I came across a Gearhart Knitting Machine cylinder that, even though unused and stored in its crate for the last 85 years, had tiny hairline cracks and some very small chips on ridges of the tracks that hold the needles. I’m not sure if you can make it out, but here is a photo of the cylinder. The cracks do not circle all 360 degrees of the cylinder.

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This leads me to Richard Candee’s statement in his book titled, The Hand-Cranked Knitter and Sock Machine. In chapter 9, page 234, he says:

Late Gearhart cylinders were made of bad pot metal, a silent testimony to the company’s coming bankruptcy.

So, did this cylinder have bad pot metal? Based on the discovery of this cylinder with hairline cracks, I do agree that somewhere along the line, the company received a batch of cylinders that did not hold up well after 85 years. For the second part of the statement, the company actually went out of business due to a hostile takeover, not a batch of bad cylinders from the Herman Doehler’s Die Casting Company. I think the health of the cylinder points more to production problems with the Doehler company or the cylinder’s subsequent storage then to the Gearhart Knitting Machine company. But, more on hostile takeovers and die-casting processes in future posts.

Richard also noted that he had observed Gearhart cylinders that had sagged or bent. And one of them was even molded to the square shape of the box it came in.

Wow! A square cylinder. If a cylinder was received by the Gearhart Knitting Machine company per the photo above, I wonder if the company would have shipped them out? I’m inclined to say they would have rejected the batch and demanded a replacement. So far, I haven’t found anything which indicates the company knew about bad cylinders. My own cylinders are perfect. no chipping, no hairline cracks, perfectly round.

I’d assume that the cylinder deteriorated over time. Either bad metal or low casting pressure might explain the progression of hairline cracks. Sagging? I don’t know. If I were stored in an attic that could reach over 150 degrees (like my attic), I might end up square and sagged too. Its hard to say without knowing more about the environment.

Pot metal is a slang term that covers many alloys of aluminum, zinc, copper and a few others in various combinations. It’s also referred to as Die Cast metal. It’s hard to just look at it and differentiate it from an aluminum alloy except in most cases, it’s heavier because of the zinc.

A scratch and daub test will usually tell you if it’s die cast alloy. A drop of battery acid (sulfuric) will foam if it has zinc. If it has zinc, it’s most likely a die cast alloy or “pot metal”. So, I performed this test on one of my cylinders, and sure enough the metal did contain zinc.

While Pot Metal does melt at low foundry temperatures, its most important property is it’s strength right out of the mould. It is this property that makes it useful in high speed die casting. Casted objects can be ejected out of the mould almost immediately after the molten alloy is injected into the mould. However, with die casting, high pressure is important because too little pressure will leave pin holes and chips, and produce a casting suseptible to gradual cracking and separation over time due to low density.

There are many slightly different alloys that have the desirable character of Pot Metal but none are strong. Probably the strongest is one called ZYMAC, an alloy I believe made up of zinc, magnesium, aluminium and copper. The Alcoa Aluminium Company was started in 1888, in Pittsburgh. So, the Doehler company in Cleveland would have had access to the material at the time they contracted with the Gearhart Knitting Machine Company to cast their cylinders. This contract was awarded in 1915. I’m not sure if the cylinders were a ZYMAC alloy, but I’m sure a metallurgist could tell me if one of the ingredients is Aluminium.

Pot metal is not junk. Zinc alloy is the least expensive metal for making light castings. Steel rusts faster than zinc alloy pits, so in that sense it beats steel. Zinc oxide turns to a white-ish powder. It forms pits and can turn into crumbly cubes before disintegrating.

So, I think in the absence of white powder, crumbly cubes, distortion or abuse, the cylinder in the photo might have been a victim of low pressure during casting. This produced a low density cast, which could have developed cracks over the years. That’s my theory.

As for the square cylinder, this is a mystery. It takes 650 degrees to melt pot metal. I wonder if large swings in temperature over years would cause this? From the posting titled Assignment of Refund Guarantee (click here), we know that the company guaranteed their product for 10 years. So, I think they had sufficient confidence in the cylinders at least over the short term.

The original question was, did the cylinders have bad pot metal? I don’t have any evidence yet that points to this, although I do have evidence that indicates a low casting pressure.

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