49 year-old headlamp glass turned yellow

[Tomnistuff]

Here's a subject that I can't find on our site.  My 49 year-old European LHD headlamp glass has yellowed with time (Not French Yellow Headlamp Glass).  I did not know until I broke the lip where a couple of those infamous clips hold it to the frame.  I bought a new Bosch lens and noticed instantly that it is clearer than the original 49 year-old lens.  See the two side-by-side photos.  The color change is apparently internal to the glass and I suppose has occurred over time with heat, light and other types of internal and external radiation.  I say that because the yellowed lens in the photo has been through the following procedure:

1.  Machine washed in the dishwasher,
2.  Hand washed with OXYCLEAN ( I couldn't resist that loud-mouthed salesman on TV),
3.  Overnight soaked in 1 part white vinegar to 3 parts water solution,
4.  Overnight soaked in water with two denture cleaner tablets,
5.  Hand washed and lightly scrubbed using a thin paste of Sodium Bicarbonate and Water,

The last four attempts were applied only to the lower half of the lens in order to try to see any change - there was no noticeable change.

I have not tried CLR nor Oven Cleaner, but by the same token, I have not tried nuclear radiation either.

Does anyone have any experience with this phenomenon?

I suspect that the only action that will result in a completely clear lens is my writing a $200 check for another new Bosch Lens.

Tom Kizer

[scoot]

Interesting.  I have absolutely no idea.

[114015]

Does anyone have any experience with this phenomenon?

Yep, Tom,

Had the same effect too when I started to renovate my headlight units.

Compared the NOS (new old stock) lens (left) with the NNS (new new stock) lens (right) and found out the same.

My NOS lens is NOT 49 years old but was from the general Czech stock (Automotive Lighting) which had delivered the original Bosch and Mercedes headlight lenses until about 2-3 years ago. So, probably  6 - 10 years old or so.
Found this weird as well and couldn't explain this.
Guess they used more yellow color or dirtier sand for the old stock lenses ...

Found also out that the NNS lens was very slightly smaller than the NOS lens - barely visible.
In my case it was visible in that I had a little gap between headlight frame and lens, which was not there with the NOS lens.
Very annoying.
Probably only in my case ...

Just my 2 cents ....


Achim
(Euroheadlightcollector)

[tel76]

Another suggestion cut additional slots in the aluminium frame and fit your original lenses with new clips.
I have sent you some pictures of how overcame the problem of a chipped glass.

[Tomnistuff]

Another suggestion cut additional slots in the aluminium frame and fit your original lenses with new clips.
I have sent you some pictures of how overcame the problem of a chipped glass.

Thanks for all your replies.  That's interesting about the NOS glass vs the NNS glass, Achim.  I would not have thought that they would yellow so fast.

Tel76, regarding cutting new slots for the clips, that's actually what I did for the yellow lens in the photo.  It is the one that is broken, so I was not afraid to experiment with trying to cut new slots or to try make it clear again.  Even after all that work, I was only able to fit four clips instead of six and those were irregularly spaced and didn't retain the glass very well.  I decided to go with new glass for that headlight.  The other lens for the other headlight is fine except that I now realize that it is yellowed also.  Oh well.  It's only money and time.  For now, I'll probably reinstall one new lens and one yellow lens and change the yellow lens after the restoration is finished.

I guess I should check to see if they are the same size as well.

Tom Kizer

[Jack Jones]

Give bleach a try since it will not harm the glass. Just make sure you do not mix anything else with it especially any type of acid as it can be deadly.  8 oz of 5% household bleach in one gallon of straight water and let it soak for a couple of days.
No guarantees but it will not make it any worse.

Jack

[jpinet]

Tom,
just curious, but is this the lens I sold you?
If it does not work for you, I'll refund it.
Let me know.

I also had bits of broken glass where the clips hold and I simply ran a bead of silicone around the glass. Nothing moves and I'm sure that it's water tight.

[Tomnistuff]

Hi Jocelyn,

I'm not sure it's the one I bought from you, since the one I bought from you is exactly like the one that I dropped and broke.  It doesn't matter anyway, since I would never ask you to refund me the cost.  I broke the edge where the clips go in several places during my fumble-fingered clip installation learning process.  I now know that I should:

1.  install the clips loosely in the slots,
2.  hook it over the glass (probably with a little tape to protect the glass from the sharp edge of the clip),
3.  using a small screwdriver or a long tapered awl, stretch the frame side of the clip to engage the frame.

But, most importantly, I should first have verified that my frame and lens fit properly together by hand without clips.  In my case, I finally noticed that my frame was warped, probably from my, or a previous owner's heavy handed mishandling of the frame or sloppy installation of the clips.  My fender spot welds, or absence thereof on the driver's side tells me that the car experienced a fender bender before I bought it.  The headlight frame warpage resulted in my having to compress the assembly too hard and pull some of the clips too far to get them to engage, because there was a variable width gap between the rubber gasket and the glass.

Finally, I disassembled everything from the frame again and, using a straight edge, the glass lens and a flat surface, I hand straightened the frame a little at a time (at the risk of breaking the die cast pot metal frame).  Now the broken lens fits the frame like it was meant to and I should have no more problems installing the clips.

I also learned that it is important that the lens not slide around while installing the clips one at a time.  That's how I experienced at least one of the breaks.

