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