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Headlight lenses changed by time and sun


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I've heard that the manganese in the glass reacts over long exposure to sunlight.  Strictly as an unscientific observation, lenses most affected seem to be the heavy fluted Bausch & Lomb lenses such as you have on your Cadillac and I have on my 1925 Pierce.

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A web search turned up some interesting information. It seems a lot of glass contains a small amount of iron which gives it a green tint. Glassmakers neutralized the iron by adding manganese in the form of glass maker's soap. The result was perfectly clear glass.


But over time, especially when exposed to ultraviolet light the glass can turn purple due to a further chemical change. This explanation comes from the Corning  Museum of Glass website.


Many glassmakers through the centuries have attempted to produce clear, colorless glass. Impurities, especially iron oxide, in the batch ingredients that were melted to make the glass often resulted in glass that was greenish instead of the desired "water clear."

An interesting characteristic of colorless glasses which contain manganese dioxide as a decolorizer is their tendency to turn different shades of purple when exposed to the rays of the sun or to other ultra-violet sources. It is a photochemical phenomenon that is not yet perfectly understood. It is generally accepted that the ultra-violet light initiates an electron exchange between the manganese and iron ions. This changes the manganese compound into a form that causes the glass to turn purple.

It was in the mid 19th century that manganese dioxide, popularly called "glassmaker's soap," began to be used by American glass manufacturers as a decolorizer. By including a small amount of this ingredient in the melt, they could produce glass that appeared virtually colorless. An 1899 publication by Benjamin Biser remarked,
"The especial use of manganese in glass is to mask or neutralize the greenish color imparted to the glass by the protoxide of iron. Manganese imparts to glass a pink or red tint, which being complementary to green, neutralizes the color and permits the glass to transmit white light. Pellat refuted this theory, and claimed that the green tint of iron was not neutralized by the pink of manganese, and thus subduing it; but by the iron taking another charge of oxygen from the manganese and becoming per-oxide of iron, and producing a reddish yellow tint, while the protoxide produces a green tint."

Glass scientists today generally agree with Apsley Pellat, explaining that an ion exchange between the iron and the manganese molecules changes the observed color of the glass.

This process is sometimes reversible by gently heating the glass to about 200°C.

In the early 20th century, changes in manufacturing processes, as well as more pure batch materials, dictated different ways to decolorize glass, and the use of manganese oxide for this purpose dwindled.

Edited by Rusty_OToole (see edit history)
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Thank you, especially, Rusty. Fascinating!


Another aspect of the history involved in our antique cars. Understanding gives me an even deeper appreciation of our "Time Machines."


Often I sit in one of our antique cars. Looking through a windshield where people have sat for tens, scores and even a hundred years, smelling the "old car smell" of leather, faint tobacco;  better knowing the Past custodians who lived in a different world. I feel the honor of being a present custodian of these vehicles that were such important to people who rode in them. A brochure about our 1920 Cadillac remarking the "side flaps inside the doors where ladies could store their hats, veils and party attire..."


Sometimes I sit in our 1914 Model T. THE car that put the average American on motorized wheels. The roads were non-existent in many areas. People were able to speed along up to 30mph in a good flat road, but usually much slower. 


My wife and I take back roads in our antique cars like our cars have done by their previous owners. We can smell the scents. See every blade of grass. Wave to all the people sharing the rich history of the past, and each other. Stop and talk with people when we stop for gas, for food, just to stop and just enjoy each other. Wave and share thumbs ups. 

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It can be cyanide reacting with small traces of iron in the original glass manufacture. I used to do some service and inspections in the old, abandoned Bausch and Lomb glass plant. It was a very secure building with special access and a call in-sign in sheet, largely due to the amount of trace cyanide from glass manufacture. I had a pair of shoes I left there to wear in the closed area. I didn't want to track it into my car.


Henry Ford made significant changes to glass manufacture in the 1920's by his technology policy. When he began to manufacture his own glass he avoided the European experienced tradesmen in favor of smart, young people, capable but not experienced. He thought that experience brought, with it, a knowledge of know limitations and an attitude of "that's the way it is done". The process he developed charged much of the industry.


