Removing AR coatings

LewisM · #1 14-03-2016, 03:49 PM

I had some lenses made in China to replace some damaged originals in a 1940 vintage military scope. They did a perfect job, but despite me asking for no AR, they applied a geen MgF coating free. Nice, but NOT period correct as it needs to be.

So, how to remove the coatings? I seem to recall soaking in hydrofluoric acid or another acid will do it? I know cerium oxide errosion will do it but i fear altering the figure.

And do not suggest living with the coatings, as being a rare and valuable item (made by Zeiss/Hensoldt) it actually devalues the item considerably despite the advantages.

PCH (Paul) · #2 14-03-2016, 04:12 PM

Lewis,

I'm no officionado but wouldn't just having replacement (Chinese) lenses be enough to devalue the item regardless of the coating?

Just a thought from someone who knows nothing about it. But I'd be interested to hear the rationale behind why my suggestion doesn't hold true.

Cheers,

Quote:

Originally Posted by LewisM

I had some lenses made in China to replace some damaged originals in a 1940 vintage military scope. They did a perfect job, but despite me asking for no AR, they applied a geen MgF coating free. Nice, but NOT period correct as it needs to be.

So, how to remove the coatings? I seem to recall soaking in hydrofluoric acid or another acid will do it? I know cerium oxide errosion will do it but i fear altering the figure.

And do not suggest living with the coatings, as being a rare and valuable item (made by Zeiss/Hensoldt) it actually devalues the item considerably despite the advantages.

alocky (Andrew lockwood) · #3 14-03-2016, 04:16 PM

Surely having Chinese replacement lenses in there would devalue it more? Presumably its for a ww2 sniper, if its going into a collection and not onto the range you might as well polish it off as the figure is not going to be that critical. The scopes from that era were pretty low power from memory, so you're unlikely to do too much damage to the figure anyway.
The atm volumes describe many ways of converting drill presses into polishing machines for small lenses as well.
HF will also dissolve the lens as it reacts nicely with SiO4, and is not something you should be able to buy, let alone play with it at home.

Cheers,
Andrew.

rrussell1962 · #4 14-03-2016, 04:32 PM

Dragging out my dim recollections of my MSc before becoming an accountant - Hydrofluoric acid reacts with glass and was stored in plastic containers. Very Very nasty stuff. Please find another solution (no pun intended) Lewis.

julianh72 (Julian) · #5 14-03-2016, 04:56 PM

Quote:

Originally Posted by rrussell1962

Very Very nasty stuff.

Yes, it is exceptionally nasty stuff, and would not be legally available to a general member of the public. It is very dangerous for direct skin contact, and the fumes can do lasting damage to all sorts of body tissues.
https://en.wikipedia.org/wiki/Hydrof...lth_and_safety

Quote:

Originally Posted by rrussell1962

Hydrofluoric acid reacts with glass and was stored in plastic containers.

My university chemistry lecturer told the (apocryphal) story that Carl Wilhelm Scheele (1742 - 1786) spent half his lifetime working out how to manufacture useful quantities of hydrofluoric acid - and then spent the rest of his life trying to find a container to put it in!

LewisM · #6 14-03-2016, 05:21 PM

Quote:

Originally Posted by PCH

Lewis,

I'm no officionado but wouldn't just having replacement (Chinese) lenses be enough to devalue the item regardless of the coating?

Just a thought from someone who knows nothing about it. But I'd be interested to hear the rationale behind why my suggestion doesn't hold true.

Cheers,

Yes, it devalues it, but not as much as totally useless chipped and cracked lenses do

It saw a hard life after being brought home after WW2 - but it is still considerably rare enough that despite the issues, it is still worth as much as a second hand Takahashi FSQ-106... the VERY few excellent condition samples extant are worth around the $5-7K US range.

blink138 (Pat) · #7 14-03-2016, 05:22 PM

if it is a glass lens lewis a soft buff will take the coatings off easily and there is no way it is going to change the glass figure
if you insist on something more gentle then the cerium oxide is the go
some optical labs have used the acid bath in the past to remove multicoating however it can pok mark the lens surface and i do not know a lab that will do it any longer
pat

LewisM · #8 14-03-2016, 05:36 PM

Thanks Pat - I have a little cerium oxide left, will do a gentle rub and see how that goes.

Kunama · #9 14-03-2016, 05:43 PM

Lewis, I just took the damaged coatings off a pair of Bushnell binos using nothing but lens cleaner on a microfiber cloth combined with a decent quantity of elbow grease. Came up beautifully.

LewisM · #10 14-03-2016, 05:48 PM

Thanks Matt - will try it. Being Chinese coatings, they will probably come off by looking at them sternly

sheeny (Al) · #11 14-03-2016, 05:57 PM

Some time ago, I was given a 2" Meade EP which had been oversprayed with sunscreen. The sun screen ate into and destroyed the coating on the EP lens. I tried a few different things unsuccessfully, but in the end I tried the offender anyway, and used some sunscreen to remove the remaining coating. Optically it is fine but obviously the performance of the EP isn't what it once was.

Use this information at your own risk!

Al.

LewisM · #12 14-03-2016, 06:58 PM

Good info thanks!

Was also told toothpaste (the old variety, not the gels) will easily and safely remove them too, since they contain extremelly fine abrasive.

Stefan Buda · #13 14-03-2016, 07:43 PM

Before you rub it with anything, try some good sticky tape on it. It is an old method for testing the adherence of optical coatings. If all else fails, you can always send it down to me and I can whip up a polishing lap and do it with cerium oxide, without changing the radius or curvature.