What are mouth-watering bits and how do they work to improve a horse's acceptance of the bit?

Mouth-watering bits are bits made from or inlaid with metals that promote salivation in the horse's mouth through their taste and the natural chemical reaction they undergo when exposed to moisture and warmth. The most commonly used mouth-watering metals are copper and sweet iron, both of which produce a mild, palatable taste that encourages the horse to mouth the bit gently, keep his jaw soft and mobile, and produce the steady flow of saliva that signals a relaxed, accepting mouth. Understanding why salivation matters — and how these metals produce it — explains why mouth-watering bits have been a staple of quality horsemanship for generations. A moist, salivating mouth is one of the most reliable indicators of genuine relaxation in a horse under saddle. When a horse is tense, anxious, or bracing against the bit, the muscles of the jaw and face tighten, salivation decreases, and the mouth becomes dry. A dry mouth is a resistant mouth — the tissues of the bars and tongue are less supple, the horse is more likely to clamp the jaw or evade through the tongue, and the bit's communication becomes less clear and less comfortable for both horse and rider. Conversely, a horse that is producing saliva freely is relaxing the jaw muscles, softening the tissues of the mouth, and maintaining the mobile, chewing jaw action that classical horsemanship has always recognized as a sign of throughness and acceptance. Copper is the most widely used mouth-watering material and appears in western and English bits across every price range and design type. It oxidizes quickly when exposed to the moisture and warmth of the horse's mouth, producing a mild, sweet taste that most horses find pleasant and that triggers salivary gland activity. Copper may be used as the entire mouthpiece, as inlays woven through a steel mouthpiece, as copper rings or bands at specific points of the mouthpiece, or as copper roller inserts. The more copper surface area in contact with the mouth, the stronger the salivating effect, which is why solid copper mouthpieces tend to produce more consistent mouth moisture than bits with only small copper accents. Sweet iron — also called cold-rolled steel or blue steel — is the other primary mouth-watering metal and is particularly popular in western performance bits, especially those influenced by the California vaquero tradition. Sweet iron rusts readily when exposed to moisture, and that surface rust has a sweet, slightly metallic taste that horses accept readily and that promotes salivation effectively. The characteristic blue-grey or rusty appearance of a sweet iron bit that has been used regularly is not a sign of neglect — it is evidence that the bit is doing its job. Trainers who use sweet iron bits deliberately allow the oxidation to develop rather than polishing the mouthpiece clean, because the rusted surface is what produces the taste the horse responds to. Many high-quality performance bits combine sweet iron mouthpieces with copper inlays or copper crickets, stacking both mouth-watering materials for a pronounced salivating effect. The combination produces a consistently moist, mobile mouth even in horses that are naturally tense or dry-mouthed, and many trainers consider this combination the most effective option available for horses that need help relaxing the jaw and accepting contact willingly. Stainless steel bits, by contrast, are inert — they do not oxidize, have no particular taste, and provide no mouth-watering benefit, which is one reason stainless steel is more common in English disciplines where other aspects of contact and acceptance are managed through training rather than through the bit's material properties.