Flag conservation

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

Monday, May 16, 2016

So, how do I store my magnets?

I recently returned from the 5th Mount Maker's Forum, held at the Cleveland Museum of Art, Cleveland, Ohio.  It was a great meeting, full of enthusiastic mount makers, all sharing great ideas and solutions with one another.

I was fortunate to be able to both give a talk, "Stick to it, magnetic mount-ineers!" and present a poster, "Magnets as an Alternative to Velcro". The mount makers had many questions regarding the use of magnets.  "What is the best way to store them?", was one of the most frequent questions I was asked.  I  realized this topic made for a perfect blog post, therefore, here it is!

As mentioned in earlier blog posts about magnets, there are four permanent magnets. Each type of magnet has its own needs for long-term use and continual performance. Which is no different from museum collections, or any other equipment that you might use. Some magnets are effected by shock or mechanical action, others are brittle and break easily, and others are effected by temperature or moisture. All of these are issues of handling and environment, which conservators and other museum professional are especially suited to understand. Depending on the class of magnet, the care will vary slightly, but, with proper care, little decay should be noticed.


Various magnets held in film style containers and separated by foam disks.

Coercivity (Hc) is the process where a magnetic field is reduced or eliminated. Each permanent magnet has its own coercivity rating. The higher the Hc, the greater the resistance to demagnetization. Understanding the Hc of permanent magnets, and other materials and equipment that surrounds us, is necessary when working with strong magnets. Rare-earth magnets currently have the highest coercivity values.

What causes coercivity?

MECHANICAL SHOCK

Several magnet types are brittle* and can easily fracture. This is especially the case with rare-earth magnets, when impact and tensile forces affect them. In fact, many suppliers do not guarantee against poor handling due to this fact.  Since a sharp hammering, or any physical shock, can cause demagnetization, it is necessary to prevent magnets from quickly jumping to one another or dropping to the floor from a raised height. Once a magnet is broken or cracked, it is highly susceptible to moisture and corrosion. Do not attempt to use them by positioning them together or gluing them together. Chipped or cracked magnets with peeling or spalling surfaces should not be used since the protective coating has been disrupted (Campbell, 1994).
*NOTE:  Brittleness increases as the grade number of the magnet increases.


cracked rare earth neodymium magnets should not be used in art conservation applications
Cracked magnets should not be used.


HEAT and Curie Temperature (Tc)

Each permanent magnet has a Curie temperature (Tc) that identifies the point where the material’s magnetism is eliminated. Neodymium magnets are very sensitive to high temperature* and therefore have the lowest Tc of the permanent magnets; Alnico and samarium have the highest Tc values. This is one of the reasons why Alnico magnets are still used. Be sure to stay well below the Tc of each permanent magnet used.
*NOTE: This is why hot glue can be dangerous when used to adhere rare earth magnets to a surface.

MOISTURE
As stated earlier, Neodymium is easily oxidized. In a magnet, an oxidized surface lowers the pull force of the affected layer, therefore allowing that region to demagnetize more readily (Campbell 1994, Drak & Dobrzanski 2007). A coating of nickel-plating, or epoxy, is applied to prevent this from occurring. Blistering and spalling of the surface can be seen, more readily with two-layer copper nickel plating (Drak & Dobrzanski 2007). Even during the manufacturing process, oxidation prevention measures are required, often using a vacuum or argon gas environment. A sintered magnet is less stable than a bonded magnet against oxidation induced demagnetization corrosion (Campbell 1994; Trout n.d.). If a neodymium magnet is used in a raised relative humidity location, a bonded magnet is recommended (Drak & Dobrzanski 2007).


A N52 magnet that was used in a salt water environment;  the magnet is corroded and is no longer usable.

DEMAGNETIZING FIELD

Some types of permanent magnets influence or weaken other magnets. One such case is when a ceramic (including flexible type) or samarium magnet is demagnetized by a neodymium magnet. As a result, neodymium rare-earth magnets should always be stored away from other magnet types. Similarly, electronics systems that rely on magnets to hold information, such as hard drives and disks, can be altered or demagnetized by a neodymium magnet that is placed nearby. Magnetic strips on credit cards and other cards can also be affected, as can electronic devices.


The statement above appears on stickers that we adhere to the magnet cases at SAC.

Ferrite magnets can be demagnetized when their poles are alternated, a reason to carefully stack the magnets. This is especially the case with the bonded flexible type; sliding a magnet side-ways perpendicular to the polar rows demagnetizes the array. Alnico type magnets are unique in that they can be remagnetized by realigning the internal domains via another strong magnetic field. This is not the case with other magnets, especially neodymium ones, where once demagnetized, the magnetism cannot be recovered.

Each type of permanent magnet should be segregated and spaced well outside other magnetic fields. As more magnets are concentrated together, the field increases. A safe approach is to separate each type in the work area.

To summarize this information, here is a table of the different categories with the various permanent magnets:



Alnico
Ferrite
SmCo
Neodymium
Use keeper for Horseshoe shape
X



Wrap to prevent abrasion

X


Group by size

X
X
X
Stack, orienting N to S

X
X
X
Place separator between


X
X
Moisture and RH sensitive



X
Demagnetizing Field (Hci)
Can be easily demagnetized. When repetitively placed north-pole-to-north-pole ends together, it quickly weakens itself.
Keep them away from Rare earth magnets.
Can be demagnetized by NdFeB magnets. But they do not weaken others.
Tough to demagnetize. This also means that they can easily demagnetize other classes of magnets like SmCo or Alnico or Ferrite. Shock can demagnetize.

Finally, with all of this information, let me show a few images of how I store my magnets.




Magnets are stored with a separator (black foam) between and in compartments lined with foam.  These small magnets are placed in "day of the week" pill containers.
-Individual small containers clearly labeled with type, grade and size.

-storing in divided boxes of a wide range of types.

-contact lens containers are wonderful to keep strong individual magnets separated from others.

-interleave magnets stored together with cardboard, foam or matte board for ease of separation

-Neodymium magnets are separated from other types of permanent magnets as that they effect their coercivity when in near proximity.

NEVER store you magnets next to a heated surface, like an oven or radiator; the location is too hot. Why? because some rare earth magnets have a low Curie temperature and thus, will demagnetize (and become completely useless) with heat.



















_____________________________
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.   To contact her, please visit her website.













1 comment:

  1. Perfect timing for this post as we are doing some spring cleaning within our vault and supply cupboards. Thanks as always for sharing!

    ReplyDelete