What does this sentence mean?

[Cooliegirly]

The avidness with which merchants and landowners in early-nineteenth-century Maryland and Virginia sought Joshua Johnston’s professional services attests to his artistic skill as a portrait painter.

What does this sentence mean? What does the "which" indicate here?

[Mister Micawber]

.
Which refers to avidness.

Merchants & landowners eagerly wanted Johnston's paintings, so he must have been a good painter.
.
_________________
Native English teacher at Mister Micawber's

[Cooliegirly]

It's still kind of confusing to me.

[Alan]

Hi Cooliegirly,

Let me try and explain this:

The avidness with which merchants and landowners in early-nineteenth-century Maryland and Virginia sought Joshua Johnston’s professional services attests to his artistic skill as a portrait painter. What does this sentence mean? What does the "which" indicate here?

In the early nineteenth century merchants and landowners from Maryland and Virginia wanted to use J J's professional services. The fact that (because) they tried to get these services with so much enthusiasm/determination (avidness) proves (attests to) that he was a very skilful (clever) painter.

Hope this helps.

Alan
_________________
English as a Second Language
You can read my ESL story Present Simple

[Cooliegirly]

Ah...Thank you! And thank you, too, Mr Micawber.