Flag conservation

Flag conservation
Textile conservator, Gwen Spicer of Spicer Art Conservation at work

Wednesday, December 31, 2014

What is your base knowledge about Rare Earth or Neodymium magnets?

Sometimes we just need to begin with the basics.

There has been lots of talk recently amongst conservators about Rare Earth Magnets, specifically rare earth magnets composed of the element Neodymium. These neodymium magnets have gained a foothold as the "go-to" magnet for mounting artifacts for exhibits and display. That is not say that this particular type is the only suitable rare earth for conservators to use, just that our literature indicates it is the most popular choice. Besides Neodymium, the other rare earth magnet for conservators is a Samarium Cobalt.

Rare Earth?  Neodymium? Samarium Cobalt? What do these terms really mean, and is there a big difference from one magnet to another?

To begin with, rare earths are not really rare, nor are they precious. They are actually as common in the earth as lead or tin. What makes them "rare" is that the elements that make up the rare earths are hard to come by, meaning mining for these elements is no easy task, and when they are found, the process to isolate them from the surrounding materials is quite difficult.

On the periodic table they are the upper row of elements that sit below the table (the plum color). (The lower fuscia colored row are the radioactive elements). They are part of the Lanthanide group of elements. Of the four permanent magnets, all developed in the twentieth century, two are made of elements from the Lanthanides; Samarium and Neodymium.

Conservators may be partial to Neodymium magnets for several reasons. They are cheaper than Samarium magnets, but more importantly, Neodymium magnets are strong, compact, permanent magnets, and they have the highest magnetic field strength as well as a higher coercivity (which makes them magnetically stable). The downside is that they have a lower Curie temperature (tolerance to heat  exposure) and are more vulnerable to oxidation than samarium-cobalt magnets. Samarium magnets, are more prone to corrosion and are far more brittle.

So it is an easy choice for many conservators, the neodymium magnet is the logical choice. The drawbacks (i.e your neodymium magnet being demagnetized because it is exposed to high heat) can easily be avoided. Simply do not use hot-melt glue with your neodymium magnet. Other options are available, like countersunk magnets to accommodate a screw for instance.

Hot melt glue and neodymium magnets just don't go together. The temperature of the glue
(even the kind marked "low temp") is just too high and will render your magnet useless.
Neodymium magnets begin to lose strength if heated above their maximum operating temperature,
which is 176°F (80°C) for standard N grades. They will completely lose their magnetization if
heated above their Curie temperature, which is 590°F (310°C) for standard N grades.

So now that we have established why neodymium is the logical choice, lets talk about the variety of neodymium magnets that are available. Not only do they come in a variety of sizes and shapes, but they are available in many strengths as well. Essentially that means these magnets are labeled in a way that tells you how strongly they "stick" to a ferromagnetic surface, which is perhaps the most useful information for conservators as we are often looking for a delicate balance of supporting an artifact, but being careful not to cause any harm to the artifact.

A very small sample of the sizes and shapes of Neodymium magnets available.

Neodymium magnets are marked N35, N38, N42, N52…but what does that mean? According to our favorite magnet distributor, K & J Magnetics, "Neodymium magnets are all graded by the material they are made of. As a very general rule, the higher the grade (the number following the 'N'), the stronger the magnet. The highest grade of neodymium magnet currently available is N52. Any letter following the grade refers to the temperature rating of the magnet. If there are no letters following the grade, then the magnet is standard temperature neodymium".

One of the smallest Rare Earth Neodymium magnets available.   This N52 measures 1/16" in diameter and only 1/32" thick!

The magnet above is an incredibly small magnet, yet as a N52 magnet it's pull force is quite strong.  Finding a balance between size, shape, pull force, and other factors to accomplish a mount is challenging. One would wonder if you could simply weigh your artifact and then figure out how many magnets would hold it up, and maybe add a few extra as a safety feature. But as we all know, nothing is ever that simple. For Conservators it is not just about holding the weight of the object, but how intact is the object, and what is it's ability to hold it's own weight. Sadly, there is no way to measure that. So we build in safety factors, like an angled display board, or a display fabric to provide some built-in friction for a textile.

So often I hear, "oh, magnets?! I know all about those" only to discover that the real magnet knowledge of the speaker is limited, it is only the term "magnet" that is familiar. Understanding that magnets are part of a broad and diverse world is the first step in using them properly.  Being familiar with magnets and their properties is the first part of creating a successful mounting system. In future blog entries we will discuss the other parts of a magnet system: The gap (or space in between the magnet and the ferromagnetic material) and the ferromagnetic material (what the magnet is attracted to).  Once these parts are understood, their cooperation together and the ways in which they can be altered, can be utilized to create inventive and successful systems.
Gwen Spicer is a textile conservator in private practice.  Spicer Art Conservation specializes in textile conservation, object conservation, and the conservation of works on paper.  Gwen's innovative treatment and mounting of flags and textiles is unrivaled.   Her current research focuses on the use of rare earth magnets in conservation. To contact her, please visit her website.

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