Sweet iron will rust when it comes into contact with (air) humidity; the surface rust tastes sweet and naturally stimulates the saliva production. This will cause the horse to salivate more which in turn encourages better acceptance of the bit. The rusting process (oxidation) changes the blue colour of the TRUST-bit to a brownish-grey.
If a sweet iron bit is not used for a while, an orange-brown rust layer may develop on the mouthpiece. Simply remove this by wiping the bit with a damp cloth.