Yes, this is precisely the effect of Sweet Iron! Sweet Iron develops surface rust through contact with (air)humidity. This surface rust tastes sweet and naturally stimulates the saliva production. This causes the horse to salivate more, which in turn encourages a better acceptance of the bit. The rusting process (oxidation) transforms the blue colour of the TRUST Bits into a brownish-grey. If a sweet iron bit is not used for a while, an orange-brown rust layer may form on the mouthpiece. Simply remove this by wiping the bit with a damp cloth. Of course, this surface rust is not harmful to the horse.