I was sent an Industar 11n 450mm large format lens from the Ukraine on a trade deal. It was disclosed that it had fungus that was not cleanable and was thrown in as a bonus item. It may function ok depending on lighting angles etc. --"good for soft focus"--that sort of thing.
I repaired my lens not thinking about taking before pictures (like an idiot). I sent the photos of the fixed lens to my Ukrainian vendor friend. He stated in a facebook chat, " The industar - WOW! It looks like a miracle. When I send it to you, I'm wrote you, that it looks inside like relic from fungus, which someone cant clean. You definitely need to make new tutorial on this!"
My vendor thankfully had photos before he sent it to me and here you can see the fungus.
Muratic Acid- available from decent hardware stores and swimming pool supply outlets. Often sold as concrete floor cleaner and toilet bowl cleaner
Baking Soda--most folks have this in their kitchen. a teaspoon or 2 should do
5-6 Q-tips or generic "cotton buds" as they are called outside of the USA in general
a shot glass with some water
a microfiber cloth--eyeglass cloth or lens cleaning cloth
Latex or Nitril gloves
Ceramic or glass dish
CAUTIONARY : Muriatic acid is nasty stuff. Wear your grubbiest t-shirt and holy jeans. Do all measuring-pouring over a sink and wear gloves and optionally eye protection. Measure out only a capfull of the acid and have additional baking soda handy to neutralize any spills. Keeping everything in a glass dish with a fairly high wall or lip will minimize any chance of damage to any environmental surfaces.
These steps may not be suitable for all lenses. You should NOT undertake disassembly of electronic autofocus lens or get in over your head on your mechanical capabilities. I'm showcasing a very simple barrel lens with an iris that required NO TOOLS to disassemble. You might need things like a spanner wrench to make disassembly possible. If you fumble or invert a lens that's been disassembled the iris blades can come out of their positions and that's a lot of fun getting the layering right on those again.
Consider any lens with a pronounced fungus issue disposable but do proceed with all due caution. The easiest thing in the world is to be spendy with other people's money.
From constructive feedback on my process lets look at the harsh chemicals potential effect on lens coatings. Only use the acid bath on surfaces impacted by the fungus as the fungus has already done it's damage to the coatings there. Lens coatings help newer lenses with color balance, light transmission and flare/glare. Older lenses (pre WW2) are generally uncoated. In this exercise I did NOT acid wash the front & rear elements as they had no fungus so any coatings on them have remained intact. The clarity through the lens has been restored. Most modern camera lenses are hard coated with Magnesium Flouride at high temperatures bonding it to the glass and abrasives like Cerium Oxide has to be used or Hydroflouric Acid to remove those coatings
Step 1: Disassemble the Lens and Layout Wet & Dry Areas
For the sake of an interesting tutorial I'm recreating the steps with photos
The first step is to define your environment and put everything to be treated for fungus in a wet containment area ( the pyrex dish in this case) and everything that needs to stay dry OUT. You'll want to open the lens to it's widest aperture and leave it "parked" there as shown in the 2nd photo. DO NOT get any acid-baking soda or water in the iris leaf assembly and avoid touching the iris leaf blades with bare fingers--your fingerprint oils will rust them out over time. Remove the elements and put the barrel off to the side.
In the dish with the lens elements is the capfull of the muriatic acid--if it spills it's contained.
Step 2: Acid Bath
Simply dip the q-tip or cotton bud in the capfull of muratic acid and gently swirl it on the fugus on the elements, If you have an element assembly and the fungus is between 2 elements that are cemented together that portion cannot be cleaned. Not even I will try to split cemented elements.
Spend about 30 seconds scrubbing each impacted element and set them back in the dish
Step 3: Neutralize the Acid With a Tip Spoon of Baking Soda
Place a small tip of a spoon's worth of baking soda on the element still wet with the acid--it will fizz & react. Add a few drops of water to make a slurry paste.
Step 4: Scrub the Lens With the Baking Soda Slurry
Grab a fresh q-tip and work the baking soda slurry around to make sure all the acid is neutralized--again a good 30 seconds of gentle rubbing. Remove the leftovers with a very damp paper towel
Step 5: Buff Dry
After all the baking soda/water/acid is removed use a microfiber lens cloth to give it a shine
Step 6: Post Cleaning
Here we have the lens in various lighting/angles. All that nasty orange peel cellulite fungus aftermath has been removed as well as any internal dust. I did not ruin my glass or my iris leaves and my Ukranian vendor was impressed. Now for a sample shot after making a lensboard. This will be going on an 18x24cm FKD soviet view camera
Step 7: Raw Test Image
Xray film in home made Leica News divided developer. f/90 for 5 seconds . Overcast day but scene looks good