Silver Mica Capacitors

Here's a fairly common component that contains silver in the plates of the rectangular section as shown on the right side of the photo below:



These are known as silver mica capacitors and are typically found in older TV and radio equipment and in surplus shops by the thousand. They are easy to identify by their burgundy colored purple soft epoxy shell as seen on the left side of the photo above.

You can easily remove the epoxy with a few hammer blows (or a ball mill) to free the legs and the silver rectangles as seen in the center of the photo above. The legs are mounted via two crimps on either end of the rectangular plates.

The silver plates are each separated by a thin layer of mica to form the desired value of capacitor using the proper spacing, number, length, and width of plates.

Typical yields for these types of capacitors is 1 troy ounce of silver per pound of components. Larger value capacitors will produce more silver as they require larger surface area plates. The silver can be melted directly as 999 silver without further processing if you are careful not to include any of the crimps or legs in with the harvested rectangles. The mica will melt around 900C just before the silver does @ 962C.

The legs are not silver and should not be included in the silver you harvest.

Steve

Thanks for the info! Yet another thing to keep a eye out for.

Nice Find. Thanks for sharing. I will keep my eye out for those.

I knew those 7 boxes of monitor boards were sitting around the shop for something..lol We shall see if it is worth our time. Thanks for the good info as always.

I have lots of those and many are much larger. I've posted below another type of silver mica typically found in transmitter RF circuits. Look to the left where they are flat on board, but there are also some on end. This guy is usually well soldered as the whole purpose of this type is to reduce stray lead length for better efficiency and bandwidth. The trimmer capacitor on right with the big slot head screw is also mica and may have silver in it.

I'm wondering if those old disc ceramics like the one in upper right of trimmer have any values. I've used unpotted versionswhere they soldered down to a board and used as a terminal to locate lead(s). Those seemed to have silver.

I have vintage micas in attic, but those are probably best EBAYed and I think most are unsilvered micas.

FrugalEE

I went out and melted a 100 of the rectangles with soda ash flux and came up with a silver colored bead that skinned over purple when cool. Looks like the best route to the silver may be to use either flotation to separate the lighter mica layers from the silver foil layers or to use 35% HNO3 to go straight for the silver.

I used a MAPP torch for the test and it took 30 minutes to get everything molten even with the flux. Maybe my furnace would have been a better choice. The mica was very resistant to the heat by itself, but once the flux was added the mica melted quickly and the tiny silver BBs began to collect together. Under the torch flame the silver was nice and bright colored.

I tested the foils with Schwerter's before melting and got a nice bright red that did not fade, so I'm sure I was dealing with the silver variety. I believe the flux may have liberated some other metal from the mica which combined with the silver.

I'll do some more testing tomorrow as I have several pounds of these.

Steve

Thanks Steve,

I take it your parts are new and free from solder. I would not have expected mica to melt. I'm thinking I may have seen you post on doing those caps, but a search on mica got nothing. Since I get HNO3 cheap I will probably try that approach at least for first sample. I should be able to find some clean parts to play with.

FrugalEE

FrugalEE said:

I would not have expected mica to melt.

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It is most likely being dissolved, which is a function of the proper flux when melting non-metallic substances.

The resulting metal from those caps, once collected by melting, would be excellent for inquartation, assuming it contains no lead. That way processing can be considered free, assuming one has need for silver for inquartation.

Harold

FrugalEE said:

I have lots of those and many are much larger. I've posted below another type of silver mica typically found in transmitter RF circuits. Look to the left where they are flat on board, but there are also some on end. This guy is usually well soldered as the whole purpose of this type is to reduce stray lead length for better efficiency and bandwidth. The trimmer capacitor on right with the big slot head screw is also mica and may have silver in it.

I'm wondering if those old disc ceramics like the one in upper right of trimmer have any values. I've used unpotted versionswhere they soldered down to a board and used as a terminal to locate lead(s). Those seemed to have silver.

I have vintage micas in attic, but those are probably best EBAYed and I think most are unsilvered micas.

