Is it rude to ask via email to a professor if there are any internship opportunities? I've worked for an university for one year after my master's degree. Now I'm searching another job. For the moment I've tried to ask to several professors via email if there is any internship opportunities but only a couple of them answered me. I'm wondering if I'm doing something wrong or this is not the praxis or the formal manner for this type of requests. This is a general template I use:

Dear Professor Surname,
I'm a young chemist for the conservation of Cultural Heritage.  In my career I've studied mainly non-invasive techniques for art diagnostics and authentication, such as reflectography and Raman spectroscopy. I've also learned, on my own, to use GIS software for organizing data for Cultural Heritage management. I've seen that one of name of a Project I'm interested tasks is the maintenance of cultural heritage. Therefore, I would like to ask you if there are any internship opportunities in which my expertise could be useful to your current research? Thank you for your time.
Best regards,

How can I propose myself for an internship? Which is the most accepted praxis?
I don't think this practice is rude, but if these are not people with whom you have any connection, and who have not advertised a position, then I don't think you should be especially surprised that you aren't getting many responses.  In all likelihood, the answer is "no" and they don't feel like anything will be achieved by telling you that.  

A few things which might be hurting you:


The email above really reads like a mass form email.  You aren't making any argument about how your expertise is relevant to their specific work, and you're putting the burden on them to think about how your skills might fit with what they're doing rather than thinking about it yourself.
Maybe this is different in other fields, but it's not clear to me what "internship" is supposed to mean here.  A paid position?  Unpaid?  You may be being deliberately vague, but if you're not responding to an ad for a specific position, you have have some clarity about what you're looking for.
I think that the slightly awkward English doesn't really help.  "Chemist for the conservation of Cultural Heritage" sounds very strange to me.  I think "Chemist specializing in/interested in/working on the conservation of Cultural Heritage" is much better.