Sweet iron will rust when in contact with (air) humidity; the surface rust tastes sweet and naturally stimulates the saliva production. This will cause the horse to salivate more which will encourage the horse to take more contact. The process of rusting (oxidation) changes the blue colour of the TRUST-bit to a brownish-grey. A sweet iron bit will develop an orange-brown rust layer on the mouthpiece if it is temporarily not used. This is easily removed by wiping it off with a damp cloth.