Forming Wet Electrolytic capacitors
What is forming, and why is it needed?
You can all that burn in, though it's not done under heat, but under maximum voltage. It is needed for "wet" capacitors.
1. Final production step.
Electrolytic capacitors are made from treated aluminum foil, isolated by a paper-like foil, soaked in conductive, water based fluid, which contains acid. We all know of course, the capacitance of a capacitor depends on it's plate surface. There are a few ways to increase the surface, so you can roll a larger surface foil into the same can. Such as: Make the foil and isolation layer as thin as possible, and also making the surface of the foil raw, which increases the surface considerable. With electrolytic capacitors, the foil is not just raw, but covered with crystals, or also amorphous processes, which increases the effective surface with a multiple factor, so not just a little bit. We don't want to start a capacitor book here, but the main advantage of electrolytic capacitors is low cost, small size, and little risk on defects by a short. So if one of those reasons apply, you'll find an electrolytic capacitor in the circuit.
After the rolled foil is put in it's can, at first use, the electrical values will not be even near to what it should be. Reason is, the capacitor is not "formed" yet. At forming, the isolation layer gets created by a process called electrolysis. This process uses electric current, and a chemical composition is formed, made up from the electrode metals, and the salty of acid fluid in between. The distance between the plates is set by the a layer of paper in between. Here comes the interesting thing with electrolytic capacitors. This layer which is formed, is none conductive. Initially, when the layer is not present yet, applying a voltage will result in a current. The electrolysis process which takes place now, will result in a non-conductive layer, and this will stop the process by itself. In the beginning, the voltage can not be very high, because current would be too high, and do damage. The layers can be grown thicker, during which process the used voltage can increase, until at some point no growth can be achieved any more. The natural limit seems around 630V isolation voltage. The isolation property of the formed layer is incredible high, it is 700...1000 Volts per micrometer.
2. Long time storage of NOS capacitors
These are factory burned i, but burn-in slowly disappears after storage. If you find some NOS capacitors, you will see d that capacitance has increased some 5...10% , and isolation voltage has reduced down to 30...60% of what is printed om them. That is good sign, so they are probably still good. The best way to damage them, is simply connect the maximum allowed voltage to them, with a power supply capable of high current. Unknowing , this is what many people do, and after that the famous "recapping" begins. The best way to save them, is apply this voltage with a high series resistor, thus limiting the current in the uA range. Voltage will not begin to rise slowly. Once done, data is within specs, and maximum voltage can be achieved.
I have 80 years old NOS capacitors here, which tested "bad" and were fine condition after forming them. I used those to repair old equipment. They tested just as good as any new, just bigger size.
3. Revive long time stored equipment.
This is a thing, no fool will understand easily. But if you find old equipment which was known to be working when it was stored decades ago, you can choose to destroy the capacitors, or save them. It's your personal choice what you want to do. If you want to destroy them anyway, I would recommend to take them out before you do so, and not risk to damage the rest of the old equipment by trying to destroy the capacitors all at the same time. By switching the equipment on, unprepared, some will damage immediately, some after a few weeks or longer, and some will not damage. If they seem to work after this, you can not see, which are the ones which are going to fail soon, which will fail later, and which will not fail. So if the device still works after you did do, you may already begin with the re-capping now, or just wait for the first defects.
The way to save the capacitors is, desolder them one by one, and format them before first use. Then you will see, all of them were not good initially, and you can format normally all of them again to be fine. This may seem the more "difficult" way, which is true when you want to sell it on Ebay anyway. But it saves you the cost and the work of recapping the device. Also you have the original caps still inside, and in many cases these are fine brands and quality. Myself, I much rather have 60 year old USA Mallory capacitors inside, if they test fine, as new made caps, whatever brands are printed on them.