A horseshoe is a U-shaped item made of metal or of modern synthetic materials, nailed or glued to the hooves of horses and some other draught animals. Like a shoe on a human, it is used to protect the animal's feet from wear and tear. Professional horseshoers, called farriers, usually place horseshoes on the solar (outer) surface of the hoof by nailing through the insensitive hoof wall, which is analogous anatomically to the human toenail, though much larger and thicker.
Horseshoes are available in a wide variety of materials and styles, developed for different types of horses and the work they do. The most common materials are steel and aluminium, but specialized shoes may include use of rubber, plastic, magnesium, titanium or copper. Steel tends to be preferred in sports where a strong, long-wearing shoe is needed, such as polo, eventing, show jumping, and western riding events. Aluminum shoes are lighter, making them common in horse racing, where a lighter shoe is desired; and often facilitate certain types of desired movement, and so are favored in the discipline of dressage. Some horseshoes have "caulkins", "caulks", or "calks": protrusions at the toe and/or heels of the shoe, to provide additional traction.
Since the early history of domestication of the horse, it was noted that working animals were exposed to many conditions that created breakage or excessive hoof wear. Ancient people recognized the need for the walls (and sometimes the sole) of domestic horses' hooves to have additional protection over and above any natural hardness. Because iron was a valuable commodity, and any worn out items were generally melted down and reused, it is difficult to locate clear archaeological evidence of the earliest horseshoes. From archaeological finds in Great Britain, it appears that the Romans attempted to protect their horses' feet with a strap-on, solid-bottomed "hipposandal" that has a slight resemblance to the modern hoof boot, and earlier people may have used rawhide boots or other wrappings to protect horse's feet. The nailed shoe was a relatively late invention.
According to the Encyclopædia Britannica Eleventh Edition (1911),
There is very little evidence of nailed-on shoes prior to AD 500 or 600, though there is speculation that the Celtic Gauls were the first to nail on metal horseshoes. The ancient Greek horse trainer Xenophon mentioned nothing about horseshoes in his treatise on the care of military cavalry, nor did the Digesta Artis Veterinariae by Vegetius Renatus, written in AD 480, mention nailed-on shoes, though he accurately enumerated everything connected with an army forge in the time. There are early literary references in the Koran, circa AD 632, to "war-horses… which strike fire, by dashing their hoofs against the stones…" which arguably is an effect that could only be obtained by shod horses. The earliest clear written record of iron horseshoes is a reference to "crescent figured irons and their nails" in AD 910. By the time of the Crusades (1096 – 1270), horseshoes were widespread and frequently mentioned in various written sources.
Many changes brought about by domestication of the horse have led to weakened feet and a need for shoes, including:
Nonetheless, domestic horses do not always require shoes. There is near-universal agreement among professionals that when possible, a barefoot hoof, at least for part of every year, is a healthy option for most horses. Farriers usually agree that some horses may even be able to go without shoes year-round, using temporary protection such as hoof boots for short-term use. However, some farriers are equally adamant that horseshoes have their place and can help prevent excess or abnormal hoof wear and injury to the foot.
Recently, there has been a renewed debate over the traditional role of horseshoes. Observations of feral horses and barefoot domestic horses in natural boarding situations (including being kept on roomy pasture, not in stalls) have provided additional evidence that domesticated horses can grow hooves as healthy as those of feral horses and may not need shoes as often as many people think. Proponents of this idea, also known as the barefoot horse movement, argue that with proper care, horses may never need shoes at any time once they have been properly transitioned. Thus, the debate of when, where, why and if to use horseshoes is a hot topic today.
Shoeing, when performed correctly, causes no pain to the animal. Farriers trim the insensitive part of the hoof, which is the same area into which they drive the nails. This is analogous to a manicure on a human fingernail, only on a much larger scale.
Before beginning to shoe, the farrier removes the old shoe using pincers (shoe pullers) and trims the hoof wall to the desired length with nippers, a sharp plier-like tool, and the sole and frog of the hoof with a hoof knife. Shoes do not allow the hoof to wear down as it naturally would in the wild, and it can then become too long. The coffin bone inside the hoof should line up straight with both bones in the pastern. If the excess hoof is not trimmed, the bones will become misaligned, which would place stress on the legs of the animal.