FrugalEE

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Good point about Ebay. HAM Radio operators and their ilk like to use them because their low thermal coefficient and their high voltage rating. They're pretty expensive retail, if you can find the values you need.

FrugalEE,

I have pounds of the flat capacitors you showed in the pics, When I took them apart I was testing the peices and the small square peices inside seem to indicate Palladium. I havn't tested with DMG to give me a positive result. but I used harolds test of heat, and they came out a pinky purple rainbow colour. I then read in an old dental metallurgy book that an alcoholic solution of potassium iodide will leave a black stain on palladium. When I checked it with this it left a black stain that could not be rubbed off.Any possibility you could test yours and see if you get similar results? I will need to source some DMG.

Lunker,

Good to hear you're exploring this part and I'm impressed with your testing. You're way ahead of me. I was hoping to put a couple of the caps in some nitric today, but the wife wanted to shop for drapes. As I think more about it I'm afraid the high copper content will just cement out the silver and I'll get a mess. I'll probably have to add HCl to get Ag to drop as a chloride. Need to study more or get some guideance. I have a scale capable of .01 gram resolution and small assay types of experiments is what I'm interested in.

I've got lots of electronic parts that I've been accumulating for 56 years now. I haven't done much building the past few years so I can't always find something real quick, but I did find my assortment of the subject cap. My memory is that I've seen only one vendor for that part. The board I photoed has UNELCO brand (Underwood electric company?). I found my hoard of parts and all 65 of them are also UNELCO. That should reduce the variations of how this type of part is built.

I bought a 10 gram bottle of DMG from Carolina a couple of months ago, but haven't done anything with it yet. PM me your address and I'll mail you some.

FrugalEE

FrugalEE said:

Lunker,

Good to hear you're exploring this part and I'm impressed with your testing. You're way ahead of me. I was hoping to put a couple of the caps in some nitric today, but the wife wanted to shop for drapes. As I think more about it I'm afraid the high copper content will just cement out the silver and I'll get a mess. I'll probably have to add HCl to get Ag to drop as a chloride. Need to study more or get some guideance. I have a scale capable of .01 gram resolution and small assay types of experiments is what I'm interested in.

I've got lots of electronic parts that I've been accumulating for 56 years now. I haven't done much building the past few years so I can't always find something real quick, but I did find my assortment of the subject cap. My memory is that I've seen only one vendor for that part. The board I photoed has UNELCO brand (Underwood electric company?). I found my hoard of parts and all 65 of them are also UNELCO. That should reduce the variations of how this type of part is built.

I bought a 10 gram bottle of DMG from Carolina a couple of months ago, but haven't done anything with it yet. PM me your address and I'll mail you some.

FrugalEE

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Don't add HCL but keep adding nitric in small amounts and the copper will get used up and the silver will go into solution, filter your solution and then use copper to cement out the silver. Then should have clean silver with no trash in it.

Thanks Barren I'll do just that. I hope that works as it will be much easier to dry and weigh metal silver. The ratio of copper to silver is going to be very high, maybe even 100 to 1. I remember years ago dissolving a silver dime in nitric, but there we have 90% Ag with only 10% Cu.

It should allow you to filter your solution out and not have to deal with any trash.

I just bought a gaylord of these vintage rf boards. Good to know about these mica caps. ;-)

FrugalEE said:

Thanks Barren I'll do just that. I hope that works as it will be much easier to dry and weigh metal silver. The ratio of copper to silver is going to be very high, maybe even 100 to 1. I remember years ago dissolving a silver dime in nitric, but there we have 90% Ag with only 10% Cu.

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Lunker,
I had not heard of alcohol and potassium iodide testing of palladium before. I will have to look into that a bit.

FruugalEE,
Barrens advice to digest in nitric is what I would do, partially because I detest dealing with silver chloride. But in this case it seems likely there will also be some lead. Using HCl added to a nitric solution to remove your silver as a chloride will also give you lead chloride that will co-precipitate if lead is present. You will not have any lead as a solid if you cement to copper. When you go to cement if there is palladium present as well as silver they will both cement on copper, you will notice that Pd is present if your cement is darker than usually seen when just cementing silver on copper. You may wish to repeat your silver dime experiment so you can see how silver cement looks on copper in nitric. Use just enough 50/50 nitric to digest your dime so you are not having to digest alot of copper to get all of your silver cemented.

