Modern painters use copper and sulfur in traces (7)

I believe the answer is:

cubists

I'm a little stuck... Click here to teach me more about this clue!

'modern painters' is the definition.
'cubists' can be an answer for 'painters' (cubist is a kind of painter). I am unsure of the 'modern' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for cubists that I've seen before include "Painters" , "Disciples of Picasso, perhaps" , "group of artists" .)

I've seen this clue in the Sydney Morning Herald.

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