Trainers use bits of various metals and alloys in an effort to encourage a horse or mule to accept the bit. They believe that if the equine is encouraged to salivate, his mouth will be soft and receptive to bit cues. This idea in itself is debatable. Many equines have been trained with great success using bits that do not stimulate increased salivation.
Certain metals cause increased salivation in the equine mouth by reacting with moisture, warmth, and air in a process called oxidation. Oxidation is the formation of metal oxides that occurs when oxygen from air or moisture combines with a metal. This electrolytic process—for which the warmth and moisture in the equine's mouth act as a catalyst—either increases the positive valence or decreases the negative valence of the bit's metal, causing a reaction in the equine's mouth. ]In technical parlance, valence is the capacity of one element to combine with another, as measured by the number of hydrogen atoms one atom of the element will combine with; oxygen has a valence of two, since one atom of oxygen combines with two atoms of hydrogen.]
A bit with a mouthpiece made of two or more combined metals--like a German silver bit with a copper center roller, or a sweet iron bit with copper inlays—creates an unpleasant reaction. If you have fillings in your teeth and you bite down on aluminum foil—yikes! That's an electrolytic reaction caused by two or more metals.
Your fillings alone can cause an unpleasant tingling sensation or nasty metallic taste. And if you put a copper penny in your mouth (not recommended), after a few minutes you with experience a rather nasty taste. In each case the sensation is caused by an electrolytic reaction similar to what happens when you put certain bits in a horse's mouth.
Copper mouthpieces have long been used to stimulate salivation in equines. Copper is an electro-reactive metal with a low oxidation rate, as evidenced by its discoloration over time. German silver is an alloy of copper, nickel, and zinc. Brass is a copper/zinc alloy, and bronze is a copper/tin alloy.
Aurigan is a patented alloy comprised of 85% copper with 15% zinc and silicone. Its manufacturer claims that it "tastes sweet, has an extreme oxidation rate, and is strong." Proponents of these expensive bits claim they are superior because they contain no nickel.
Sweet iron is likely not pure iron, but a mixture of iron and carbon combined to create some form of a carbon steel. Bit makers claim that it tastes sweet to the animal and therefore keeps the animal's mouth moist. In my own experience sweet iron does not seem to cause extreme amounts of salivation, but this no doubt varies from equine to equine. Living in the dry climate of New Mexico, I have seen little if any rust (oxidization) on sweet iron bits. Whether sweet iron tastes good or bad, sweet or bitter, is academic. The truth may never be known.
Stainless steel is a steel alloy containing chromium and nickel. It is a non-reactive, non-oxidizing metal that is popularly referred to as 18/8 stainless (e.g. containing approximately 18% chromium and 8% nickel). Stainless steel is widely used in the medical profession to make pins and other parts that are implanted into the human body. Since it is non-reactive, it has no taste caused by electrolytic reactions and is considered a safe metal to be in contact with living tissue.
Aluminum is not an inert metal. That black smudgy stuff you can rub off the bit are aluminum oxides—the metal reacting with oxygen.
Chrome-plated bits are not reactive, as long as the plating is intact on the whole surface of the mouthpiece.
Plastic bits are now available. Their manufacturers claim they taste sweet or have apple flavors impregnated into them—something I have not personally been able to detect. Of course, no electrolytic reactions at all occur with plastic or rubber bits.
Getting your horse or mule to salivate through electrolytic reaction does not guarantee that he will accept the bit. Bit acceptance ultimately comes through kind and patient training. As you teach and physically condition your equine, keeping him relaxed and calm, you'll find that he will quietly champ the bit and maintain a relaxed jaw. When this happens, your equine will have naturally increased salivation and a soft, responsive mouth—the result of his trust in you and contentment with his work