Hi Sami, hi Anthony, what I need to do is actually what Sami says. I do not want to always archive the artifacts of all builds, since I don't have so much space. That instance of Jenkins is meant to be populated with dozen projects and it might be used to build "test" builds or to make "release" builds. I found it a good solution to create a promotion job called "Release" that I manually trigger when I want to promote a build to be a "Release" build. Then, I want artifacts from that build to be archived, so that they are downloadable even if the workspace is later used for making other new builds.
I then found this bug: https://issues.jenkins-ci.org/browse/JENKINS-13447 Artifacts can actually be archived at promotion time, the problem is that the Jenkins UI is not updated in order to make any link to the archived artifacts available. I think I have to find another solution meanwhile... :-( Thanks for your help, Mauro.