What is meant by this use of quotation marks?
Solution 1:
The quotation marks are signalling unusual usage:
Quotation marks are often used to alert readers that a term is used in a nonstandard, ironic, or other special sense.* [CMoS]
A drawing is normally made with crayon, chalk or pen on papaer. Not with gunpowder on a wood screen. Hence the "what I (or the artist) will now call a drawing".
The colon is used to let the viewer know that his wall sized screens are the drawings mentioned
A crop circle is flattened corn on a field, but one of the "drawings" resemble a crop circle so it is in quotation marks