Untreated materials may degrade faster than treated ones, often due to exposure to environmental factors. If you leave that untreated metal outside, it could rust quickly.
Pro Tip 2/3
Natural State
'Untreated' often implies that the substance is in its natural, original state without any alterations. I prefer furniture made from untreated wood for a more rustic look.
Pro Tip 3/3
Environmental Concern
Using 'untreated' may refer to concerns over chemicals or processes that are harmful to the environment. Many prefer untreated food products to avoid pesticides.