A while back, I finally managed to obtain 'Architecture, dead language' by Giorgio Grassi from my university library after it had been on long term loan. Now that I've had chance to familiarise myself with his drawings, it's clear they're not watercolours as I had suspected from reduced image sizes but have in fact been rendered with a form of marker pen. I have a set of Faber Castell PITT artist pens but on any given paper stock, non of these seem to be able to produce a translucent watercolour effect as Grassi manages to do.

I have worked a fair bit with watercolour but would love to know of the pen sets (or similar) that Grassi uses. Are they unique to Italy or am I missing an obvious trick here!?

Any help appreciated, thanks.