John Cook recalls the Heat Treatment Room, c.1950s

Oil, water, cyanide, and more...

John Cook Heat Treatment 1
John Cook Heat Treatment 1
John describes how metal was hardened and softened for processing
John Cook Heat Treatment 2
John Cook Heat Treatment 2
John describes the cyanide pot and the heat treatment room
John Cook Heat Treatment 3
John Cook Heat Treatment 3
John describes the colour of sparks given off when grinding steel and quenching metals in different fluids

In these three clips, John Cook recalls the Heat Treatment Room in the 1950s and the processes for hardening and softening metals.  Heat Treatment was part of the Tool Making Room, where they specialised in making tools for use on the factory floor.  There they used cyanide, a furnace, oil, and water to hardened and soften metals for the factory.  When Brian worked there as an apprentice, he remembers Brian Kemmett as the man responsible for Heat Treatment.

Hardening metal

In the first clip, John describes how metals would be hardened.  He uses the example of piston which would need to fit into a cylinder.  The piston would be machined slightly oversize and then put through a heat treatment where it would be case hardened.

“Case Hardening meant that the piston would be dipped into a pot.  It was a big pot full of cyanide that was white hot.  You just dropped the item into the cyanide pot and by charts you could read you could tell the depth of hardness that you were actually putting onto it.”

The amount of excess metal to be ground off on the grinders in the machine shops would then be indicated.

Softening metal

To soften metal, John remembers the example of a file that needed to be made into something else.

“The chaps would bring the file to Heat Treatment and we’d drop it into a forge, then dip in oil or water which would soften it.  They could then work on it themselves, make whatever they wanted, bring it back to us then we would hardened it back up again.”

Copper pipe, used in the workshops and The Pipe Shop, would also need to be softened because copper “work hardens”.  That means that the more it is bent or worked the harder it becomes to the point it can’t be bent any further.  So, copper pipes would be brought to Heat Treatment to be softened when they became too hard to be worked on.


In the second clip, John describes the cyanide pot that was used as part of the heat treatment process.

The cyanide pot was 3 to 4 feet deep and no more than 18 inches diameter.  It was electrically heated to warm the cyanide, but, as the cyanide cooled, a crust would form on the top of the cyanide that was “like eggshell”.  This would be broken with a pick before the items were lowered into the cyanide.

Whenever the pot was used, a big long facemask and flame-proof elbow-length gloves had to be worn for protection.

“Whenever you put anything into the cyanide pot, it was usually tied with wire with a hook on one end so that it could be lowered into the pot and then we could raise it out without actually touching the item itself to quench it in water.”

When quenching the items in water, “there was always a sharp crack, like a small explosion”, which was caused by the heat from the cyanide going into cold water.  It was this cooling process which put the hardness into the metal.

After items were quenched, they were ready to use or be machined almost straight away.

John recalls that working in Heat Treatment was good as an apprentice because “if something was in a cyanide pot it might need to be in there for an hour to give it so many degrees of hardness.”  Nothing else could be done in that time, which was usually used by him to catch up on homework from technical college!

Water and Oil

In the final clip, John describes how different types of metal could be identified by the spark they gave off on the grinding machine and how quenching in water or oil gave different properties to the metal.

Most of the items quenched in heat treatment would be quenched in water but another way to quench was in oil.  Quenching the item in oil cooled the item more slowly than did water.  The speed of cooling effected the hardness of the metal.  Cooling the item quickly in water made the metal “glass hard”.  However, if you wanted your metal to be softer and “reasonably pliable” you would cool it in oil instead.

If you remember the Heat Treatment Room please share you memories by adding a comment below.

Click here to listen to John’s memories of the crane “slingers” in the Heavy Machine Shop.

Click here to listen to John’s memories of his apprenticeship and The Craft School, 1948-1955.

Click here to listen to John’s memories of the Light Machine Shop, 1948-55.

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