A few connections between Breaths and Metallic Arts. Mainly, what can be done in one system of Investiture can often be done in another system, too.
Iron/Steel -- Using Breaths to Awaken works on biological-but-nonliving things best and doesn't work on metals (mostly). In contrast, burning iron or steel in Allomancy pulls or pushes metals, and metals only (mostly).
Tin -- A person with more Breaths have increased senses (and life awareness), similar to burning tin in Allomancy.
Pewter -- A person with Breaths and proper commands can effectively strengthen one's body, similar to burning pewter in Allomancy.
Copper -- A person with many Breaths have ways to hide their Breath so it doesn't give off an aura. They can also hide their life sense. In Allomancy, burning copper hides Allomantic pulses.
The Returned have a very strong Breath but no memory of their former life. Copper stores memory in Feruchemy and steaks memory and other things in Hemalurgy. (Or maybe I should sort this under aluminum & duralumin storing identity & connection.)
Bronze -- A person of upper Heightenings can sense an object that has been awakened. Similarly, a person burning bronze can sense the use of Allomancy.
Cadmium -- Cadmium literally stores breaths... But I think it's the oxygen kind of breath, not the Biochroma type of breath.
Gold/Electrum -- The Returned, who have divine Breaths, have a very limited (but important) access to visions of the past and future. Burning gold and electrum in Allomancy reveals one's past self and future, respectively.
A person with lots of Breaths is nearly immune to diseases and poison and slow to age; The Returned are in perfect health, can survive without food, and can heal one person. Gold stores health in Feruchemy.
Pewter & Atium -- Some people with divine Breath can change their appearance, affecting things like height, body shape, age, and hair color. Some of these (but not the hair color) can be achieved to some extent by using pewter in Allomancy and Feruchemy (changes body's musculature) and Atium in Feruchemy (changes body's age).
