Pure brass is not magnetic; it does not attract a common magnet under normal conditions. This alloy is primarily a mixture of copper and zinc, neither of which possesses the atomic structure required for strong magnetic behavior. Therefore, a solid piece of high-purity brass will not stick to a magnet. Understanding why brass is non-magnetic requires examining how materials are categorized based on their interaction with a magnetic field.
Understanding Magnetic Classification
Materials are classified into groups based on how their electrons behave in the presence of an external magnetic field. The strongest form is ferromagnetism, which allows materials like iron, nickel, and cobalt to be strongly attracted to a magnet. These materials can also retain their magnetic properties after the external field is removed. This strong attraction occurs because they have uncompensated electron spins that align to form magnetic domains.
A weaker classification is paramagnetism, where materials are only slightly attracted to a strong magnetic field. Paramagnetic substances possess unpaired electrons that are randomly oriented. These electrons align temporarily when exposed to an external field, but the weak attraction disappears as soon as the field is removed. Aluminum and platinum exhibit this behavior.
The third classification is diamagnetism, a property present in all matter, but only observable in non-ferromagnetic or non-paramagnetic substances. Diamagnetic materials have all their electrons paired. When exposed to a magnetic field, they develop a weak magnetic moment that opposes the external field, resulting in a slight repulsion. This response is usually too weak to be noticed without specialized equipment.
The Diamagnetic Nature of Pure Brass
Brass is non-magnetic because its main constituents, copper and zinc, are classified as diamagnetic materials. The magnetic behavior of a metal is determined by the configuration of its electron shells. To exhibit strong magnetism, a material must have unpaired electrons in its atomic structure that can spontaneously align.
In both copper and zinc atoms, the electrons are arranged in pairs, filling their outer shells. Since there are no unpaired electrons, there are no uncompensated electron spins to form the magnetic domains characteristic of ferromagnetism. Therefore, the base alloy cannot be permanently magnetized or attract a magnet.
When pure brass is placed near a strong magnet, its electrons slightly shift their motion to create a magnetic field opposing the external one. This shift results in a weak repulsion, confirming its diamagnetic classification. The magnetic susceptibility, a measure of how much a material is magnetized in an applied field, is a small negative value for copper and zinc. This inherent non-magnetic property makes brass suitable for sensitive applications, such as marine environments or instrumentation, where magnetic interference must be avoided.
Why Brass Sometimes Appears Magnetic
The observation that some brass objects attract a magnet is not due to the brass alloy itself. This perceived magnetism is almost always the result of contamination or the inclusion of other components. During manufacturing, trace amounts of ferromagnetic elements, most commonly iron, can be introduced as an impurity.
Iron contamination can occur from the use of steel tools and machinery or from low-quality raw materials. Even a small percentage of iron in the alloy can cause a weak, localized attraction to a magnet. This effect indicates that the brass is not completely pure or is a lower-grade alloy.
Many brass items are not made entirely of brass, but are assemblies of different materials. A brass-plated object or a fixture with internal components often contains steel springs, screws, or mounting hardware. In these cases, the magnet is sticking to the internal steel component, not the brass exterior, leading to the false impression that the brass is magnetic.