The workshop Ferrous Attractions: The Science Behind the Magic was led by Gwen Spicer (Spicer Art Conservation, LLC) and Van Wood (Small Corp Inc.). The program was outlined in three sections: introduction to the magnetic system, hands-on testing, and a brief summary of testing results.
The magnetic system includes four components:
- the strength of the magnet,
- the type of ferromagnetic material receiving the magnet,
- the space between them (occupied by the artwork),
- and gap materials (those in contact with the artwork: fabric, Mylar, twill, batting, paper, or suede).
Magnetic systems can be assembled in several different ways:
- Two-part (point fastener): magnet/magnet or magnet/ferromagnetic material.
- Three-part (sandwich): ferromagnetic material/magnet/ferromagnetic material or magnet/magnet/ferromagnetic material.
- Large area pressure: flexible magnet/ferromagnetic material or magnetic slat/ferromagnetic material.
The ferromagnetic material, commonly 24 gauge steel, is called a “soft magnet” because it is magnetized in the presence of a permanent magnet.
Conservators typically use neodymium permanent magnets. Neodymium magnets are considered the strongest, yet their sourcing is ethically problematic – details of their mining and trade were addressed in Gwen’s 2015 AIC Sustainability session talk. This table compares other types of magnets you may encounter.
Hands on testing was done in pairs using a wooden jig, bucket, weights, and a kit of various magnets, metals, and gap materials. The test: choose a type of magnetic system, suspend the bucket, and fill it until the system fails (ie. the bucket drops). Images of the jig and testing results from a previous workshop are found on Gwen’s blog.
Many factors influence the strength of a permanent magnet including:
- shape (disc/cylinder, sphere, block, ring)
- size (diameter, thickness)
- grade (“strength” of the magnet – the higher the N# the stronger – N42 and N52 are common)
- pull force (force required to separate the magnet from steel or another magnet)
- pole orientation (axial or diametrically magnetized)
- Curie temperature (temperature at which the magnet is demagnetized)
- intrinsic coercive force (resistance to demagnetization).
K & J Magnetics Inc. discusses these aspects in detail in their blog and glossary. They also offer a Magnetic Pull Force Calculator to aid in selecting magnets for appropriate use!
Tips from the summary discussion:
- Do not use hot glue or irons near magnets because the heat can demagnetize them. For example, the Curie temperature for neodymium magnets is considered low: 310-400 degrees Celsius.
- Do not put cell phone near strong magnets!
- Axial magnetic attraction is stronger than diametric (side to side) attraction. Always check the pole orientation when purchasing.
- Match your magnets! Magnets of similar diameter and pull force will behave the most regularly. Magnets that differ by more than 1/8″ diameter will slip because the magnetic field is not evenly distributed.
- Neodymium magnets corrode easily and therefore require a coating. They are brittle and chip easily. They are hard to demagnetize, but they can easily demagnetize other magnets!
- Three-part systems are the strongest. Magnets can be bundled to increase pull force.
- Is the pull force additive or logarithmic?
Ferromagnetic materials –
- Steel powders were deemed ineffective by the group overall – you need A LOT of it.
- 24 gauge steel is the sweet spot – higher gauge steel is only minimally stronger.
- Using a thinner magnet on the artifact surface and a thicker one behind reduces the total pull force of the two magnets.
- Beware of steel washers. The hole significantly reduces the pull force and causes the magnet to slip.
Gap materials –
- The best gap material is decidedly Benchmark suede (to cover the magnets) due to the suede’s friction and slight give – better than Mylar or cotton. Polyester batting was the worst because the gap is too big.
- What is the optimal or minimal gap for your magnetic system?
Gwen Spicer is writing a book on magnets in conservation, supported by the AIC Kress publication fellowship. If you have a Case Study you would like to share, please get in touch!