When we talk about materials such as gold, iron, wood, cotton, etc. in general, we don’t use articles.
The cotton is soft.
- Cotton is soft.
The silver is shiny.
- Silver is shiny.
If we talk about a particular example of a material then we use the.
- The wood I bought is from a pine tree.
(particular example/wood I bought)
- The silver used to make this ring is very old.
(particular example/silver in the ring)