In "Word Focus" William Sillings writes:

The clay (pelos) that Jesus made was made by putting spittle, or saliva (ptusma), on the ground and mixing the saliva with the dirt. The ancients held a high regard for the curative value of saliva. And while we may frown on the gauche nature of such an idea today, there is little doubt that even the saliva of Jesus had healing virtue.
But Jesus was going to get into trouble for this act of making clay of dirt and saliva. Why? It was the Sabbath (v. 14). The Jewish traditional law forbade kneading on the Sabbath, and mixing any liquid with a solid in order to form anything of a dough-like or clay-like consistency was considered kneading. What Jesus did here, then, was not only to heal a man on the Sabbath, but also, in the minds of the Pharisees, to desecrate the Sabbath by working.

Discussion: The Pharisees thought Jesus was a sinner. Why don't we?
Source: Miracles of Jesus: Adult Teacher's Insights, page 46.