View Full Version : Fixing the light trap in a 100 year old 12x20 Korona plate holder

31-Aug-2017, 04:54
The light traps in my Korona 12x20 plate holder have failed. The holder is very old, so I guess it was inevitable.

I took the plate holder to a local woodworker who said he was willing to give it a crack.
I explained to him what I thought needed to be done, having never seen the inside of a plate or film holder before, and he very very gently unscrewed and pried the ancient glued wooden artefact apart. He worked so gently, working against the old glue and nails buried in the wood. He managed to get the face off the plate holder and we were met with this:


I wasn't expecting two strips of felt but it made sense as soon as I saw it.

We looked closer:


Whatever the metal is, tin, is so thin, about as thin as aluminium foil, and it is folded over in half. The felt is glued to the top of this strip and the corner of the tin is 'crimped' over the outside edge of the felt, I assume to prevent the dark slide from catching on it when it is being inserted.


The tin has corroded and lost it's reflex along the edges. In some parts it's more or less rusted away entirely.


Also, the wooden groove that this strip of tin slits in has a very very slight overhanging lip over it that the 'crimped' part of the tin sits under.

The woodworker was very impressed with the ingenuity of the design and the craftsmanship that has gone into a very simple concept. He was really excited about the age and seeing the marks from the tools used to make it.

Anyway, so we're at the point now where we realised that the thin tin needs to be replaced, and the felt too. The material needs to stand up to silver nitrate, which is corrosive to organic material. We're thinking about what could be used. Triple folded leather? Rubber sponge with velcro or a similar material glued over the top of it? The new darkslide I have for this palte holder is made out of .6mm colourbond steel. Any of you guys have any ideas as to what we could use to create a springy light trap in this little slot? We've come this far so total repair is entirely possible.

Any help would be greatly aprpecited from this point.

Steven Tribe
31-Aug-2017, 11:56
I think that replacement with commercial velvet ribbons of good quality is a better solution than trying to find a felt quality which is compressible. This material was used, not just in light traps on camera backs/slides, but also between lens barrels and lens sleeves. They were always glued into slightly sunken channels in the wood or brass, just like the ones you have, to ensure that remove the chance of insertion "lifting" the velvet. This really only important with first time insertion, until the fibers have been bent over. I think the metal edging is only important if the dark slide is completely out, which would be a challenge for the light trap.

I don't think you can make a system which is not going to be eventually attacked by silver nitrate by using modern materials! I think it would be better to invest in a reliable (quality stainless screws) mounting of the timber piece, so that you could do a regular check and repair of the light seals every few years or so. Otherwise the screws will fail in the worn wood.

Looking at the plate holder again, I think the depth of the "felt" is greater than the usual velvet. One solution might be to glue velvet onto a metal strip which fits exactly into the groove dimension. The thickness of the metal strip is selected to provide just enough space up to the dark slide, but still providing enough compression to be light tight. When the right combination is found, then the metal strip is glued into the recess.

Jim Noel
31-Aug-2017, 13:09
I don't have the same holder, but the metal in my old plate holder appears to be bronze

31-Aug-2017, 14:03
Replacement of the velvet is logical, but the issue would be the spring material underneath, as that provides the tension that pushes the velvet up to seal the darkslide... Since this will be used with wet plate, the metals used will corrode, (and fail) so replacement would be vital...

The good news is that we live in the 21st century, and have many more materials now that didn't exist then, so maybe a different approach is possible...

A (blue sky) possibility would be some form of closed cell neoprene rubber strip that would be laid in the groove as a "spring", and fabric coated or pushing felt, or maybe the fuzzy velcro strip to replace the velvet... As long as the "spring" could compress to fit, and expand back out without hesitation when the slide is pulled, and there is a proper "ramp" shape so the slide can be inserted/pulled and not distort/roll/bind while in operation...

I have made replacements for lighttraps in some dead, odd holders using neoprene/velcro that worked, but sometimes there is a couple of pinholes of light that can filter through the velcro, but for slow materials (like wet plate), or if the dark cloth is over the camera back while operating the holder, it worked fine...

Go to a HVAC supplier and see what neoprene sealing materials they might have, or even a wet suit maker (or chop up an old wet suit) for strips... That might work alone, or it can be used as the "spring"...

Good luck, and happy hunting!!!

Steve K

31-Aug-2017, 14:26
Since silver is well above every metal except gold in the electrochemical series, it will displace the others and they will be lost to corrosion products (nitrate, oxide, etc.)
The metal you found may be tin, now largely replaced by silver, but it is also possible that it was originally just silver foil. Impure silver nitrate could conceivably have had traces of nitric acid in it, which would have slowly attacked the metal.

Given the age, rarity and potential serviceability of the holder, it might not be so outlandish to look for some silver foil and use wooden forming jigs to reproduce the original traps. Anode and filter bags for electroplating are available in polyester (Dacron) and polypropylene, and these stand up to just about everything except concentrated sulfuric acid. It may be possible to find black felt or velvet in one of these materials, in which case you would have a combination that should hold up as well as or better than the wood making up the rest of the holder.

I have no idea of the range of sources for silver foil, but Rio Grande (www.riogrande.com) would be a reasonable place to start. It looks as though enough for a few holders would be less than $100 if I read their catalog correctly.

Michael Roberts
31-Aug-2017, 14:38

[Thanks to Erik Larsen]

31-Aug-2017, 16:44
Hasselblad slide light traps are a piece of folded black mylar, instead of your tin, and a piece of sponge instead of the felt. The mylar is folded into a lopsided V, with the short leg on top. The sponge is the width of the longer leg, trapped in the bottom of the V with half hanging out to make the trap. The whole thing sits in a shallow grove so that the mylar won't move and one edge of the sponge is gripped in the bottom of the V. It doesn't use any adhesive, and is a bit of a trick to hold all together while reassembling the holder.

You can probably find a video on youtube, and this might work for larger holders.

Yup: relevant section starts at 6:35-- https://www.youtube.com/watch?v=HjjxmYltCa8

31-Aug-2017, 18:10
The strips that Mike Roberts suggested are called fingerstock gaskets. Parker Chromerics (https://www.chomerics.com/products/emi/gaskets/fingerstock/index.html), amongst others, sell stainless steel strips.

Years ago, I wanted to use them to repair a light trap and gave up after not finding an appropriate size, nor being dexterous enough to wrap felt around them. I just rolled up some felt and glued it into the groove.


1-Sep-2017, 23:03
Thanks for the advice all. I found some of this at a hardware store. I think the rounded edge will do well not to get too rippes up by the dark slide i will report back. https://uploads.tapatalk-cdn.com/20170902/d0eba5993b3cadabf90f5feba1e4661a.jpg

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