What makes a good chalkboard? I've encountered lots of chalkboards through my career, and they vary widely in quality.  Some write smoothly and erase cleanly; for others, the chalk squeaks and the eraser just smears.

I assume there are differences in the materials they use, the construction techniques, how the surface is prepared or treated, and so on.  But I'm clueless as to what these differences might be, and how they affect the quality.  If I'm looking for a "good" one, what questions should I ask?  

(Academics are one of the few groups these days that use chalkboards on a regular basis, so I hope this question is suitable for this site.  Please note that I'm not asking for specific product recommendations, just general information.)
The good chalkboards appear to be made of porcelain enamel fused to a steel core. See this site for more details on the manufacturing process. As JeffE mentioned in a comment on another answer, cheaper chalkboards are made by painting wood or chipboard with a special chalkboard paint. Although these are good for crafts, they are NOT the kind you want to have in a classroom. They have to be primed with chalk before the first use and after being cleaned with a damp cloth. Also they will often have to be given another coat of paint after being used for a while.

For non-squeak writing, break long pieces of chalk in half--this will usually take care of the problem! I have noticed that regular, i.e. daily, sponging with an all-purpose cleaning solution also helps to prevent smearing while writing/erasing.