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Purple Glass Bowls On Coughtrie Bulkheads?

Posted: Wed Apr 18, 2012 2:56 pm
by Tom
I hope Colin or someone else may know this. Can anyone anyone tell me the reason behind the apparent purple tinted glass bowls seen on some of the larger Coughtrie GLS bulkhead luminaires? Is it possibly to give the GLS a cooler colour appearance?

Re: Purple Glass Bowls On Coughtrie Bulkheads?

Posted: Wed Apr 18, 2012 7:51 pm
by sailormoon01uk
Hi Tom

Good Question, possibly it' was a UV filter to reduce the yellowing of the Bowls, or possibly it's the type of Plastic used and they go purple with sunlight, like Polycarbonate goes yellow, Acrylic goes milky white etc

All the Best

Colin

Re: Purple Glass Bowls On Coughtrie Bulkheads?

Posted: Wed May 16, 2012 9:05 am
by Dave
Ive seen this on quite a lot on the old Coughtrie bulkheads, where the shades have gone this purply blue. I think it may be due to age. I know of two identical Coughtrie bulkheads, one in the open, subject to direct sun, and the other under a canopy. The one under the canopy is still white opal, however the other one has gone the purply blue. I think it may be down to the type of plastic Coughtrie used. This is now polycarbonate, but i have no idea what this weird plastic is that turns purple.

Re: Purple Glass Bowls On Coughtrie Bulkheads?

Posted: Thu Jun 28, 2012 12:07 am
by Ash
And does it remain structurally intact ? (not cracking like PC does ?)

I seen here only the 80's CPS in incandescent, PL and 2D versions, and among them all i seen were with perfectly clean covers

Re: Purple Glass Bowls On Coughtrie Bulkheads?

Posted: Thu Jun 28, 2012 9:59 am
by Dave
I'm not entirely sure. looking at the ones i've seen (and actually bending the bowl) it does seem to hold its structure well.

Re: Purple Glass Bowls On Coughtrie Bulkheads?

Posted: Sat Sep 21, 2013 9:08 pm
by James
Are you sure that the glass doesn't have this tint also when it is new? It would be unusual for a glass to turn a purplish colour as it ages, normally they become instead more cloudy, or slightly yellowish or blueish if the colour does change.

Possibly it was intentionally made that way, as were the stems of many British-made incandescent lamps during the early 1980s. The reason was simply to differentiate a new glass composition which was introduced at the time. Previously the UK made much of its glass with a composition of 30% lead oxide. The lead content is essential for the manufacturing process but also environmentally unfavourable, and for that reason it is better to use as little as possible. In the early 1980s it became possible to work with glass having a lower content of 20% lead oxide, however to handle the material, manufacturers needed to adjust their fire settings on the machines which formed the glass parts because its softening temperature was a little different. To help visually identify the new 20% lead glass and prevent it from being accidentally sent down a production line with the fires still set up for 30% lead, it was decided to give the new glass a feint purple colouration. After a few years, once all old stocks of the 30% lead glass had been used up and there was no further risk of confusion, the colouration was dropped.

This can quite often be seen on the stems of British GLS lamps made around 1982-84. Since the joint Thorn-GEC glassworks which produced much of the country's lead glass also made the glass bowls for many other luminaire companies, it is conceivable that Coughtrie was also sourcing its glass from here and the purple coloured bowls are for the reason explained above.

Re: Purple Glass Bowls On Coughtrie Bulkheads?

Posted: Tue May 06, 2014 11:49 am
by Fairlane500skyliner
Are the bowls actual glass? It may be a case of the colour shifting to "Sun-coloured amethyst" (SCA), due to UV exposure. It's caused by the presence of a certain chemical in the glass that was originally used to make it clear, but turns purple/amethyst with UV exposure. I think it was called "Manganese" or something... There are a lot of SCA glass telephone insulators about, and are quite attractive pieces! I've seen SCA lamp covers too, though they seem few and far between.

The links below both have some good information about SCA. Scroll down to the heading "Are all altered colors easily recognizable? " on the NIA webpage.

http://www.antique-bottles.net/forum/Sun-Colored-Amethyst-questions-m115952.aspx

http://www.nia.org/altered/


Hope this helps.

Chris