Author Topic: Color of Dumet Seal in Filament LEDs  (Read 1186 times)
Multisubject
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Color of Dumet Seal in Filament LEDs « on: July 10, 2025, 06:45:02 PM » Author: Multisubject
Normally in a soft glass pinch seal, the copper-coated Dumet wire feeding through the glass is first oxidized and then sealed, producing the classic red-colored seal that we all know and love.

However, in (I think) every single filament LED lamp I have ever seen, the feed through is bright copper colored, meaning the bare copper layer of the Dumet is making the seal to the glass. This means it is either not oxidized before sealing, or maybe it just got so hot that the oxide went away before the seal could be made.

Why is this? Isn't an oxide bonding layer needed for maximum reliability? I would think this is extra important considering most of these lamps are filled with helium, which is notorious for slipping through the tiniest of crevices.

Thanks!
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RRK
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Re: Color of Dumet Seal in Filament LEDs « Reply #1 on: July 11, 2025, 12:43:43 AM » Author: RRK
Cheaper lamps are probably not He or oxygen filled, so they probably cut some corners as there is no more a need for seal quality until it really starts to crack...

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Medved
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Re: Color of Dumet Seal in Filament LEDs « Reply #2 on: July 11, 2025, 03:44:56 AM » Author: Medved
Wonder if mainly the cheapest aren't filled by Hydrogen for cooling? Then maybe the seal quality does not have to be that perfect...
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James
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Re: Color of Dumet Seal in Filament LEDs « Reply #3 on: July 13, 2025, 02:25:54 PM » Author: James
This is a very good observation and question.  I do not know the exact answer but have a theory.

In the Chinese lamps I have analysed their seals still make use of the same 42% niron core with copper sheath and top coating of sodium borate.  The glass types made in China do however tend to be slightly different than the long-established compositions produced by the Western lampmakers, during the era when pretty much each lamp company also made its own glass.  That era has almost finished, and Chinese lamps seem to be made with standard commercial glass types rather than the unique compositions developed by the lampmakers for optimum compatibility with their high speed production machinery.  One of the main differences of Chinese glasses is their elimination of heavy metals such as lead and barium oxides, for environmental reasons.  That results in slightly harder glasses with higher softening and annealing temperatures.

When a Dumet seal is first made, the colour is of black copper oxide.  During prolonged heating of the seal-making process that gradually dissolves into the glass, followed by the red coating of sodium borate, and if heating is maintained long enough (or hot enough), that will eventually also disappear into the glass to leave a brilliant metallic copper interface.  According to old GE and Philips lamp training manuals, the optimum seals are made just before the last of the red borate dissolves into the glass.  However, working so close to that limit makes the manufacturing process controls very critical.  Too much heating, and the complete loss of an oxide interface between the metal and the wire results in increased risk of leakage.  Especially since the Western companies typically ran their production on high speed lampmaking machines, it was too difficult and critical to make seals in which the oxide coating had been dissolved as much as would be ideal.  The risk was too high that some seals may become overheated, which then very quickly begins to impair seal quality and increases the risk of gas leakage.  Hence the compromise was to aim for a salmon colour seal, in which some red borate was still present but the brassy colour metal just began to be visible.

In China, due to low labour costs there was so far no need to invest in high speed machinery : the high capital investment could not be recovered by reduced production costs.  So the typical Chinese lampmaking lines run very slowly.  With such a process it's easier to better regulate the heating of the glass, and dare to keep the seals hotter for longer.  The slightly higher working temperature of their glass also means that during the annealing phase, there is naturally more dissolution of the borate into the glass.  I believe these two factors, when combined, account for the fact that Chinese dumet seals show significantly different colours than in the West.
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Multisubject
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Re: Color of Dumet Seal in Filament LEDs « Reply #4 on: July 13, 2025, 06:44:04 PM » Author: Multisubject
@James

Wow, that is a lot of good info! I think that is very likely the case, since there is still some extent of red color on the edges of the seal. Very very interesting, thanks!
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