Question: I start learning the English language in a more native way. I listen to a course on the internet.
A man, who gives this course, said this:

**This is pure glass** (this sentence has been taken from a movie)

My question here, why we do not use `a` with **pure glass**?

Answer: 
For the same reason we do not say "a pure money" - the noun is [uncountable](https://learnenglish.britishcouncil.org/grammar/beginner-to-pre-intermediate/countable-and-uncountable-nouns-1).

"[Glass](https://www.oxfordlearnersdictionaries.com/definition/english/glass_1)" can refer to a number of things. The most likely reference in your quote is the material glass (def. 1 at OALD), in which case it is uncountable. In other cases, e.g. "a glass of milk," it refers to the container for the beverage, in which case it is countable.

But there are exceptions. We do come across uncountable nouns that are sometimes affixed with indefinite articles:

> 
> Pine is **a** soft **wood**.
> 
> 
> 

"Wood" as used here (the material used in woodworking) is uncountable, but the indefinite article is used because we are discussing a *type* of wood.
`Glass` is used as uncountable noun here. It means the material, not the thing made up of that material.

You can only use the `a` determiner with a countable noun so you can't use it in this context. But `glass` can also be a countable noun (something made of glass to drink from), in which case you can, and should, use the determiner:

> 
> This is a glass.
> 
> 
> 

Although `pure` can't be used here since it only applies to the material. Instead you could say something like this:

> 
> This is a glass made of pure glass.
> 
> 
>