cover with a refractory brick for when the furnace heats up.

I also keep a thin 1/4" layer of bone ash on the base slab for easy spill cleanup.

My first Vcella was the smallest 6" cube furnace, I believe the model 6. It was cheap to run, heated up quickly, and didn't give ma a suntan every time I opened the door. It could hold 9 to 12 cupels easily. The only reason I moved up to the larger furnace is it was given to me by a refiner who switched to gas.

Good luck with your assaying venture, I hope this info is helpful.

 

[lazersteve]

Chris,

I'm no furnace expert, but I've been using mine for several years now and have crossed some of the same bridges you are talking about.

My furnace has a Paragon 9" x 9" x 8" interior. The door swings open to the left and has a 1/2" peep hole with cover in it. The roof has a 1/2" hole dead center in it.

It takes about an hour to get up to heat and requires aluminumized gloves and leather body protection to work around when the door is open. I started out using regular welding gloves and got my thumb tip badly burned right through the glove on my first run. That's when I switched to aluminumized gloves.

I use 36" crucible tongs to keep my distance from the opening when the oven is hot.

I've had a few crucibles give way (usually when iron is in the mix) and had to repair the floor of the oven twice now. A few months back I started using a thin flat piece of graphite on the floor of the oven to help protect it.

The small holes don't really provide the necessary air with my oven, so I have to open the door.

Steve

 

[goldsilverpro]

4metals,

Thanks.

I like the 5 holes in the door. I think I'll drill 4, 1/2" holes in the top. When I open all 4, that's equal in area to a 1" hole and I won't have to buy 2 high dollar 1" bits. Also, it might be better to spread them out.

I can't believe I didn't think of the sleeves. I'm pretty sure I can get away with them. What do your wear on your face?

I don't think it would be too hard to modify that door. I'm thinking of two channels, with the door riding in them. The channel would have to first come towards you, to clear the door, and then up. A counter weight on a pulley could work the door, like on some furnaces. The Johnson door had one short piece of small channel iron and one short piece of 1/2" steel rod welded to each side of the door. These rode in the u shaped channel.

Steve,

You might cast a floor plate out of refractory cement. Also, 4metals mentioned covering the plate with bone ash. I put it on thick. Also, when you get a spill, cover it with a bunch of bone ash. It will absorb the flux and, when hot, it can be scrapped off. If you move fast, the floor plate isn't damaged much. I did buy an extra floor plate, though. They're pretty cheap at Legend-Reno.

One time I was doing assays in a Johnson gas furnace with an open muffle plate - no enclosed muffle. The first time I ran it, the blast blew lead out of the cupels on the sides whenever the fire kicked on. The plate was riddled with little pools of lead. When it cooled, I picked the small beads off. However, the large beads had sunk pretty deep. After that, whenever I ran the furnace, there were about 5 little bright shiny pools of molten lead.

For others, here are the furnaces I've discussed.
http://www.lmine.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=vcella_assay_furnace
http://www.lmine.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=26250&Category_Code=johnson_assay_furnace&Product_Count=0

Chris

 

[4metals]

Chris,
I haven't used face protection, other than my normal safety glasses, with my furnace except for while doing crucible fusions on low grade refinery residues. The mid fusion additions require a full suit of armor as well as a face shield.

I've been experimenting with assaying using the technique seen in this video

http://www.midstatesrecycling.com/movies/assaymed.wmv

It's from the web site of Mid States Recycling in Chicago, I've never seen the technique before and when I saw it I had to try it out. Got good results on karat material, close numbers. Ever used this technique before?

 

[markqf1]

Just to interject a thought, ... the holes on the top will draw no air but will allow the holes on the bottom to draw more air.
True, it takes thirty or forty minutes to get it up and going but, then it's worth it as the furnace will maintain heat so well.
It's perfect for routine use doing different tasks throughout the day, if I desire.
Also, I can run 5-6 assays at a time.

When I'm in my testing mode, I always try try to run three identical tests, and then divide for a more accurate result.

Mark

 

[goldsilverpro]

It's from the web site of Mid States Recycling in Chicago, I've never seen the technique before and when I saw it I had to try it out. Got good results on karat material, close numbers. Ever used this technique before?

I watched it several times and the only unusual thing I saw was the use of a dish for fusion instead of an assay crucible, except I saw no litharge. Is that what you mean? What do you think he was doing? Maybe I'm blind.

 

[4metals]

He's doing karat assays' (clean scrap) he scorifies every cup before he cupels. The scorification is to get rid of the base metals and the cupellation gets rid of the lead. Seems redundant to me but they claim better accuracy. I don't see how.

 

[goldsilverpro]

He's doing karat assays' (clean scrap) he scorifies every cup before he cupels. The scorification is to get rid of the base metals and the cupellation gets rid of the lead. Seems redundant to me but they claim better accuracy. I don't see how.