Your sealant idea is excellent.  It will guarantee a seal; it can be applied to the properly fitted frame, gasket and lens and allowed to cure before installing the clips; and it will keep the lens from sliding around while compressing the rubber gasket to install the clips.  It is unlikely that the rubber gasket will ever be reusable in the future anyway.  I'll insert the sealant idea as step #1 above.

Thanks for the offer and the ideas, but in this case, I met the enemy and it was me.  I'll gladly take the ideas, but I refuse the generous offer.

Cheers,

Tom Kizer

[jpinet]

Tom, you are a gentleman.
I had the same kind of problems installing my lenses.
I also had to straighten the frame, gently!, and try to fit the glass properly. My frame was broken and I had to fix it with POR15 joint putty which dries unbelievably hard.
Once everything as straight as can be, and with new rubbers everywhere, there was still some waves in my frame where I could not get it to touch the glass. I figured that when the glass is attached to the frame, there is no longer any reason to take it off, (besides if it breaks, but that's another story) and so I drew a nice bead of black silicone around the glass edge and with soft edged clamps, I held all together for 48 hours. Looks perfect and one could even think that they are supposed to be made this way. Truth is, modern lenses are glued to their frames. Have fun with all of this! I know I do!

[scoot]

Give bleach a try since it will not harm the glass. Just make sure you do not mix anything else with it especially any type of acid as it can be deadly.

FWIW I think it is bleach + amonia cleaners = chlorine gas which is quite poisonous.  A general caution when mixing various household cleaners since both are common to to different cleaners.

[Jack Jones]

Scoot, you are also correct about ammonia but acid and sodium hypochlorite (bleach) also releases chlorine gas. Being in water treatment it is a very big taboo and you will usually not survive more than one breath. 

[66andBlue]

....  I say that because the yellowed lens in the photo has been through the following procedure:
1.  Machine washed in the dishwasher,
2.  Hand washed with OXYCLEAN ( I couldn't resist that loud-mouthed salesman on TV),
3.  Overnight soaked in 1 part white vinegar to 3 parts water solution,
4.  Overnight soaked in water with two denture cleaner tablets,
5.  Hand washed and lightly scrubbed using a thin paste of Sodium Bicarbonate and Water,
.....

Tom,
since these thorough procedures had no effect I submit that the lens has not "yellowed" but had this yellowish tint from the beginning. We just need to find an "unmolested" 50 year old NOS lens and compare!  

To produce coloring in finished glass is difficult and requires strong ionizing radiation (X- or gamma- rays) to produce color centers, even far UV light is not enough. To make perfectly clear glass requires very clean silica sand ("quartz"); the glass used for the old headlamps is usually borosilicate glass, similar to Pyrex here or Schottglas in Germany, made from a mixture of boron oxide, silica sand, soda and lime. Adding a tiny bit of another metal oxide, for example, iron produces a green color, cadmium and sulfur produce a yellow hue, and so on. (http://en.wikipedia.org/wiki/Glass_coloring_and_color_marking).  Keeping these out from all the ingredients requires extra preparation and costs money.
As your photo demonstrates the center of the old lens is clear and the yellow hue is only visible on the thicker sides where the lightpath length is longer. Thus the lens was perfectly fine for light produced by incandescent light bulbs.

[mdsalemi]

From some antique journals I've read, there is anecdotal evidence (that would be suggestions, not proof) that some glass does yellow over time. This is reported to be an affect of the elements arsenic and selenium in the glass with exposure to UV. I don't know if it's true or not but pure, clear glass is much harder to make than "tinted" glass which does appear more greenish (as opposed to yellow). See Alfred's note below, 100% true.

My friend worked once for the now spun-off Ford Glass, and the big joke was charging extra for "tinted" glass, but clear glass was standard. The former was much less costly to make, and the latter difficult.

But back to your OEM yellowed glass--there are a few "recipes" for attempting to clear it up, but my guess is even if these worked the only way you'll get the colors to match is with age-matching units.

[Tomnistuff]

Well, I've got some news, about my headlights, not about removing the yellow of the lens.

Today, I dug through my stored parts in the garage and retrieved the parts for the other headlight to compare the lens to the yellowed one and to the new one.  "Glory be", as my Alabama-born mother would have said.  The other lens looks more like the new one than it does the yellowed one.  It was jeweler's loop time!  The yellowed one (broken) is identical to the other used one (clear and good) and the new one (also clear and good), EXCEPT, the yellowed one is missing a set of numbers that the two clear ones have.

The numbers on the clear lenses, missing from the yellowed lens, are shown in the attached sketch.  I'm thinking that, since all the other markings on the three lenses are identical, perhaps the R7 stands for Revision 7 of the lens specification or design and maybe it has something to do with the "material" of the glass.

Anyway, I'm happy that my two remaining lenses are identical and clear, at least to the naked eye.  I don't have to buy another new lens.  And this afternoon, I cleaned the parts and put the headlamps back together.  They are ready to install as soon as I get the front harness installed.  In the second photo, the new lens is on the right, the old good lens is on the left.  The old lens is not perfectly clear but it's much, much clearer than the yellowed broken one.  Another Pagoda knowledge base item to remember.

Tom Kizer

[jpinet]

I'm positive that you are VERY careful when moving these lights about your garage!