Why does Sulphur powder get used on feathers?

Flowers Of Sulphur is also known as yellow sulphur (sulfur) powder, is a bright yellow powder obtained from naturally-occurring volcanic brimstone deposits. It is a purified form of elemental sulphur called sublimed sulphur in scientific circles. Flowers of sulphur are produced by distillation. The resulting powder is chemically active and has several important uses, including preserving fruit, fertilising plants and controlling certain kinds of pests. FOS is a natural mineral that has been used as a remedy for centuries. In ancient texts such as the bible it is called brimstone, and 2,000 years ago it was used as a remedy for skin disorders and other ailments.

Flowers of sulphur is a fairly pure form of elemental sulphur, with only a few impurities. It includes some oxygen, and it slowly oxidises to form a small amount of sulphuric acid, or H2SO4. Flowers of sulphur should not be confused with flour of sulphur, the ground form of a less pure variety of sulphur called roll sulphur or cast rhombic sulphur. Sulphur is a non-metal and is the fifteenth most abundant element in the Earth's rocky crust and upper mantle.

Sulphur is flammable, with a flashpoint of 207 degrees Celsius and a spontaneous combustion temperature of 232 degrees C. It may ignite explosively if the dust becomes airborne. Airborne sulphur dust is also an inhalation hazard. It may cause skin or eye irritation. While flowers of sulphur are not poisonous, ingesting large amounts may cause sore throat, headache and nausea. Store flowers of sulphur in a tightly sealed container in a cool, dry location.

You can buy Sulphur powder at pet shops and animal stock feed stores / country stores. Farmers, those with animal stock, and pet breeders have used it for many decades. It is cheap and effective.
When you look at Flowers of Sulphur under a microscope the crystals resemble flowers, hence the name.
Flowers of sulphur is not the same as MSM (methylsulfonylmethane), which is also a source of sulphur.

 Sulphur powder has many uses for horses and their owners:

• Antiseptic. Kills bacteria and fungi on the skin. Can be applied to wounds, cuts and grazes.
• Lice. Kills and repels head lice and pubic lice.
• Mites. Treats all kinds of mites including skin mites.
• FOS treats most skin complaints in pets and farm animals. It kills and repels fleas, tics, mites and lice. Treats skin allergies, mange and scabies.
• Horses. Good for sweet itch, mud fever and skin complaints.

How does it work? 

Flowers of sulphur are effective as an insecticidal dust, commonly used against the mites that cause scabies or chorioptic mange. The powder, once applied (please see what-is-pig-oil? for more advice on an application method) , slowly oxidises to form the active ingredient sulphur dioxide. Sulphur dioxide is used as a preservative for fruit, wine-making and in many other everyday uses. 

Although it is a traditional and long used remedy and is a product in many veterinary as well as human medications some people and animals are sensitive or have allergies to sulphur and a test patch should always be carried out before using sulphur powder. With heavy horses if concerned please consult a veterinarian for advice before trying sulphur powder on a horse.