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, also called farriers or blacksmiths (more commonly used in the UK), attach horseshoes on the palmar surface of the hoof, usually by nailing through the insensitive hoof wall, which is anatomically similar 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 horse and for the work they do. The most common materials are steel and aluminum, 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.
When kept as a talisman, a horseshoe is said to bring good luck. Many believe that to hang it with the ends pointing upwards is good luck as it acts as a storage container of sorts for any good luck that happens to be floating by, whereas to hang it with the ends pointing down, is bad luck as all the good luck will fall out. Others believe that the shoe should be hung the other way, as it will then release its luck to the people around it. A stylized variation of the horseshoe is used for a popular throwing game, horseshoes.
Since the early history of domestication of the horse, working animals were found to be 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. An early form of hoof protection was seen in ancient Asia, where horses' hooves were wrapped in rawhide, leather or other materials for both therapeutic purposes and protection from wear.
The nailed shoe was a relatively late invention. 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, if taken literally, is an effect that could be obtained only by shod horses, as barefoot hooves striking stone do not create sparks.
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, the Romans appeared to have attempted to protect their horses' feet with a strap-on, solid-bottomed "hipposandal" that has a slight resemblance to the modern hoof boot.
According to the Encyclopædia Britannica Eleventh Edition (1911),Though the ancients were sufficiently impressed by the damage done to horses' hoofs to devise certain forms of covering for them (in the shape of socks or sandals), the practice of nailing iron plates or rim-shoes to the hoof does not appear to have been introduced earlier than the 2nd century B.C., and was not commonly known till the close of the 5th century A.D., or in regular use till the middle ages. The evidence for the earlier date depends on the doubtful interpretations of designs on coins, &c.
There is very little evidence of nailed-on shoes prior to AD 500 or 600, though there is speculation that the Gauls were the first to nail on metal horseshoes. The nailed iron horseshoe first appeared in the archaeological record in Europe about 5th century A.D. when a horseshoe, complete with nails, was found in the tomb of the Frankish King Childeric I at Tournai, Belgium. The earliest clear written record of iron horseshoes is a reference to "crescent figured irons and their nails" in AD 910.
Around 1000 AD, cast bronze horseshoes with nail holes became common in Europe. Common was a design with a scalloped outer rim and six nail holes. The 13th and 14th centuries brought the widespread manufacturing of iron horseshoes. By the time of the Crusades (1096–1270), horseshoes were widespread and frequently mentioned in various written sources. In that period, due to the value of iron, horseshoes were even accepted in lieu of coin to pay taxes.
By the 13th century, shoes were forged in large quantities and could be bought ready-made. Hot shoeing, the process of shaping a heated horseshoe immediately before placing it on the horse, became common in the 16th century. From the need for horseshoes, the craft of blacksmithing became "one of the great staple crafts of medieval and modern times and contributed to the development of metallurgy.” A treatise titled "No Foot, No Horse" was published in Great Britain in 1751.
In the mid 19th century Canada, marsh horseshoes kept horses from sinking into the soft intertidal mud during dike-building. In a common design, a metal horseshoe holds a flat wooden shoe in place. 
Reasons for use of horseshoes
Many changes brought about by domestication of the horse have led to a need for shoes for number of reasons. Overall in captivity, horses' hooves harden much less and are more vulnerable to injury. In the wild, a horse may travel up to 50 miles per day to obtain adequate forage. While horses in the wild covered large areas of terrain, they usually did so at relatively slow speeds, unless being chased by a predator. They also tended to live in arid, steppe climates. The consequence of slow but nonstop travel in a dry climate is that horse's feet are naturally worn to a small, smooth, even and hard state. The continual stimulation of the sole of the foot keeps it thick and hard. However, in domestication, the ways horses are used differs from their natural environment. Domesticated horses were moved in large numbers from the arid steppes to colder and wetter areas. These softer and heavier soils soften the hooves and have made them prone to splitting, making hoof protection necessary. Consequently, it was in northern Europe that the nailed horseshoe arose in its modern form.
