Shedding Some Light

Production engineering is an entire specialty in and unto itself.  Making one of something or even a handful is comparatively easy.  Making thousands of them–all alike, in a mass production environment, is another matter entirely.  Even something as ubiquitous as a common incandescent lightbulb takes on a whole new dimension when placed into a production environment.

Thomas Edison’s early lamps were all handmade.  There was a team of glassblowers at Menlo Park tasked with making the envelopes.  Another group made the stems, which held the lead-in wires and filament structure.  Through the early years and even when dealing with experimental and prototype variations, this is how it was done.  Once a design to mass-produce was chosen, the procedure was different.  Someone had to devise a methodology that would result in hundreds or thousands of them being made all alike and as efficiently as possible.

In the case of common lightbulb envelopes, a set of simple molds were made.  Each mold had a fitting to allow it to be held in the semi-automated equipment.

In simplest of explanation, the process was along the lines of this:  Glass gathered from the furnace onto a hollow tube would be aligned with the mold.  The machine would close the halves of the mold around this and air would flow through the tube, causing the glass to take the shape and dimensions of the machined mold surface.  Typically these molds were coated with a graphite material similar to the modern equivalent called “Aquadag.”  This served to lubricate and prevent the glass from sticking.  In addition, a close look at these molds reveals the minute air holes in them.  These allowed air to escape as the glass expanded to fill the cavity.

Shortly afterwards, the mold would open and the tube with glass shape would be withdrawn.  The next steps would involve fire-polishing to eliminate the seam line, cutting off of the tube, followed by fire-polishing the cut end, and at least one good annealing cycle to prevent stress cracking later.  After this, the stem assembly with filament (or entire filament, grid, plate structure in the case of vacuum tubes) would be inserted and welded into the bottom of the envelope by a burner while the machine rotated it for even heating.

From there, it was onward to the pumps and processing stage…….envelope evacuated, envelope and elements heated to drive out impurities, then once sealed off the pumps the lamp would be fitted with the base and then given a final test prior to packaging and shipping.

Many details and many machines have been omitted from this short description, but you get the idea.  Browsing through an old Eisler catalog gives a glimpse at the machines used to perform each step.  Such equipment would have been in place at any lamp or tube manufacturer.

These old iron molds represent a look into how something so common was able to be made in quantities sufficient enough for it to become so common that we don’t often give it a second thought.