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Bendy bond

My 12-year-old son has been asking me how flexible fridge magnets are made (such as the ones with printed advertising on one side). And what sort of material are these magnets made from?

• A flexible fridge magnet consists of barium or strontium ferrite powder mixed with a flexible plastic or rubber binder. It is formed either by extrusion (for a magnetic strip) or calendering (for magnetic sheeting) and is then magnetised by applying a magnetic field. Finally it is laminated with vinyl or adhesive, cut to size, rolled onto a core and boxed for shipment.

Johan Uys

Bellville, South Africa

Woodland wonder

The ice crystals in the photo were found on small branches lying on the ground in mixed woodland. The crystals had only formed where the bark was missing and there was very little frost elsewhere on the ground. Can anybody explain how they formed?

• Essentially the same phenomenon came up in the Last Word of 28 April 2001 when a photograph showed thin, hair-like filaments growing out of rocks in Frome, Somerset. The explanation behind this new photograph of fine, frilly ice filaments growing from small branches lying on the woodland floor (once again, coincidentally, in Somerset) is very similar.

The hairs of ice are generated from water held in the pores of the decaying wood, which is sucked out by the freezing action to form filaments. For this to happen there would have to be no free water on the outside of the wood. Indeed, the questioner notes that there was little frost elsewhere on the ground.

I have seen the same ice crystals on rotting elder wood alongside the river Barle, downstream of Wimbleball dam, also in Somerset.

David Stevenson

Newbury, Berkshire, UK

Web of intrigue

Is the silk of a spider’s web synthesised at high speed as required, or is it stored in readiness? If the former, how does the spider perform this feat? And what is the chemical composition of the filaments that makes them so strong?

• The main component of spider silk is similar to keratin, the structural protein found in hair and horns. The spider’s silk protein is synthesised by special cells and stored in various abdominal storage glands where different silks are produced for different functions, such as dragline silk, egg-sac silk and capture silk. The silk spigots behave as high-pressure valves that extrude the viscous liquid-crystalline silk.

The great force required to extrude the protein through a small orifice causes some of the protein chains to align and form ordered crystals, while the remainder of the protein is in a more disordered helical or spring-like state. In essence, the silk filament is a composite material in which the helices provide its elastic properties while the crystals impart strength. Spider silk is comparable in strength to Kevlar or steel, and is also extremely elastic. A typical thread is only 1 micrometre in diameter, but I once heard that if a spider’s web could be scaled up so the silk strands were the diameter of a pencil it would be strong enough to capture a passenger jet.

A. M. Monro

Canterbury, Kent, UK

• Weight-for-weight, spider silk has a much higher tensile strength than steel, and is about five times as strong as a piece of steel of the same diameter. This enormous strength is the result of the structure and composition of the silk. Its filaments are mainly composed of a polymerised protein called fibroin made up mostly of the amino acids glycine, alanine, serine and tyrosine. This gives the silk not only its strength, but also a fantastic elasticity.

The fibroin molecule is composed of a tightly folded crystalline lattice, as well as loose spring-like chains of glycine peptides. The highly ordered crystal structure makes up approximately 25 per cent of the polymer, and is bonded together by strong alanine peptide bonds. It is the overall arrangement of these crystals and the bonds between them that gives silk its great strength.

Nick Sutton

Dorking, Surrey, UK

This week’s questions

Pond life

A neighbour advised me to put a bale of barley straw into my pond to prevent weed growing. It worked, but how?

P. J. Wood

Inverurie, Aberdeenshire, UK

Moonflight

If I were an Olympic high jumper would I be able to jump high enough to escape the moon’s gravity? I suspect not, but astronauts on the moon’s surface seemed to float in the air for a long time. Exactly how high and how fast would I have to jump to escape the moon’s pull, or simply to fly over its surface? As there is no atmosphere, are my aerodynamics irrelevant?

Based on a question by Sandie

Mountford-Jones

Stoke-on-Trent, Staffordshire, UK