Domesticated horses are also subject to inconsistent movement between stabling and work, they must carry or pull additional weight, and in modern times they are often kept and worked on very soft footing, such as irrigated land, arena footing, or stall bedding. In some cases, management is also inadequate. The hooves of horses that are kept in stalls or small turnouts, even when cleaned adequately, are still exposed to more moisture than would be encountered in the wild, as well as to ammonia from urine. The hoof capsule is mostly made from keratin, a protein, and is weakened by this exposure, becoming even more fragile and soft. Wearing shoes does not prevent or reduce damage from moisture and ammonia exposure. Rather, they protect already weakened hooves. Further, without the natural conditioning factors present in the wild, the feet of horses grow overly large and long unless trimmed regularly. Hence, protection from rocks, pebbles, and hard, uneven surfaces is lacking. A balanced diet with proper nutrition also is a factor. Without these precautions, cracks in overgrown and overly brittle hoof walls are a danger, as is bruising of the soft tissues within the foot because of inadequately thick and hard sole material.
Physical stresses requiring horseshoes
- Abnormal stress: Horses' hooves can become quite worn out when subjected to the added weight and stress of a rider, pack load, cart, or wagon.
- Corrective shoeing: The shape, weight, and thickness of a horseshoe can significantly affect the horse's gait. Farriers may forge custom shoes to help horses with bone or musculature problems in their legs, or fit commercially available remedial shoes.
- Traction: Traction devices such as borium for ice, horse shoe studs for muddy or slick conditions, calks, carbide-tipped road nails and rims are useful for performance horses such as eventers, show jumpers, polo ponies, and other horses that perform at high speeds, over changing terrain, or in less-than-ideal footing.
- Gait manipulation: Some breeds such as the Saddlebred, Tennessee Walking Horse, and other gaited horses are judged on their high-stepping movement. Special shoeing can help enhance their natural movement.
- Racing horses with weakness in their foot or leg require specialized horseshoes. 
Horseshoeing theories and debates
Horseshoes have always been viewed as an aid to assist horses' hooves when subjected to the various unnatural conditions brought about by domestication, whether due to work conditions or stabling and management. Countless generations of domestic horses bred for size, color, speed, and many other traits with little regard for hoof quality and soundness make some breeds more dependent on horseshoes than feral horses such as mustangs, which develop strong hooves as a matter of natural selection.
Nonetheless, domestic horses do not always require shoes. There is near-universal agreement among farriers that when possible, a barefoot hoof, at least for part of every year, is a healthy option for most horses. However, other farriers are equally adamant that horseshoes have their place and can help prevent excess or abnormal hoof wear and injury to the foot. Many farriers 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.
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 whether to use horseshoes is a hot topic today.
Process of shoeing
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 pliers-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 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.
Mistakes are sometimes made by even a skilled farrier, especially if the horse does not stand still. This may sometimes result in a nail coming too close to the sensitive part of the hoof (putting pressure on it), or a nail that is driven slightly into the sensitive hoof, called "quicking" or nail pricking. This occurs when a nail penetrates the wall and hits the sensitive internal structures of the foot. Quicking results in bleeding and pain and the horse may show signs of lameness or may become lame in following days. Whenever it happens, the farrier must remove the offending nail. Usually a horse that is quicked will react immediately, though some cases where the nail is close to sensitive structures may not cause immediate problems. These mistakes are made occasionally by anyone who shoes horses, and in most cases is not an indication that the farrier is unskilled. It happens most commonly when horses move around while being shod, but also may occur if the hoof wall is particularly thin (common in Thoroughbreds), or if the hoof wall is brittle or damaged. It may also occur with an inexperienced or unskilled horseshoer who misdrives a nail, uses a shoe that is too small, or has not fitted the shoe to the shape of the horse's hoof. Occasionally, manufacturing defects in nails or shoes may also cause a misdriven nail that quicks a horse.
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. 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.
The horseshoe is presented as a talisman in The True Legend of St. Dunstan and the Devil; Showing how the Horse-Shoe came to be a Charm against Witchcraft, written in 1871 by Edward G. Flight, with illustrations by George Cruikshank and engravings by John Thompson.
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