While the term 'potmetal' was used for many low melting point alloys the modern name for the potmetal we are familiar with is 'die cast zinc.' Zinc is actually a wonderful metal to cast with a melting point of 419.53 °C or 787.15 °F. This is low enough so that a mold can easily be made of many materials. Today the purity of zinc is very high, around 99.999% before it is mixed with other metals to make alloys. Aluminum is common but any alloy commonly raises the melting point so pure zinc is preferred for most applications. Some metals cannot be added to the zinc in even tiny amounts without creating a dimensionally unstable alloy.
The potmetal problems we see on phonographs is mostly due to 'inter granular crystal growth' and is not temperature related. The old timers called this 'dezincification' which is a misnomer since the zinc really does not go anywhere unless a piece falls off. If even a tiny amount of certain elements like tin, lead, cadmium, sulfur, or other contaminate is introduced into the liquid casting metal the result with time will be slow deformation of the casting as atomic bonds dissociate along crystal lines while other crystals actually grow. Thus, some portions of the solid casting may contract while others expand. A very pure potmetal will be dimensionally stable for very long periods of time. The contaminated batches may have been inadvertent. Old pipe organ pipes made of zinc could have been used for scrap in the melting pot without the people thinking about the lead/tin solder seams. Just that small amount of contamination is enough to ruin a full batch of zinc. Some batches were very pure and that is why you can find a perfect potmetal reproducer body with some regularity. Another problem is the cooling rate when the casting was removed. A rapid cooling of a die cast zinc will leave stress in the metal. A slow cooling allows for the metal to naturally anneal at lower stress. Die cast zinc today that needs to be accurate to a high degree is removed from the mold and allowed to go through a series of cooler furnaces before reaching room temperature. There are environmental elements that can add to a bad batch of zinc. If the casting was exposed to salt air (a.k.a. sodium chloride) or acidic air such as the sulfur dioxide coal plants produce, then any porosity may become a source of oxidation and the casting will suffer. Zinc oxide will form along the microscopic lines between zinc crystals and they too will cause expansion but much less than a bad batch would cause. But HEY, these things were not supposed to last more than a couple of centuries anyway! ! ! May all your finds be rare ones, Al _______________________________________________ Phono-L mailing list http://phono-l.org
