Even pure, acid free papers may be contaminated by the polluted environment around them, causing them to eventually no longer be acid free. There are two different systems used to describe the weight of different papers. If the weight of the paper is stated in pounds, the weight was determined by weighing one ream of the basic size of that paper. Both of those papers are actually the same thickness. The alternate method of describing paper weight is to weigh one square meter of the paper, and list its weight in grams.
Watercolor paper is available in as many as four different surfaces.
Hot Press is the term used to describe the smoothest surface of watercolor paper, as if it had been ironed using a hot iron to make it perfectly smooth. Soft Press paper has slightly more texture, but is still smooth, as if the hot ironing had been done, but using less pressure. Cold Press paper has even more texture than the previous two, as if it had been ironed using a colder iron, leaving more bumps. At the paper mill, these textures are actually achieved by putting the paper through big rollers, which basically do iron the paper. Uses for decorative papers are limited only by your imagination.