What causes cleaning marks?

[Joe Smigiel]

There are so many references to lens cleaning marks as well as a current discussion as to whether they actually impact performance (except of course on eBay where they make no difference at all to performance), but how exactly do these marks originate? Quartz has a mineral hardness of 6 and you can use powdered limestone (whiting, rottenstone, Bon Ami cleanser, etc.,) with a hardness of 3 to polish glass. Other than rubbing sand into the lens or using emory cloth (hardness 9 IIRC) or diamond dust (10), what is actually scratching the lenses or coatings? I would think cotton or other fibers would be much too soft to cause the actual damage. But, it obviously happens for some reason.

This question puzzles me almost as much as how those clothes hangers seem to accumulate without any purchase.

[David A. Goldfarb]

Old guys with trench coats, stogies, and fedoras cleaning their lenses with their polyester neckties.

[Paul Fitzgerald]

Joe,

brass rivets do wear down iron brake drums. the brass will wear down about 4X faster than iron BUT it still wears down the iron. I guess the dust wears down faster than glass or coating BUT it still wears down the glass or coating.

Just a thought

[E. von Hoegh]

Bits of abrasive material on/in whatever you are cleaning you lens with OR bits of abrasive dust on the surface of the lens when you wipe it.

They should be called something else; say "abuse marks" as that is what they are.
It is better to keep your lens clean, than to keep cleaning it. Use a protective filter and lens caps!! I bought one lens new in about 1988. It has been cleaned a grand total of 5 times.

Edit: Optical glass can be soft when compared to, say, window glass. It really doesn't take much to mark it.

[Brian Ellis]

"Cleaning marks" is often just a euphemism used to describe almost any defect in the glass or the coating. Real damage from cleaning can be caused by any number of mistakes, such as using an abrasive cloth, using a cloth with extreme dust or similar particles on it, trying to clean a lens without first removing any abrasive materials that are on it by blowing them off, cleaning the glass too often, applying too much pressure to the glass/coating, all sorts of things like that.

[IanG]

Some optical glass is actually quite soft. The glass used fot Leica Summar lenses is some of the worst, which is why you find very few good unscratched Summars.

Early coating also sometimes softened the glass surface too, while most modern Multicoating tends to form a harder tougher surface. There are no hard and fast rules.

Cleaning a lens with a dry tissue or wipe can cause minute scratches that build up over time, the liquid cleaners are often far less abrasive as well as being more effective. One problem with cleaning "dry" is that often the dust & dirt builds up at the edge of the glass, over the years this can become quite thick and hard, so much so that one lens I bought with a camera was described as having separation. In fact a good "wet" clean showed that the problem was actually 70 years of dirt.

The only lenses I've had that suffer from cleaning marks were both new enlarger lenses, and used in a commercial darkroom, I wasn't the only person to clean those lenses, I have no idea what employees used to clean then but they have a lot of fine scratches, I've only kept them for parts.

Ian

[aduncanson]

Old guys with trench coats, stogies, and fedoras cleaning their lenses with their polyester neckties.

Yes, I have heard that cigar or cigarette ash makes an effective, very fine, abrasive.

[Ole Tjugen]

... Quartz has a mineral hardness of 6 and you can use powdered limestone (whiting, rottenstone, Bon Ami cleanser, etc.,) with a hardness of 3 to polish glass. ...

Quartz has a hardness of 7, by definition. Even the hardest glasses reach only 6.5, which is easily scratched by quartz which is a common constituent of dust.

[Joe Smigiel]

Oops. My bad. How could I forget this?

Must have lost my apatite.

[vann webb]

There are so many references to lens cleaning marks as well as a current discussion as to whether they actually impact performance (except of course on eBay where they make no difference at all to performance), but how exactly do these marks originate? Quartz has a mineral hardness of 6 and you can use powdered limestone (whiting, rottenstone, Bon Ami cleanser, etc.,) with a hardness of 3 to polish glass. Other than rubbing sand into the lens or using emory cloth (hardness 9 IIRC) or diamond dust (10), what is actually scratching the lenses or coatings? I would think cotton or other fibers would be much too soft to cause the actual damage. But, it obviously happens for some reason.

This question puzzles me almost as much as how those clothes hangers seem to accumulate without any purchase.

Wear from abrasion behaves in a way that is counterintuitive to what you would expect to happen. I build machinery for a living and wear from sliding surfaces is always a source of concern especially in conveying applications. When two surfaces are in contact and an abrasive condition exists between them, the harder surface always wears first. This is because the abrasive material embeds or adheres more to the softer material turning it into a sort of sandpaper if you will. I have seen conveyors where the soft plastic chain has worn almost completely through 1/4" thick steel. Happens all the time. Studies have been conducted by major conveyor belt manufacturers that confirm this, bizarre as it sounds. The same thing happens with the lens surface. The soft cleaning surface picks up the abrasive dust and scratches the lens surface. Thus ends today's obscure lesson on abrasion.