Since you have DMG, test a small sample after digesting. I would suggest you use a test tube for this since you think your solution will be rather dark with copper. Put a small sample of your solution in your test tube then add a couple of drops of your DMG solution. Agitate it a bit to mix it then let it rest for 10 minutes. The reaction itself is fast, but it can take a bit to settle if the amount of Pd is small. If you do not see a precipitate after this, it would be a good idea to dilute your test with some distilled water to make your solution more transparent. I recommend the distilled water in this case to be sure you do not introduce any chlorides that could give you a sediment of silver or lead chloride that you may mistake as a Pd reaction in a colored solution. You are looking for a light yellow precipitate as a positive for Pd, but its color may be masked by your solution color.

Since you bought DMG I would also suggest you buy or make some Schwerter’s solution for testing silver.

Barren and Oz,

Thanks again for your replies and they tell me that the dissolving of the metals is an entirely separate operation and the cementing won't happen until I put in fresh metal Cu. One part of my brain was trying to tell me that I would either get a mess of Cu sticking to Ag or that the Ag would go into solution and immediately precip out as metal. Oz, when you say go easy on the HNO3 when dissolving dime it implies to me that Cu added is going to just go into solution until pH drops to a certain point and then we're going to have cementing start to happen. It also implies that weak HNO3 solutions may not dissolve without some cementing happening right away.

I'm going to start with clean new parts that shouldn't have any solder unless it was used to make the part. I have Schwerter's solution as part of a gold testing kit, but I haven't used it yet. I also have the raw chemical to make more.

I doubt that these micas are still being used very much. They have mostly been replaced by ceramic chip caps. Recovering values from these guys is going to be tough with the solder to deal with. Steve cleverly eliminated the Cu & solder from his melting of the epoxy micas and I'm anxious to take a hammer to some of my old dipped micas. To recover from the UNELCOs we might want to remove the heavy Cu although it is obviously silver plated and there might be more silver on the Cu than on the mica. Anyway, it's an interesting project.

FrugalEE

Oz
I found it in a old student text book on dental metallurgy. It was for distinguishing palladium from platinium.From what I have been able to gather. It dosn't stain stainless, aluminium steel nickel silver. You can buy it at the pharmacy as a antiseptic.If the parts are palladium it could be one more way of narrowing down what a material is or is not.

FrugalEE,
I dissolved some in nitric and cemented them out on copper. I melted the silver powder.I don't know if my mapp torch is weak ir if there is palladium in the mix but i couldn't get it to melt. So I got angry and melted it with a oxy acetylene torch. I know that was a bad move! the resulting bead is covered with pox marks from the gas bubbling out on cooling.Still nor sure if it was just the silver spitting or the palladium spitting.Without the DMG I am running blind!

I would gladly buy some test solution from you. But i live in Ontario Canada.Shipping may be tough.But thank you for the offer!

Lunker,

Good report. What I had in mind was putting something like a half gram of DMG powder in a plastic enveope inside a normal letter envelope and mailing it to you for FREE. I know some people get excited about white powder in envelopes. Would that be a problem for Canada?

I just picked out two 510 Pf parts and weighted them---- 3.318 grams so I need about 18 ml of 50/50 HNO3.

I would expect the more Pf the part is rated for the more values to be had. I don't recall seeing parts higher than 1000Pf and I found one I had at 12 Pf which seems unusually low for that type of part. I remember these parts as being rather expensive when they first started showing up.

FrugalEE

FrugalEE said:

I just picked out two 510 Pf parts and weighted them---- 3.318 grams so I need about 18 ml of 50/50 HNO3.

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Remember when you do your calculations that you expected a good deal of copper in these parts. It takes 2.38ml of 50/50 nitric to digest a gram of silver, however it takes 8.10ml to digest a gram of copper. In other words it takes 3.4 times more 50/50 nitric to digest copper than it does the same amount of silver.