Quick Question RE: Cleaning and Old Brass Lens

[Punkinhed7]

A number of years ago I stumbled across this large Darlot magic lantern petzval at an antique store and, of course, quickly added it to my collection. The problem was I didn't have a camera of sufficient weight or with a large enough lens board to accommodate it's sizable flange. Fast forward to this week and I've finally got it mounted on a board and ready to go but the intervening years (both since I acquired it and it's original manufacture) have left a rather thick layer of dust/grime on the elements -- that's tough enough to withstand a blower and camel hair brush.

I was wondering if anyone might be able to recommend a good method of cleaning the elements reliably and safely. I was thinking of just disassembling the lens and using water/my fingers on the elements to spiff it up. Is there any danger to this and, if so, what might be a better method? I was particularly concerned that this might somehow affect the integrity/seal of the cemented doublet pair. Thanks so much for the help!

[Fotoguy20d]

Water is fine. Windex in moderation, just try to keep it off the balsam. Don't drop the glass into your sink (don't ask).

Dan

[Steven Tribe]

Canada Balsam is basically insoluble in water, and pretty resistant to organic solvents too. If there is ink/pencil "Darlot" identification written on the GG edges of the lenses, these are quite resistant as well.

Don't be alarmed if the front achromat has developed a yellowish hue due to aged balsam - the clarity (or overwise!) is very obvious when this is removed from the front cell. You might want to check the allignment of the rear pair at the same time (see CCHarrison's website on Pretzval's design). These lenses have been cleaned before and it is very common that someone has mounted the pair incorrectly.

[Fotoguy20d]

As Steven mentioned, pencil is even resistant to acetone.

[goamules]

I use lens cleaning tissues most of the time, rather than soapy water (unless there's decades of crud). I place a drop of cleaning fluid in the center of the lens, lay a tissue in it, then rotate the LENS around the tissue, from inner to outer parts. Then repeat 4-5 times using a clean tissue each time. Let the surface tension be the only pressure on the tissue, don't press down until you're sure everything is off. Then one time gently pressing, will get any stubborn haze off.

[Punkinhed7]

Thanks for all the advice guys. One more question on the topic of the balsam...and I'm pretty sure I won't like the answer. It appears that there is some separation occurring between the front element pair, just off of center: how much is this going to wreck the image quality and is there any (reasonable) way of fixing it?

[goamules]

If it's just a small spot, it shouldn't cause any problems. The only way to fix it is to completely separate and reglue. I've never done that, but others have.

[Steven Tribe]

Balsam joining of two lenses was never meant to last a hundred years!
As Garrett wrote, it takes quite a lot of breakdown before the lens becomes useless. I would guess that well over 50% of Petzval achromats have edge separation (over about 2mm) or restricted problems like yours. I can see from a number of mine that have lens edge writing ( a very French activity!), that the fading pencil or india ink are due to previous rebalsaming in the 20th century.

Balsam life is finite , but like light-bulbs, there is a difference in their longevity for (apparently) identical lens designs!

Darlot is an easy achromat to get out of the cell - Dallmeyers are very difficult.
Although there are small risks with the separation and re-glueing process, it takes just a few hours (plus a waiting period up to a month for the solvent to penetrate into the centre).