Making a Padlock and Key

This term, as a first year student in the metalwork conservation graduate diploma program, I've been exploring the properties of ferrous metals. From making, hardening, and tempering steel tools to practicing chemical and mechanical cleaning techniques on samples of corroded mild steel, I have been getting a hands-on understanding of iron alloys.

The most challenging project was to design and make a steel padlock and key, which would then be "destroyed" by our tutor, Jon Privett, so that we could put our conservation skills to use in cleaning, stabilizing, and restoring it. (See Bill's post from last week about a related earlier project here.)

Since I wanted to incorporate both my Chinese heritage as well as my jewelry-making experience in the design, I made a padlock that would also function as a necklace. In pre-revolutionary China, at a time when infant mortality was high, young children often wore necklaces shaped like locks, due to the belief that such an accessory would lock their health and luck in place. The escutcheon (the plate that surrounds the keyhole) was inspired by traditional bat motifs. Bats are considered auspicious animals in Chinese culture, because the word for bat is a homophone of the word for fortune.

The process:

I hot forged the hasp/neckpiece out of mild steel square bar, decorated with water twists and shaped around a round mandrel to fit comfortably on the shoulders.

The front and back plates were cut with a jeweler's saw from mild steel sheet, as was a strip that would become the walls of the lock. This strip had four tabs along one edge to be inserted through the back plate, which had four corresponding slots. This created a wall that would be more sturdily attached to the back plate. Each of the tabs on the wall piece were pierced and then formed with needle-nose pliers. The tabs were then bent down flat against the back plate, making stylized cloud designs (which often accompany bats in traditional Chinese art, since the word for cloud is a homophone of the word for luck).

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I brazed the wall piece and back plate together for additional strength. The tabs were covered with brazing material, so that they could be filed down at the end, revealing the steel cloud design nestled within the yellow brass filler material.

Because mill scale (the dark oxide that develops on the surface after being heated) protects steel from corrosion, I tried to remove as little of it as possible from my padlock during the cleanup phase after brazing. This resulted in quite a motley texture on the back of the lock. As a former jeweler, not finishing surfaces to an even texture makes me uncomfortable- but I had to remember the lock needed that protective layer of mill scale for when it would be aged and corroded!

I made the components for a very simple locking mechanism. The components were either brazed or riveted in place. These included:

  • A spring from high carbon steel gauge plate, which was hardened and tempered to about 320°C, the ideal temperature for springs, conveniently indicated by the steel turning a beautiful gray-blue color.
  • A fulcrum for the spring.
  • A cam, which would be pushed by the key to activate the spring. I attached a length of clock spring to one end of the cam so that it would bounce back into position after being pushed with the key.
  • A pin, to smoothly guide the key into the keyhole and act as leverage.

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The bat design was etched into the brass escutcheon with ferric chloride, with nail varnish used as a resist. When this was finished, I riveted the escutcheon onto the front plate. When I was sure that no more work needed to be done on the interior, I used four long steel rods to rivet the whole structure together. Finally, I made the steel key, hot forging the bow into the shape of a peach. Again, I drew inspiration from traditional Chinese art, where a peach, a symbol of longevity, is often grouped with four bats.

After I finished making the lock, it was aged and pitted in an electrolytic bath and is currently hanging outside to corrode in the damp English winter, sprayed regularly with a sodium chloride solution.

In a few months, I will be describing the conservation process on the same padlock, when the time comes to bring it back into the metals workshop. So please watch this space!