Please do not take this on as a do-it-yourself project UNLESS the lenses are already a write-off due to fungus or other damage.
It is self-evident that you DO NOT boil the entire lens, barrel and all. Your endeavour will fail with scunge and grunge from emulsified grease all over the glassware necessitating dismantlement and a clean.
Cleaning lens glass in itself is a distinct skill that requires learning and practice.
To dismantle your lens to get at the individual elements, you will need some trade tools like a Rolyn Spanner and jeweller screwdrivers. Chiselling away at ring retainers with screwdrivers is not on. Likely you will have the point end of a jeweller's screwdriver stuck in your thumb or the web of your hand bleeding all over the glass and the rest of the household running to your assistance when your howls of protest are heard. If there is any doubt that the ring retainer cannot be removed, there will be none after you have damaged the slot or hole machined into the retainer for the specialised tool needed.
With older lenses, the compound elements ( two of more pieces of glass bonded together ) may be balsamically bonded. Boil them and they may come apart or the bond between will discolour or separate partially. They can be cleaned and rebonded but this is a distinct skill which requires learning and practice. Using modern UV cure optical cement, bonding has to be done right everytime first time or it is all over. That stuff does not come apart easliy.
Boiling of optical elements is fraught with bad possibilities. The chief cultprit is taking the element out of the boiling water to have a bit of a look at it. Due to shock cooling from ambient air temperature and the evaporation of the water, you will become suddenly amazed as the thing shatters before your very eyes, hopefully not blinding you. Optical glass is very brittle you will thus observe
Some internal groups of elements are fastened together as a removable module inside a swaged brass cylinder. To get at each element in the group, the brass cylinder has to be sacrificed to open it up and a new one made to very exact dimensions. This is a distinct skill which has to be learned and practiced.
Working with lenses is a distinct specialist craft and even for the most skilled artisans, the renovation of old lenses does not always end well. You will be both learning, practicing and doing the deed in your first go at it. You have the laws of percentages and gambling working against you.
That said, I did take on two old Cooke lenses which had been given to me in a ruined state. Fungus etching had got into the glass surface itself and it is usually game over because you will alter the figure of the glass by trying to remove this. I did it with what limited skills I had from polishing thin optical blanks for 35mm spinning disk adaptors. Despite all care, if one miniscule flake comes off from the rim of the element into the polish slurry, you've got yourself some tiny scratches before you know it and that did happen.
I got lucky with the Cookes but with a 90mm Zoomar Macro Kilar, the pits from the fungus etching were way too deep. The figure of the front element became altered. The lens became unacceptably soft and of course flary along with it.