I used to have a block of clear scientific grade glass around here, but haven't seen it in a while. It was about 80 years old and flawless.


Back to the question, cyanide and iron are probably reacting.


Now I am remembering the 6" copper air line that ran through that glass plant and how deadly the pyramid of dust on the top looked. I used to think of the unsuspecting scappers breaking in a stealing it.


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6 hours ago, Bill Caddyshack said:

We can smell the scents.

New York RT 62 from Radolph to Gowanda in a 1960 Buick flat top with all the windows open on a hot summer day. People waving and smiling from their yards and clotheslines. Always slowing for the horse drawn buggies. First thought when I read "scent".

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I can assure you that it is not age, but sunlight.     When I was rebuilding my first car, a 1927 314B Cadillac 4 passenger phaeton,  the local engine reconditioner pointed me towards  Henry Formby, who was driving his 1923 Cadillac with a pair of incorrect headlights.  Henry had one correct headlight with the lens darkened as your photos show.   Well when I was in Melbourne at university, I used to walk everywhere, and  one day I went into a city new car dealer for GM cars, and Cadillac was cited in the old signage,  though I doubt they handled any for a long time.  No; they had no old stock parts.  But they gave me the phone number of a retired gent, Mr Johnson, who had been the Cadillac agent long ago.   When I phoned and indicated my interest,  He told me to hold the line and wait till he came back. He had just put out all the new old stock parts for the municipal corporation to remove as "hard rubbish"; and while the phone was silent, he was bringing them all back inside.  It all cost me 30 pounds in 1961.  It included a single brand new1923 headlight with unblemished black enamel, and a pure, clear, un-darkened Bausch and Lomb lens.   So I gave Henry the new headlight so he had a set on his car.  ( This is the way we worked before swap meets, and people became mercenary;  and always since  I have had most satisfaction through helping other people who need, deserve, and appreciate;  and are prepared to help me or other people  when possible.)     Well, there was some other good stuff there too.  There was one bank of new 30 thou oversize V16 pistons with rings and pins in original boxes.  There was no longer a V16 Cadillac in Australia, so I gave them to Bud Catlett in April 1970 when two Harrah cars were out here for the FIVA Sydney to Melbourne run.  I had incorrect headlights on my 1918 Mercer on that event;  And Bud took the original headlights off one of Harrah's Mercers, and got Vic Billstrom to make a correct pair for me, which were beautifully nickel plated, complete with the left & right hand threaded sleeves to attach them to the mounting posts.    Henry has been gone since 1981,  but his twin grandsons Scott and Craig Emmerson share the car which is in a finer condition than Henry could ever manage and afford.  Next time I see the car I will take note whether the lenses have equalised in colour.    Incidentally, I have a Bausch & Lomb 2 axis digital readout on my milling machine, but it is not as reliable as their old headlights.  I have to make a datum point I can re-set to;  because it un-predictably gets a blank look on its face and loses its memory.  That is a lot of nuisance when I might be machining new castings for a Stutz dual throat carby.   Maybe I shall replace it with an honourable oriental digital position readout,   which hopefully does not have that bad habit.

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  • 3 weeks later...

How beautiful and elegant! I was noticing this over the summer on I believe a Pierce Arrow and thought it was supplied that way, kinda like how Lexus made green/blue-hued lenses to stand apart about a decade ago.


I wonder if this will become a desirable trait, much like how sun-faded Rolex and IWC watch dials are more sought-after by a subset of collectors over their pristine counterparts. 

Edited by MarrsCars (see edit history)
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I found the same issue on the cowl mounted parking lamp lenses that were on my Studebaker. I had no idea why they would have a purple tint.  I ended up swapping them out for some spares that were clear but kept the tinted ones. I'll have to see if the replacements change over time. This has been a very interesting and informative post. Thanks to all for sharing.


Edited by Stude Light (see edit history)
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