Shoes are then measured to the foot and bent to the correct shape using a hammer and anvil, and other modifications, such as taps for shoe studs, are added. Farriers may either cold shoe, in which he bends the metal shoe without heating it, or hot shoe, in which he places the metal in a forge before bending it. Hot shoeing can be more time-consuming, and requires the farrier to have access to a forge, however it usually provides a better fit, as the mark made on the hoof from the hot shoe can show how even it lies. It also allows the farrier to make more modifications to the shoe, such as drawing toe- and quarter-clips. The farrier must take care not to hold the hot shoe against the hoof too long, as the heat can damage the hoof.
Hot shoes are placed in water to cool them off. The farrier then nails the shoes on, by driving the nails into the hoof wall at the white line of the hoof. The nails are shaped in such a way that they bend outward as they are driven in, avoiding the sensitive inner part of the foot, so that they emerge on the sides of the hoof. When the nail has been completely driven, the farrier cuts off the sharp points and uses a clincher (a form of tongs made especially for this purpose) or a clinching block with hammer to bend the rest of the nail so it is almost flush with the hoof wall. This prevents the nail from getting caught on anything, but also helps to hold the nail (and therefore the shoe) in place.
The farrier then uses a rasp (large file), to smooth the edge where it meets the shoe and eliminate any sharp edges left from cutting off the nails.
However, the term "farrier" implies a professional horseshoer with skill, education, and training. Some people who shoe horses are untrained or unskilled, and likely to do more harm than good for the horse. Incompetent horseshoers are not true farriers. People who do not understand the horse's foot will not trim the hoof correctly. This can cause serious problems for the animal, resulting in chronic lameness and damage to the hoof wall. Poor trimming will usually place the hoof at an incorrect angle, leave the foot laterally unbalanced and may cut too much off certain areas of the hoof wall, or trim too much of the frog or sole. Some horseshoers will rasp the hoof down to fit an improperly shaped or too-small size of shoe, which is damaging to the movement of the horse and can damage the hoof itself if trimmed or rasped too short. A poor horseshoer can also make mistakes in the shoeing process itself, not only quicking a horse, but also putting shoe on crooked, using the wrong type of shoe for the job at hand, shaping the shoe improperly, or setting it on too far forward or back.
Horseshoes are considered a good luck charm in many cultures. The shape, fabrication, placement, and manner of sourcing are all important. A common tradition is that if a horseshoe is hung on a door with the two ends pointing up then good luck will occur. However, if the two ends point downwards then bad luck will occur. Traditions do differ on this point, though. In some cultures, the horseshoe is hung points down (so the luck pours onto you); in others, it is hung points up (so the luck doesn't fall out); still in others it doesn't matter so long as the horseshoe has been used (not new), was found (not purchased), and can be touched. In all traditions, luck is contained in the shoe and can pour out through the ends.
In some traditions, any good or bad luck achieved will only occur to the owner of the horseshoe, not the person who hangs it up. Therefore, if the horseshoe was stolen, borrowed or even just found then the owner, not the person who found or stole the horseshoe will get any good or bad luck. Other traditions require that the horseshoe be found to be effective.
One reputed origin of the tradition of lucky horseshoes is the story of Saint Dunstan and the Devil. Dunstan, who became the Archbishop of Canterbury in AD 959, was a blacksmith by trade. The story relates that he once nailed a horseshoe to the Devil's hoof when he was asked to reshoe the Devil's hoof. This caused the Devil great pain, and Dunstan only agreed to remove the shoe and release the Devil after the Devil promised never to enter a place where a horseshoe is hung over the door.
Some believe that if guests come to a house where a horseshoe is above the door, they must leave by the same door through which they entered or they will take the luck from the horseshoe with them from the house.
Another theory concerning the placing of horseshoes above doorways is to ward off Faeries; the theory being that Faeries are repelled by iron and as horseshoes were an easily available source of iron, they could be nailed above a door to prevent any unwanted, otherworldly guests. One can see how the custom, as people began to forget the stories concerning the Fair Folk, eventually morphed into a simple good luck charm.