Zirconium Crucibles

Zirconium Crucibles

Zirconium crucibles are rapidly becoming a standard piece of equipment, particularly in laboratories where fusions using strongly alkaline fluxes are common. This Center has found no better material to use for the thousands of sodium peroxide fusions that are done here every year.

Zirconium crucibles usually arrive from their suppliers in a polished, silvery state. They resemble a polished iron or nickel crucible very closely. If they become confused with iron or nickel crucibles, they can be identified by tapping them with a pencil or spatula and listening to the tone produced. Zirconium crucibles produce a pleasant, almost musical tone when tapped. The first use of the crucible in a flame will produce a dark-gray coating over the metal that seems to enhance the crucible’s resistance to sodium peroxide. It is thought that this coating is a nitride or a nitride-oxide mixture, but no attempt has been made to confirm or refute the idea.

The use of zirconium crucibles should be limited to alkaline fluxes and fusions done over a burner. Zirconium crucibles should not be used for extended fusions or ignitions in a muffle furnace unless an atmosphere free of nitrogen and oxygen can be maintained in the furnace. The hottest part of the flame produced by a Fisher or similar design gas-air mix burner is proper for most sample-flux mixtures. If a high carbon, sulfide, or metallic content sample is being fused, it is usually desirable to begin the fusion gently in case the sample goes into autofusion. The crucible may be supported on a wire triangle over the flame or held with tongs, depending on the analyst’s preference.

Autofusion occurs when the sample-flux reaction is exothermic, and the effect can range from barely detectable to explosive. Any sample known to contain a reduced species or a reducing agent should be fused in sodium peroxide moderated by the addition of anhydrous sodium carbonate, and extra care must be taken in assuring complete mixing of the sample and flux. A well-mixed fusion will seldom eject material from the crucible even if the fusion becomes quite active. If an autofusion becomes too active or skyrockets, remove the crucible from over the burner as quickly as safety will allow. Place or hold the crucible with tongs where it will not contaminate any other work, until the melt solidifies, and then drop the crucible into a large beaker or a sink partially filled with water. Allow the leaching reaction to subside and then retrieve the crucible, clean it, and check it for damage.

With proper care, zirconium crucibles can last for more than 100 fusions. They should be checked before their first use for manufacturing flaws such as cracks or pits. During normal use, the bottom of the crucible is the part that receives the most severe abuse and the edge of the bottom is where most crucibles fail. As the crucible is used the bottom becomes thinner, and when it is thin enough so that it can be flexed with finger pressure, the crucible should be checked before each use for pinholes around the edge of the bottom. Discard any crucible that has a pinhole; the thin edge of a pinhole is particularly susceptible to attack by sodium peroxide. Also, do not use crucibles with thin bottoms to fuse samples that may autofuse. Extending the fusion time for a sample increases the attack on the crucible, which in turn shortens the life of the crucible and increases the amount of zirconium introduced into the sample. In general, the crucible and its contents will come to red heat within about 3 min. The melt in the crucible should then be swirled and the bottom examined for sample particles. The fusion is assumed complete when no more sample particles can be found. Some samples are not so easily distinguished, and only experience with that sample type can form a basis for judging completion.

Sodium peroxide fusions should not be attempted outside a fume hood. At the minimum, an operator should wear safety glasses and a lab coat or apron. Durable gloves would be advisable if samples known to be prone to autofusion are to be used.