Fresco (buon fresco in Italian) was the technique of painting on wet plaster, which produced a very durable decoration on the wall, as opposed to fresco secco, which tends to flake off in time.

Fresco is quite a complex process, so it is a good question which 'medium' you have here. It could also be understood as 'the medium of expression' - 'expressing himself through the medium of fresco'.

The wall was rough-plastered, then a layer of plaster called the arriciato was applied. On this the outline for the fresco would be drawn full size.
Each day, an area large enough for one day's work would be covered with the final layer of plaster, called intonaco, and the colours applied to that area.

The pigments were mixed with plain water or lime-water, allowing for the fact that when they dried, they appeared much lighter.

Besides working in fresco, Leonardo painted in oils and experimented. In fact one of his most famous paintings, the Last Supper, was already deteriorating in his lifetime, because here, his experiments with with using oil colours on the fresco were not successful.

I hope you can use some of this information - you may have a lot of it already!


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Note added at 23 hrs 9 mins (2005-10-14 10:20:35 GMT)
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To Roald - I meant Leonardo - I didn't know you were left handed, but there you are!

Michelangelo was left handed too ... We're in good company