I have tried it all, boiled egg yolks, bleach, liver sulphate, and have had some brilliant results. However, I've had many more rubbish results, without changing any of the obvious variables, so lord knows what's going on there? The very best I've had was from a natural re-tone, achieved when living in our caravan by the sea! I could turn a coin very nearly black, just by leaving it on a high shelf. Strangely, though, as you point out, some coins just wouldn't budge, and some would tone quite nicely, but the tone would smudge off with a thumb, whilst others would be a permanent and good quality tone. Equally, some would tone black, and others a brownish colour, almost like copper??? Not helpful, unfortunately, but, whatever you try, make sure the coin is decontaminated with acetone first, then the surfaces have a chance of toning evenly. Ps I did buy some silver ageing 'juice' used by creative jewellers to make antique-effect jewellery...I wouldn't advise it, it was very unpredictable in its action and outcomes, mostly leaving a brown/grey surface that, more often than not, would wash off! Good luck with it all! Something I haven't yet tried is burial in a plant pot, with various mediums and moisture contents...I reckon a formula could be found there?