The dielectric in an electrolytic cap is a microscopically thin layer of aluminum oxide, formed during manufacturing by passing current through the electrolyte (mostly water and borax). As the forming process continues, the current becomes less and less, and finally becomes insignificant. But ALL electrolytic capacitors leak, at least a tiny bit, and the leakage current is filling in any remaining gaps in the dielectric layer. Leakage current will be more if it hasn't been powered in a while. If that "while" is YEARS, the leakage current may be enough to destroy the cap by overheating it... unless the current is limited. Which is "re-forming" the dielectric. A cap tester can do this, or a high-voltage supply with current limiting (a resistor will do), but a variac is hit-or miss (no current limiting). If there are defects in the foil, corrosion, other contamination, there may be hot spots that will fail sooner or later (if you see jumps in current while it's "re-forming", that cap is TOAST or will be soon...). Many 50-year old can caps will measure like new after reforming. But many won't re-form. Small electrolytics will almost ALWAYS be bad, as they contained less electrolyte - it evaporated long ago.