I thought of scorification but things just didn't look right to me. They looked like scorifying dishes, but the ones I always used were pink. I notice the A.P. Green ones are white, though. What is the black coarse looking stuff in the dishes (test lead?) and what is the white salt he is adding? That doesn't make sense to me unless the white stuff is the sample? He added the white stuff with a spoon, so I doubt if that's the sample. Where is the karat gold sample?

I noticed that there were both lead buttons and lead foil cubes in the cupels. I assume the buttons are for gold + silver and the foil is for gold only (with added silver). Interesting the way he numbers the cupels. Looks like a magic marker.

I don't understand the claimed better accuracy either. The assayer probably is an old ore assayer that loves scorification.

I assume that you talked to them and found out exactly what they are doing. The video was short and not that clear.

BTW, I just noticed that the Bugbee and Shepard & Dietrich assay books sell at Legend-Reno for $20 each. Both are excellent for ores and bullion. Not much good for electronic scrap unless you read between the lines.
http://www.lmine.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=assaying

 

[4metals]

What he is doing, which isn't specific in the video, is weighing a karat sample into a scorifier, adding the silver, adding test lead, adding borax, and scorifying. After pouring into a cone mold the lead cone is de-slagged and in the process made into cubes. (pounding to square them often helps remove the thin slag on the sides)

Now the video shows him weighing the lead, I don't know why, if you notice the balance is around 5 grams, that seems light for the sample, silver and remaining lead, perhaps the filming sequence is out of order. (after all the purpose is to sell their services, often to people who don't know what they're looking at anyway)

Finally the parting. I've been told they add 1 gram samples and 3 grams of silver but the way the coronets hold together doesn't look that way, unless they hold the shape of an annealing cup, that is a possibility.

I'll try and get more information on the process to make more sense of it.

 

[goldsilverpro]

Finally the parting. I've been told they add 1 gram samples and 3 grams of silver but the way the coronets hold together doesn't look that way, unless they hold the shape of an annealing cup, that is a possibility.

Do they use annealing cups or is that the form they take by just dissolving the silver out of the silver/gold bead? In the video, the coronets look big. In reality, they are much smaller.

I don't know. Maybe, with very weak nitric to start, you can first keep the gold from breaking up and produce the coronets shown in the video. Personally, I have never feared the breaking up of the gold. I add a lot of silver (5X to 6X) and let the gold break up. I part with 7/1, water/nitric, in a 30 ml Coors crucible and then swirl the particles until they settle together on the front bottom edge of the crucible. I then use a stronger acid( 3 HNO3/2 H2O), rinse it several times, and dry it slowly on the hot plate. In my life, 10,000 plus assays, I have never poured off a particle of gold doing this. I like this breaking up thing because it guarantees the elimination of everything but gold. With the old US mint method, keep it in one piece, 2/1 Ag/Au method, you are never sure unless you do it exactly as they do it, with jeweler's rolls and perforated platinum parting crucibles. I don't have these advantages.

BTW, I am really enjoying this discussion. Keep it up.

Chris

 

[4metals]

The coronets do look big, probably a close-up. Judging from the grain of the coronet, it probably takes the shape of the annealing vessel. The coronets I've produced from the rolling procedure (low silver) have a much tighter grain.

I've done it with the 2:1 silver, annealing the beads, rolling them in the mill, and coiling the rolled beads before parting. They never fell apart in parting but it was real easy to lose a sliver of metal in the rolling mill unless you annealed after every pass. When I was taught, the 2:1 silver rolling and annealing method was referred to as the "long" procedure, the 3:1 silver that falls apart in parting was the "short" procedure.

The small granules of gold behave well and it's relatively easy to decant without losing metal. Parting in Coors crucibles makes it easier to keep track of the gold granules because of the white color and you can anneal in the same vessel. I like the "short" procedure much better. I agree that the parting is more complete when the coronet breaks up.

I'll try and get more details on the assaying procedure as I'll be in Chicago most of next week.

I still have my rolling mill but now I use it to roll out wax impressions of gill plates so I can age the fish I catch in my pond.

 

[Chumbawamba]

I look forward to the day that I understand what 4metals and GSP are talking about.

In the meantime, I still have to make time to wire up a 220V outlet for my kiln so I can test it out. I appreciate the talk of the amount of heat that these things can generate. I already knew that protective clothing would be required but didn't really think about it until reading this discussion. My kiln opens from the top with a big heavy door, so I will probably have to be wearing a full suit of leather armor when this thing gets up to temperature to move things in and out of it.

I think I'll probably have more fun with the gas powered furnace that I plan to construct at some point. Right now I'm still trying to get my backyard work shed going so I can continue my experiments. 21 more days of shortening length, then we plateau and the days start getting longer again. Yaaay!