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Acrylic replica 4th Order Fresnel for Pottawatomie Light

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Postby Optics » Tue Apr 19, 2005 6:34 am


Dan also has the capability of making replicas in glass, although they are much more expensive.

Have you ever noticed that when you take a photo of a lens that it looks slightly orange or tan. Some of that is due to lighting and film, but some is in the glass itself. The glass reacted to ultra-violet as Terry said. It started out very clear with only a slight tinge of yellow-green, older lenses (pre-1880) were more green. But, after many years in the sun all lenses turned a slight orange color due to the chemical composition of the glass. Also on cheap pressed glass buoy lenses you will often note a decided purple color. That was because manganese was added to the glass which reacts with ultra-violet to produce purple.
Last edited by Optics on Wed Apr 20, 2005 5:39 am, edited 1 time in total.
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Postby mikev » Tue Apr 19, 2005 12:32 pm


Brent, don't ever, EVER put silicone near a Fresnel unless some misguided soul has put some there before. Silicone bonds with the glass at a molecular level, not only altering the glass but making it impossible to use anything but silicone ever after.

Litharge does have a chemical lifespan, and at the ALCC we worry a lot about the possible catastrophic failure of Fresnels even though most seem to be holding on well past the 65 years or so you can count on. My guess is a lot of prisms are being held in place by the wood shims, behind powedering litharge (a lead hazard, by the way).

The putty of choice these days seems to be DAP, a vinyl compound. But I'm no expert. There are only four lampists in the country I'd unquestioningly trust with my lens, and one of them just joined this group.

In the meantime, the best rule of thumb always is: Do No Harm, and call an expert.

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Postby Pharoslvr » Tue Apr 19, 2005 1:24 pm


It seems that wood shims would be unstable because of loss of moisture content followed by shrinkage, in an invironment like that...plus the close tolerances. ('Just wondering aloud)

DAP is a great compound to work with but have to admit that I am surprised that something isn't used with a little more lifespan. (again, just wondering aloud)

Thanks for the information, especially the note on silicone!
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Postby Weasel58 » Tue Apr 19, 2005 2:38 pm


It seems funny, Brent, but I keep answering your questions. Yes they use wooden shims to align the prisms when putting a lens together or in the case of the pictures I have, back together. About 2 years ago I got to visit the Ponce Inlet Lens shop while they were rebuliding the original First Order lens. If you check the link to the Harbour Lights collectors forums below, scroll down about halfway and there is a picture of the prisms being held in by little wooden pegs.

http://www.lighthousekeepers.com/cgi-bi ... 647#000003
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Postby beachbum1616 » Tue Apr 19, 2005 2:53 pm


Nice close up shots!

Now, it that the 1st order lens from the Cape Canaveral Light?
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Postby Hersh » Tue Apr 19, 2005 7:04 pm


Great shots, it's a rare treat to be so up close and personal with a first order.... Someday I'll have my shot, but until then you guys are the next best thing. :D
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Postby Terry_Pepper » Tue Apr 19, 2005 8:02 pm


Brent, your comment on wood shims being unstable due to loss of moisture content followed by shrinkage is a good one.

From what I understand, the material from which shims are made is particularly critical. Using a material that is too hard would increase the likelihood of chipping the glass when inserting the shims. Using a material that was too soft would prevent adequate securing of the prisms in the frame after the alignment process.

I believe that this was the reason that wood was chosen, as it is a good compromise from both perspectives, and was readily available, and easy to modify either in the lamp shop or in the field by the Lampist.

I further understand that this is the specific reason that shims are made of end-grain walnut, as its' grain characteristics make it less susceptible to expansion and contraction than most other woods.
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Postby Pharoslvr » Wed Apr 20, 2005 4:31 am


Thanks Terry, Eric, Tom, and everyone else who has contributed to this tread. It has (and is) been quite an educational experience reading your various comments.
Having been a structural-designer many years ago (buildings) I have a interest on how things are designed and assembled, especially on how it relates to strength-of-materials and stresses.
I have an abiding respect for the challenges that the lampists of old (as well as of today) have to face on a continual basis.
'Amazing work that they do....but I am sure it is a labor of love! It has to be... :)

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Postby Weasel58 » Wed Apr 20, 2005 5:12 am


Stepehen, those pictures are of the Ponce Inlet first order lens. The Cape Canaveral lens can just barely be seen in the edge of the first 2 pictures on the May 26th post.
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Postby Pharoslvr » Wed Apr 20, 2005 5:09 pm


Hopefully someone can answer this question.
Are these acrylic prisms initially...
Cast?
Laser Cut?
Machine Cut?

Thanks,
Brent
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Postby Weasel58 » Wed Apr 20, 2005 5:28 pm


The video Dan showed at the Florida Lighthouse Associtation meeting, showed the prisms being lathed, out of a round block of acrylic. I wish he had that video on his website.
Afterward, the lens parts are polished and in cases of repairwork, tinted to match the remaining original parts of the lens.
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Postby Pharoslvr » Wed Apr 20, 2005 5:42 pm


Damn, that was a fast response! :)

Thanks, Eric
Brent
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Postby Terry_Pepper » Wed May 11, 2005 7:18 pm


I received an email from Tim Sweeet with FORI - Friends of Rock Island tonight.

In his email, Tim indicated that acrylic Fourth Order lens from Artworks Florida was transported to Rock Island on Tuesday May 10, and installed in the lantern that same .

Here are a few images chronicling the installation - if the lens were any larger there would be no way it would have fit through that door in the lantern floor!

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Thanks to Tim for sharing the images.
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Postby Jen » Wed May 11, 2005 8:33 pm


Thanks for the update and for sharing the pix, Terry. I was at the Door County Festival last year and had the opportunity to climb Pottawatomie as well as meet Tim Sweet during the dedication and restoration completed ceremony. That is indeed a very tight fit into that lantern room!
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Postby Terry_Pepper » Wed May 11, 2005 8:37 pm


Jen - Tell me about it. I made it through that scuttle back in 2000 - and I weigh 200 lbs!
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