delamination

[paulhofseth]

I notice that the canadabalsam of the Septon is mentioned as giving up its grip (in the thread on this lens) I have noticed the same on the earlier Nokton 1,5 and have wondered about the reasons; is it due to very different thermal expansion rates of the cemented glasses. Hence these and others with similar composition should be kept well away from rapid changes of temperature.

p.

[Bosun Higgs]

I suspect that thermal expansion of differential glass types does play a part, and also possibly the shape of the mating surfaces if not plano/plano.

I can fully attest that rapid temperature change can certainly cause element separation, I have seen it happen.

A while ago I was cleaning the elements of 4:2 Petzval lens that had fungus growth, this involved immersing the elements in the usual chemical bath and then rinsing in running water.

As I was holding the front group in the water stream I saw Newton's Rings suddenly appear spreading from the centre of the lens. Then there was a really rapid "flash" and a silvery area of separation appeared in a crescent around the element centre.

I have fixed many lenses with cemented element separation, but I have never actually seen the separation occur before my very eyes before.

Then I realised what had happened. It was winter here at the time and genius here took a large cemented group at 20C and ran it under water at 2C. The rapid dissimilar contraction of the glass types and the element shapes did the rest.

Even more annoying was that the element was glued using synthetic cement, not Canada Balsam, so getting it fully unstuck was a much more lengthy and hazardous process.

Lesson learned, I always check the rinse water is at room temp now.

[BrianSVP]

Certain cement formulations like that used on the Septon have more problems with delamination - I think it has much more to do with the particular cement formulation than with the curvature of the two mating surfaces. The affected doublet (rearmost in front cell) doesn't have a particularly radical curvature that would cause more stress than those used by its contemporaries:



An illustrative example of how different cements can lead to increased separation can be found with the many 55mm f/1.4 "Tomioka" lenses that all use the same optical blanks but different coatings/cements.

Certain variants (e.g. the Chinons and Cosinons/Revuenons) nearly always have some degree of separation, while others (Rikenons, Mamiyas) almost never do. Even more interesting, both brands use Canada balsam, but the separation manifests completely differently in the two brands - Chinons tend toward subtle delamination, whereas the Cosinons/Revuenons tend to degrade much more radically into a yellow, flaky mess.

If you use heating to separate doublets with synthetic cements, the "instant delamination" phenomenon can commonly be seen. On occasions where I've resorted to using the method of placing the doublet on an incandescent bulb, I specifically have watched for this to occur and pull the lens off the instant I see see the rings form. If you can grab the doublet with a cloth to protect your hands from the heat, you can usually separate the elements much more easily at this point than if you let the group cool down, as the cement will begin to partially adhere again as it cools. It usually leaves the cement clinging to one side, and it can be scraped off with a clean fingernail or soft tool, followed by light polishing with cerium oxide to remove any lingering residue.

I of course wouldn't use this method with any very high value/rare lens or one that has any obvious flaws, since it can lead to cracking of the elements, but now that methylene chloride has become basically impossible to get as a consumer here in the U.S., I've had to resort to it more.

It's also particularly important not to run any element made of fluorite/ED glass under cold water, since they are more brittle than typical crown/flint glass, and the rapid contraction from cold is known to actually crack elements.

The separation on the Septon is particularly unfortunate, since the doublet is swaged into its housing, making it very difficult to remove/repair if necessary.

[jamaeolus]

Interesting tip Brian thanks.

[Bosun Higgs]

Your incandescent bulb method must generate some pretty high temperatures!

I tried to separate two synthetically bonded doublets with heat alone, an old Ross and a much more modern ISCO. I used an oven, as I do when I separate Canada Balsam doublets, but the synthetics would not budge. The oven topped out at 240C, so I suspect your bulb method must be going a lot higher than that.

I mainly convert projection lenses and the doublets in these are usually the front groups from Petzvals, these are generally large, the Ross for example was 60mm diameter, so this would be difficult to manage with a bulb.

Although I routinely use methylene chloride now, I still heat/cool the doublets before immersion as I have found it shortens the separation time. I suspect this degrades the cement and makes it more porous, this helps as there is only a tiny seam between the elements for the MC to attack. One unheated doublet took a month to part, but so far the heated ones have not taken more than about a week.

[jamaeolus]

Be sure to use adequate PPE and ventilation with methylene chloride. That stuff is a potent carcinogen. And just toxic as hell.

[Bosun Higgs]

[stevemark]

[Bosun Higgs]

Yeah, when I was working we used chemicals that were so nasty you had to get a special Home Office licence just to import them into the country.

My favourites were the stuff we had synthesised to order, I always remember the big warning stickers on the packages "WARNING: the toxicological and teratogenic properties of this substance are untested and unknown".

Then there was the time I decided to defrost an old freezer in the unit and found a full cannister of nerve agent - like 250ml of it.

The entire area was evacuated and the army NCB disposal team came out to deal with it.

Ah, Happy Days!