| In fact, the reason they were called etches is really because the ink was physically etched into the glass surface. Unfortunately though the process is quite critical, and very often not done well.
The ink itself is a highly complex material uniquely prepared by the lamp industry, via an elegant chemical process that takes about 2 weeks to complete. It basically involves dissolving the oxides of silver, lead and sometimes also copper into an organic syrup, laced with boric acid, which has the most wonderful smell you never forget after you have been in any lamp plant. The ink is applied to the glass via conventional rubber stamping methods, and then burned in with a gas flame. That is the critical part. The temperature has to be high enough that the glass becomes soft, and triggers a reaction in which the lead borate chemically etches its surface, and draws the silver and/or copper ions below the surface of the glass. The problem is that slightly excessive heating will cause the glass to deform. It is very difficult to find the right balance, because there can be variations of one or two tenths of a millimetre in glass wall thicknesses from one bulb to the next, and the thinner bulbs are prone to deformation. To avoid that, most setters of the Sealex machines err on the side of caution, and tend not to set the etching fires quite hot enough. The result is that the etching reaction does not proceed fully, and the ink sometimes be rubbed away.
Many years ago this reached such a problem that too many lamps were losing their etches, which was considered a safety issue because the customer might not be able to read the wattage and know if they were installing a lamp within the the power rating of their fixtures. The various standardising bodies such as the IEC and ANSI then developed a mandatory test for the robustness of the etch, which was taken into the IEC 61 standard for incandescent lamps and to this day has been copied over to most other lamp standards. In the factory we have to take a soft cloth, soak it with hexane (a solvent for the ink syrup), and rub the etch softly for 15 seconds. After testing it must still be visible. I do think that since the 1970s most lamps probably meet this requirement, but of course we all know several exceptions where it has not been etched firmly enough.
GE lamps were another matter. Some time ago they tried to shift to different ink types as a way of getting rid of the lead content, for employee safety reasons in handling the lead borate marking paste. They developed a black air-drying ink based on copper oxide which did not have to be burned into the glass. Some years later Philips also developed an air-drying ink with a golden appearance, which was rather better. And Sylvania developed red colour inks which were extremely robust, but contained cadmium borate which also had to be dropped after a few years due to the environmental restrictions.
The reason internal etches was dropped was quite simple, because it was found that they can cause chemical contamination of the lamp atmosphere and lead to reduced life.
The problem with the glass frit decals is the same as the etch materials - they require high temperature firing. That is fine for heavy-walled borosilicate lab glassware, but very difficult to control for 0.5mm thick soda-lime glass bulbs and tubes. Also the speed of applying the decals is too slow and not really compatible with high speed automated production.
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